diff --git a/.travis.yml b/.travis.yml
index cb9bf42225a..4126683eb62 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,7 +8,7 @@ node_js:
 sudo: false
 
 env:
-  - workerCount=4
+  - workerCount=3
 
 matrix:
   fast_finish: true
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 74a1e17bd6b..bec8686ab93 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4416,10 +4416,19 @@ namespace ts {
             }
             const propTypes: Type[] = [];
             const declarations: Declaration[] = [];
+            let commonType: Type = undefined;
+            let hasCommonType = true;
             for (const prop of props) {
                 if (prop.declarations) {
                     addRange(declarations, prop.declarations);
                 }
+                const type = getTypeOfSymbol(prop);
+                if (!commonType) {
+                    commonType = type;
+                }
+                else if (type !== commonType) {
+                    hasCommonType = false;
+                }
                 propTypes.push(getTypeOfSymbol(prop));
             }
             const result = <TransientSymbol>createSymbol(
@@ -4429,6 +4438,7 @@ namespace ts {
                 commonFlags,
                 name);
             result.containingType = containingType;
+            result.hasCommonType = hasCommonType;
             result.declarations = declarations;
             result.isReadonly = isReadonly;
             result.type = containingType.flags & TypeFlags.Union ? getUnionType(propTypes) : getIntersectionType(propTypes);
@@ -7798,8 +7808,39 @@ namespace ts {
             return false;
         }
 
-        function rootContainsMatchingReference(source: Node, target: Node) {
-            return target.kind === SyntaxKind.PropertyAccessExpression && containsMatchingReference(source, (<PropertyAccessExpression>target).expression);
+        // Return true if target is a property access xxx.yyy, source is a property access xxx.zzz, the declared
+        // type of xxx is a union type, and yyy is a property that is possibly a discriminant. We consider a property
+        // a possible discriminant if its type differs in the constituents of containing union type, and if every
+        // choice is a unit type or a union of unit types.
+        function containsMatchingReferenceDiscriminant(source: Node, target: Node) {
+            return target.kind === SyntaxKind.PropertyAccessExpression &&
+                containsMatchingReference(source, (<PropertyAccessExpression>target).expression) &&
+                isDiscriminantProperty(getDeclaredTypeOfReference((<PropertyAccessExpression>target).expression), (<PropertyAccessExpression>target).name.text);
+        }
+
+        function getDeclaredTypeOfReference(expr: Node): Type {
+            if (expr.kind === SyntaxKind.Identifier) {
+                return getTypeOfSymbol(getResolvedSymbol(<Identifier>expr));
+            }
+            if (expr.kind === SyntaxKind.PropertyAccessExpression) {
+                const type = getDeclaredTypeOfReference((<PropertyAccessExpression>expr).expression);
+                return type && getTypeOfPropertyOfType(type, (<PropertyAccessExpression>expr).name.text);
+            }
+            return undefined;
+        }
+
+        function isDiscriminantProperty(type: Type, name: string) {
+            if (type && type.flags & TypeFlags.Union) {
+                const prop = getPropertyOfType(type, name);
+                if (prop && prop.flags & SymbolFlags.SyntheticProperty) {
+                    if ((<TransientSymbol>prop).isDiscriminantProperty === undefined) {
+                        (<TransientSymbol>prop).isDiscriminantProperty = !(<TransientSymbol>prop).hasCommonType &&
+                            isUnitUnionType(getTypeOfSymbol(prop));
+                    }
+                    return (<TransientSymbol>prop).isDiscriminantProperty;
+                }
+            }
+            return false;
         }
 
         function isOrContainsMatchingReference(source: Node, target: Node) {
@@ -8090,6 +8131,25 @@ namespace ts {
             return source.flags & TypeFlags.Union ? !forEach((<UnionType>source).types, t => !contains(types, t)) : contains(types, source);
         }
 
+        function isTypeSubsetOf(source: Type, target: Type) {
+            return source === target || target.flags & TypeFlags.Union && isTypeSubsetOfUnion(source, <UnionType>target);
+        }
+
+        function isTypeSubsetOfUnion(source: Type, target: UnionType) {
+            if (source.flags & TypeFlags.Union) {
+                for (const t of (<UnionType>source).types) {
+                    if (!containsType(target.types, t)) {
+                        return false;
+                    }
+                }
+                return true;
+            }
+            if (source.flags & TypeFlags.EnumLiteral && target.flags & TypeFlags.Enum && (<EnumLiteralType>source).baseType === target) {
+                return true;
+            }
+            return containsType(target.types, source);
+        }
+
         function filterType(type: Type, f: (t: Type) => boolean): Type {
             return type.flags & TypeFlags.Union ?
                 getUnionType(filter((<UnionType>type).types, f)) :
@@ -8228,7 +8288,7 @@ namespace ts {
                 if (isMatchingReference(reference, expr)) {
                     type = narrowTypeBySwitchOnDiscriminant(type, flow.switchStatement, flow.clauseStart, flow.clauseEnd);
                 }
-                else if (isMatchingPropertyAccess(expr)) {
+                else if (isMatchingReferenceDiscriminant(expr)) {
                     type = narrowTypeByDiscriminant(type, <PropertyAccessExpression>expr, t => narrowTypeBySwitchOnDiscriminant(t, flow.switchStatement, flow.clauseStart, flow.clauseEnd));
                 }
                 return createFlowType(type, isIncomplete(flowType));
@@ -8236,6 +8296,7 @@ namespace ts {
 
             function getTypeAtFlowBranchLabel(flow: FlowLabel): FlowType {
                 const antecedentTypes: Type[] = [];
+                let subtypeReduction = false;
                 let seenIncomplete = false;
                 for (const antecedent of flow.antecedents) {
                     const flowType = getTypeAtFlowNode(antecedent);
@@ -8250,11 +8311,17 @@ namespace ts {
                     if (!contains(antecedentTypes, type)) {
                         antecedentTypes.push(type);
                     }
+                    // If an antecedent type is not a subset of the declared type, we need to perform
+                    // subtype reduction. This happens when a "foreign" type is injected into the control
+                    // flow using the instanceof operator or a user defined type predicate.
+                    if (!isTypeSubsetOf(type, declaredType)) {
+                        subtypeReduction = true;
+                    }
                     if (isIncomplete(flowType)) {
                         seenIncomplete = true;
                     }
                 }
-                return createFlowType(getUnionType(antecedentTypes), seenIncomplete);
+                return createFlowType(getUnionType(antecedentTypes, subtypeReduction), seenIncomplete);
             }
 
             function getTypeAtFlowLoopLabel(flow: FlowLabel): FlowType {
@@ -8280,6 +8347,7 @@ namespace ts {
                 // Add the flow loop junction and reference to the in-process stack and analyze
                 // each antecedent code path.
                 const antecedentTypes: Type[] = [];
+                let subtypeReduction = false;
                 flowLoopNodes[flowLoopCount] = flow;
                 flowLoopKeys[flowLoopCount] = key;
                 flowLoopTypes[flowLoopCount] = antecedentTypes;
@@ -8296,6 +8364,12 @@ namespace ts {
                     if (!contains(antecedentTypes, type)) {
                         antecedentTypes.push(type);
                     }
+                    // If an antecedent type is not a subset of the declared type, we need to perform
+                    // subtype reduction. This happens when a "foreign" type is injected into the control
+                    // flow using the instanceof operator or a user defined type predicate.
+                    if (!isTypeSubsetOf(type, declaredType)) {
+                        subtypeReduction = true;
+                    }
                     // If the type at a particular antecedent path is the declared type there is no
                     // reason to process more antecedents since the only possible outcome is subtypes
                     // that will be removed in the final union type anyway.
@@ -8303,13 +8377,14 @@ namespace ts {
                         break;
                     }
                 }
-                return cache[key] = getUnionType(antecedentTypes);
+                return cache[key] = getUnionType(antecedentTypes, subtypeReduction);
             }
 
-            function isMatchingPropertyAccess(expr: Expression) {
+            function isMatchingReferenceDiscriminant(expr: Expression) {
                 return expr.kind === SyntaxKind.PropertyAccessExpression &&
+                    declaredType.flags & TypeFlags.Union &&
                     isMatchingReference(reference, (<PropertyAccessExpression>expr).expression) &&
-                    (declaredType.flags & TypeFlags.Union) !== 0;
+                    isDiscriminantProperty(declaredType, (<PropertyAccessExpression>expr).name.text);
             }
 
             function narrowTypeByDiscriminant(type: Type, propAccess: PropertyAccessExpression, narrowType: (t: Type) => Type): Type {
@@ -8323,10 +8398,10 @@ namespace ts {
                 if (isMatchingReference(reference, expr)) {
                     return getTypeWithFacts(type, assumeTrue ? TypeFacts.Truthy : TypeFacts.Falsy);
                 }
-                if (isMatchingPropertyAccess(expr)) {
+                if (isMatchingReferenceDiscriminant(expr)) {
                     return narrowTypeByDiscriminant(type, <PropertyAccessExpression>expr, t => getTypeWithFacts(t, assumeTrue ? TypeFacts.Truthy : TypeFacts.Falsy));
                 }
-                if (rootContainsMatchingReference(reference, expr)) {
+                if (containsMatchingReferenceDiscriminant(reference, expr)) {
                     return declaredType;
                 }
                 return type;
@@ -8355,13 +8430,13 @@ namespace ts {
                         if (isMatchingReference(reference, right)) {
                             return narrowTypeByEquality(type, operator, left, assumeTrue);
                         }
-                        if (isMatchingPropertyAccess(left)) {
+                        if (isMatchingReferenceDiscriminant(left)) {
                             return narrowTypeByDiscriminant(type, <PropertyAccessExpression>left, t => narrowTypeByEquality(t, operator, right, assumeTrue));
                         }
-                        if (isMatchingPropertyAccess(right)) {
+                        if (isMatchingReferenceDiscriminant(right)) {
                             return narrowTypeByDiscriminant(type, <PropertyAccessExpression>right, t => narrowTypeByEquality(t, operator, left, assumeTrue));
                         }
-                        if (rootContainsMatchingReference(reference, left) || rootContainsMatchingReference(reference, right)) {
+                        if (containsMatchingReferenceDiscriminant(reference, left) || containsMatchingReferenceDiscriminant(reference, right)) {
                             return declaredType;
                         }
                         break;
@@ -8500,9 +8575,7 @@ namespace ts {
 
             function getNarrowedType(type: Type, candidate: Type, assumeTrue: boolean) {
                 if (!assumeTrue) {
-                    return type.flags & TypeFlags.Union ?
-                        getUnionType(filter((<UnionType>type).types, t => !isTypeSubtypeOf(t, candidate))) :
-                        type;
+                    return filterType(type, t => !isTypeSubtypeOf(t, candidate));
                 }
                 // If the current type is a union type, remove all constituents that aren't assignable to
                 // the candidate type. If one or more constituents remain, return a union of those.
@@ -8512,13 +8585,16 @@ namespace ts {
                         return getUnionType(assignableConstituents);
                     }
                 }
-                // If the candidate type is assignable to the target type, narrow to the candidate type.
-                // Otherwise, if the current type is assignable to the candidate, keep the current type.
-                // Otherwise, the types are completely unrelated, so narrow to the empty type.
+                // If the candidate type is a subtype of the target type, narrow to the candidate type.
+                // Otherwise, if the target type is assignable to the candidate type, keep the target type.
+                // Otherwise, if the candidate type is assignable to the target type, narrow to the candidate
+                // type. Otherwise, the types are completely unrelated, so narrow to an intersection of the
+                // two types.
                 const targetType = type.flags & TypeFlags.TypeParameter ? getApparentType(type) : type;
-                return isTypeAssignableTo(candidate, targetType) ? candidate :
+                return isTypeSubtypeOf(candidate, targetType) ? candidate :
                     isTypeAssignableTo(type, candidate) ? type :
-                        getIntersectionType([type, candidate]);
+                    isTypeAssignableTo(candidate, targetType) ? candidate :
+                    getIntersectionType([type, candidate]);
             }
 
             function narrowTypeByTypePredicate(type: Type, callExpression: CallExpression, assumeTrue: boolean): Type {
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 28ededebb0d..a6e860450c6 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2160,6 +2160,8 @@ namespace ts {
         mapper?: TypeMapper;                // Type mapper for instantiation alias
         referenced?: boolean;               // True if alias symbol has been referenced as a value
         containingType?: UnionOrIntersectionType; // Containing union or intersection type for synthetic property
+        hasCommonType?: boolean;            // True if constituents of synthetic property all have same type
+        isDiscriminantProperty?: boolean;   // True if discriminant synthetic property
         resolvedExports?: SymbolTable;      // Resolved exports of module
         exportsChecked?: boolean;           // True if exports of external module have been checked
         isDeclarationWithCollidingName?: boolean;    // True if symbol is block scoped redeclaration
diff --git a/tests/baselines/reference/controlFlowBinaryOrExpression.symbols b/tests/baselines/reference/controlFlowBinaryOrExpression.symbols
index e76cfbf4cab..5251973005f 100644
--- a/tests/baselines/reference/controlFlowBinaryOrExpression.symbols
+++ b/tests/baselines/reference/controlFlowBinaryOrExpression.symbols
@@ -86,8 +86,8 @@ if (isNodeList(sourceObj) || isHTMLCollection(sourceObj)) {
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
 
     sourceObj.length;
->sourceObj.length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>sourceObj.length : Symbol(NodeList.length, Decl(controlFlowBinaryOrExpression.ts, 10, 27))
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
->length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>length : Symbol(NodeList.length, Decl(controlFlowBinaryOrExpression.ts, 10, 27))
 }
 
diff --git a/tests/baselines/reference/controlFlowBinaryOrExpression.types b/tests/baselines/reference/controlFlowBinaryOrExpression.types
index 0bf5ab524b9..d24462316b4 100644
--- a/tests/baselines/reference/controlFlowBinaryOrExpression.types
+++ b/tests/baselines/reference/controlFlowBinaryOrExpression.types
@@ -106,7 +106,7 @@ if (isNodeList(sourceObj) || isHTMLCollection(sourceObj)) {
 
     sourceObj.length;
 >sourceObj.length : number
->sourceObj : NodeList | HTMLCollection | ({ a: string; } & HTMLCollection)
+>sourceObj : NodeList
 >length : number
 }
 
diff --git a/tests/baselines/reference/controlFlowInstanceof.js b/tests/baselines/reference/controlFlowInstanceof.js
new file mode 100644
index 00000000000..3f2287b4628
--- /dev/null
+++ b/tests/baselines/reference/controlFlowInstanceof.js
@@ -0,0 +1,184 @@
+//// [controlFlowInstanceof.ts]
+
+// Repros from #10167
+
+function f1(s: Set<string> | Set<number>) {
+    s = new Set<number>();
+    s;  // Set<number>
+    if (s instanceof Set) {
+        s;  // Set<number>
+    }
+    s;  // Set<number>
+    s.add(42);
+}
+
+function f2(s: Set<string> | Set<number>) {
+    s = new Set<number>();
+    s;  // Set<number>
+    if (s instanceof Promise) {
+        s;  // Set<number> & Promise<any>
+    }
+    s;  // Set<number>
+    s.add(42);
+}
+
+function f3(s: Set<string> | Set<number>) {
+    s;  // Set<string> | Set<number>
+    if (s instanceof Set) {
+        s;  // Set<string> | Set<number>
+    }
+    else {
+        s;  // never
+    }
+}
+
+function f4(s: Set<string> | Set<number>) {
+    s = new Set<number>();
+    s;  // Set<number>
+    if (s instanceof Set) {
+        s;  // Set<number>
+    }
+    else {
+        s;  // never
+    }
+}
+
+// More tests
+
+class A { a: string }
+class B extends A { b: string }
+class C extends A { c: string }
+
+function foo(x: A | undefined) {
+    x;  // A | undefined
+    if (x instanceof B || x instanceof C) {
+        x;  // B | C
+    }
+    x;  // A | undefined
+    if (x instanceof B && x instanceof C) {
+        x;  // B & C
+    }
+    x;  // A | undefined
+    if (!x) {
+        return;
+    }
+    x;  // A
+    if (x instanceof B) {
+        x;  // B
+        if (x instanceof C) {
+            x;  // B & C
+        }
+        else {
+            x;  // B
+        }
+        x;  // B
+    }
+    else {
+        x;  // A
+    }
+    x;  // A
+}
+
+// X is neither assignable to Y nor a subtype of Y
+// Y is assignable to X, but not a subtype of X
+
+interface X {
+    x?: string;
+}
+
+class Y {
+    y: string;
+}
+
+function goo(x: X) {
+    x;
+    if (x instanceof Y) {
+        x.y;
+    }
+    x;
+}
+
+//// [controlFlowInstanceof.js]
+// Repros from #10167
+function f1(s) {
+    s = new Set();
+    s; // Set<number>
+    if (s instanceof Set) {
+        s; // Set<number>
+    }
+    s; // Set<number>
+    s.add(42);
+}
+function f2(s) {
+    s = new Set();
+    s; // Set<number>
+    if (s instanceof Promise) {
+        s; // Set<number> & Promise<any>
+    }
+    s; // Set<number>
+    s.add(42);
+}
+function f3(s) {
+    s; // Set<string> | Set<number>
+    if (s instanceof Set) {
+        s; // Set<string> | Set<number>
+    }
+    else {
+        s; // never
+    }
+}
+function f4(s) {
+    s = new Set();
+    s; // Set<number>
+    if (s instanceof Set) {
+        s; // Set<number>
+    }
+    else {
+        s; // never
+    }
+}
+// More tests
+class A {
+}
+class B extends A {
+}
+class C extends A {
+}
+function foo(x) {
+    x; // A | undefined
+    if (x instanceof B || x instanceof C) {
+        x; // B | C
+    }
+    x; // A | undefined
+    if (x instanceof B && x instanceof C) {
+        x; // B & C
+    }
+    x; // A | undefined
+    if (!x) {
+        return;
+    }
+    x; // A
+    if (x instanceof B) {
+        x; // B
+        if (x instanceof C) {
+            x; // B & C
+        }
+        else {
+            x; // B
+        }
+        x; // B
+    }
+    else {
+        x; // A
+    }
+    x; // A
+}
+class Y {
+}
+function goo(x) {
+    x;
+    if (x instanceof Y) {
+        x.y;
+    }
+    x;
+}
diff --git a/tests/baselines/reference/controlFlowInstanceof.symbols b/tests/baselines/reference/controlFlowInstanceof.symbols
new file mode 100644
index 00000000000..31894827990
--- /dev/null
+++ b/tests/baselines/reference/controlFlowInstanceof.symbols
@@ -0,0 +1,232 @@
+=== tests/cases/compiler/controlFlowInstanceof.ts ===
+
+// Repros from #10167
+
+function f1(s: Set<string> | Set<number>) {
+>f1 : Symbol(f1, Decl(controlFlowInstanceof.ts, 0, 0))
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s = new Set<number>();
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+
+    if (s instanceof Set) {
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+        s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+    }
+    s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+
+    s.add(42);
+>s.add : Symbol(Set.add, Decl(lib.es2015.collection.d.ts, --, --))
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+>add : Symbol(Set.add, Decl(lib.es2015.collection.d.ts, --, --))
+}
+
+function f2(s: Set<string> | Set<number>) {
+>f2 : Symbol(f2, Decl(controlFlowInstanceof.ts, 11, 1))
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s = new Set<number>();
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+
+    if (s instanceof Promise) {
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+>Promise : Symbol(Promise, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --))
+
+        s;  // Set<number> & Promise<any>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+    }
+    s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+
+    s.add(42);
+>s.add : Symbol(Set.add, Decl(lib.es2015.collection.d.ts, --, --))
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+>add : Symbol(Set.add, Decl(lib.es2015.collection.d.ts, --, --))
+}
+
+function f3(s: Set<string> | Set<number>) {
+>f3 : Symbol(f3, Decl(controlFlowInstanceof.ts, 21, 1))
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 23, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s;  // Set<string> | Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 23, 12))
+
+    if (s instanceof Set) {
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 23, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+        s;  // Set<string> | Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 23, 12))
+    }
+    else {
+        s;  // never
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 23, 12))
+    }
+}
+
+function f4(s: Set<string> | Set<number>) {
+>f4 : Symbol(f4, Decl(controlFlowInstanceof.ts, 31, 1))
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 33, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s = new Set<number>();
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 33, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 33, 12))
+
+    if (s instanceof Set) {
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 33, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+        s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 33, 12))
+    }
+    else {
+        s;  // never
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 33, 12))
+    }
+}
+
+// More tests
+
+class A { a: string }
+>A : Symbol(A, Decl(controlFlowInstanceof.ts, 42, 1))
+>a : Symbol(A.a, Decl(controlFlowInstanceof.ts, 46, 9))
+
+class B extends A { b: string }
+>B : Symbol(B, Decl(controlFlowInstanceof.ts, 46, 21))
+>A : Symbol(A, Decl(controlFlowInstanceof.ts, 42, 1))
+>b : Symbol(B.b, Decl(controlFlowInstanceof.ts, 47, 19))
+
+class C extends A { c: string }
+>C : Symbol(C, Decl(controlFlowInstanceof.ts, 47, 31))
+>A : Symbol(A, Decl(controlFlowInstanceof.ts, 42, 1))
+>c : Symbol(C.c, Decl(controlFlowInstanceof.ts, 48, 19))
+
+function foo(x: A | undefined) {
+>foo : Symbol(foo, Decl(controlFlowInstanceof.ts, 48, 31))
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>A : Symbol(A, Decl(controlFlowInstanceof.ts, 42, 1))
+
+    x;  // A | undefined
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+
+    if (x instanceof B || x instanceof C) {
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>B : Symbol(B, Decl(controlFlowInstanceof.ts, 46, 21))
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>C : Symbol(C, Decl(controlFlowInstanceof.ts, 47, 31))
+
+        x;  // B | C
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+    }
+    x;  // A | undefined
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+
+    if (x instanceof B && x instanceof C) {
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>B : Symbol(B, Decl(controlFlowInstanceof.ts, 46, 21))
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>C : Symbol(C, Decl(controlFlowInstanceof.ts, 47, 31))
+
+        x;  // B & C
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+    }
+    x;  // A | undefined
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+
+    if (!x) {
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+
+        return;
+    }
+    x;  // A
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+
+    if (x instanceof B) {
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>B : Symbol(B, Decl(controlFlowInstanceof.ts, 46, 21))
+
+        x;  // B
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+
+        if (x instanceof C) {
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>C : Symbol(C, Decl(controlFlowInstanceof.ts, 47, 31))
+
+            x;  // B & C
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+        }
+        else {
+            x;  // B
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+        }
+        x;  // B
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+    }
+    else {
+        x;  // A
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+    }
+    x;  // A
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+}
+
+// X is neither assignable to Y nor a subtype of Y
+// Y is assignable to X, but not a subtype of X
+
+interface X {
+>X : Symbol(X, Decl(controlFlowInstanceof.ts, 78, 1))
+
+    x?: string;
+>x : Symbol(X.x, Decl(controlFlowInstanceof.ts, 83, 13))
+}
+
+class Y {
+>Y : Symbol(Y, Decl(controlFlowInstanceof.ts, 85, 1))
+
+    y: string;
+>y : Symbol(Y.y, Decl(controlFlowInstanceof.ts, 87, 9))
+}
+
+function goo(x: X) {
+>goo : Symbol(goo, Decl(controlFlowInstanceof.ts, 89, 1))
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 91, 13))
+>X : Symbol(X, Decl(controlFlowInstanceof.ts, 78, 1))
+
+    x;
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 91, 13))
+
+    if (x instanceof Y) {
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 91, 13))
+>Y : Symbol(Y, Decl(controlFlowInstanceof.ts, 85, 1))
+
+        x.y;
+>x.y : Symbol(Y.y, Decl(controlFlowInstanceof.ts, 87, 9))
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 91, 13))
+>y : Symbol(Y.y, Decl(controlFlowInstanceof.ts, 87, 9))
+    }
+    x;
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 91, 13))
+}
diff --git a/tests/baselines/reference/controlFlowInstanceof.types b/tests/baselines/reference/controlFlowInstanceof.types
new file mode 100644
index 00000000000..e52d1a25b1d
--- /dev/null
+++ b/tests/baselines/reference/controlFlowInstanceof.types
@@ -0,0 +1,256 @@
+=== tests/cases/compiler/controlFlowInstanceof.ts ===
+
+// Repros from #10167
+
+function f1(s: Set<string> | Set<number>) {
+>f1 : (s: Set<string> | Set<number>) => void
+>s : Set<string> | Set<number>
+>Set : Set<T>
+>Set : Set<T>
+
+    s = new Set<number>();
+>s = new Set<number>() : Set<number>
+>s : Set<string> | Set<number>
+>new Set<number>() : Set<number>
+>Set : SetConstructor
+
+    s;  // Set<number>
+>s : Set<number>
+
+    if (s instanceof Set) {
+>s instanceof Set : boolean
+>s : Set<number>
+>Set : SetConstructor
+
+        s;  // Set<number>
+>s : Set<number>
+    }
+    s;  // Set<number>
+>s : Set<number>
+
+    s.add(42);
+>s.add(42) : Set<number>
+>s.add : (value: number) => Set<number>
+>s : Set<number>
+>add : (value: number) => Set<number>
+>42 : number
+}
+
+function f2(s: Set<string> | Set<number>) {
+>f2 : (s: Set<string> | Set<number>) => void
+>s : Set<string> | Set<number>
+>Set : Set<T>
+>Set : Set<T>
+
+    s = new Set<number>();
+>s = new Set<number>() : Set<number>
+>s : Set<string> | Set<number>
+>new Set<number>() : Set<number>
+>Set : SetConstructor
+
+    s;  // Set<number>
+>s : Set<number>
+
+    if (s instanceof Promise) {
+>s instanceof Promise : boolean
+>s : Set<number>
+>Promise : PromiseConstructor
+
+        s;  // Set<number> & Promise<any>
+>s : Set<number> & Promise<any>
+    }
+    s;  // Set<number>
+>s : Set<number>
+
+    s.add(42);
+>s.add(42) : Set<number>
+>s.add : (value: number) => Set<number>
+>s : Set<number>
+>add : (value: number) => Set<number>
+>42 : number
+}
+
+function f3(s: Set<string> | Set<number>) {
+>f3 : (s: Set<string> | Set<number>) => void
+>s : Set<string> | Set<number>
+>Set : Set<T>
+>Set : Set<T>
+
+    s;  // Set<string> | Set<number>
+>s : Set<string> | Set<number>
+
+    if (s instanceof Set) {
+>s instanceof Set : boolean
+>s : Set<string> | Set<number>
+>Set : SetConstructor
+
+        s;  // Set<string> | Set<number>
+>s : Set<string> | Set<number>
+    }
+    else {
+        s;  // never
+>s : never
+    }
+}
+
+function f4(s: Set<string> | Set<number>) {
+>f4 : (s: Set<string> | Set<number>) => void
+>s : Set<string> | Set<number>
+>Set : Set<T>
+>Set : Set<T>
+
+    s = new Set<number>();
+>s = new Set<number>() : Set<number>
+>s : Set<string> | Set<number>
+>new Set<number>() : Set<number>
+>Set : SetConstructor
+
+    s;  // Set<number>
+>s : Set<number>
+
+    if (s instanceof Set) {
+>s instanceof Set : boolean
+>s : Set<number>
+>Set : SetConstructor
+
+        s;  // Set<number>
+>s : Set<number>
+    }
+    else {
+        s;  // never
+>s : never
+    }
+}
+
+// More tests
+
+class A { a: string }
+>A : A
+>a : string
+
+class B extends A { b: string }
+>B : B
+>A : A
+>b : string
+
+class C extends A { c: string }
+>C : C
+>A : A
+>c : string
+
+function foo(x: A | undefined) {
+>foo : (x: A) => void
+>x : A
+>A : A
+
+    x;  // A | undefined
+>x : A
+
+    if (x instanceof B || x instanceof C) {
+>x instanceof B || x instanceof C : boolean
+>x instanceof B : boolean
+>x : A
+>B : typeof B
+>x instanceof C : boolean
+>x : A
+>C : typeof C
+
+        x;  // B | C
+>x : B | C
+    }
+    x;  // A | undefined
+>x : A
+
+    if (x instanceof B && x instanceof C) {
+>x instanceof B && x instanceof C : boolean
+>x instanceof B : boolean
+>x : A
+>B : typeof B
+>x instanceof C : boolean
+>x : B
+>C : typeof C
+
+        x;  // B & C
+>x : B & C
+    }
+    x;  // A | undefined
+>x : A
+
+    if (!x) {
+>!x : boolean
+>x : A
+
+        return;
+    }
+    x;  // A
+>x : A
+
+    if (x instanceof B) {
+>x instanceof B : boolean
+>x : A
+>B : typeof B
+
+        x;  // B
+>x : B
+
+        if (x instanceof C) {
+>x instanceof C : boolean
+>x : B
+>C : typeof C
+
+            x;  // B & C
+>x : B & C
+        }
+        else {
+            x;  // B
+>x : B
+        }
+        x;  // B
+>x : B
+    }
+    else {
+        x;  // A
+>x : A
+    }
+    x;  // A
+>x : A
+}
+
+// X is neither assignable to Y nor a subtype of Y
+// Y is assignable to X, but not a subtype of X
+
+interface X {
+>X : X
+
+    x?: string;
+>x : string
+}
+
+class Y {
+>Y : Y
+
+    y: string;
+>y : string
+}
+
+function goo(x: X) {
+>goo : (x: X) => void
+>x : X
+>X : X
+
+    x;
+>x : X
+
+    if (x instanceof Y) {
+>x instanceof Y : boolean
+>x : X
+>Y : typeof Y
+
+        x.y;
+>x.y : string
+>x : Y
+>y : string
+    }
+    x;
+>x : X
+}
diff --git a/tests/baselines/reference/discriminantPropertyCheck.errors.txt b/tests/baselines/reference/discriminantPropertyCheck.errors.txt
new file mode 100644
index 00000000000..41593c4b224
--- /dev/null
+++ b/tests/baselines/reference/discriminantPropertyCheck.errors.txt
@@ -0,0 +1,77 @@
+tests/cases/compiler/discriminantPropertyCheck.ts(30,9): error TS2532: Object is possibly 'undefined'.
+tests/cases/compiler/discriminantPropertyCheck.ts(66,9): error TS2532: Object is possibly 'undefined'.
+
+
+==== tests/cases/compiler/discriminantPropertyCheck.ts (2 errors) ====
+    
+    type Item = Item1 | Item2;
+    
+    interface Base {
+        bar: boolean;
+    }
+    
+    interface Item1 extends Base {
+        kind: "A";
+        foo: string | undefined;
+        baz: boolean;
+        qux: true;
+    }
+    
+    interface Item2 extends Base {
+        kind: "B";
+        foo: string | undefined;
+        baz: boolean;
+        qux: false;
+    }
+    
+    function goo1(x: Item) {
+        if (x.kind === "A" && x.foo !== undefined) {
+            x.foo.length;
+        }
+    }
+    
+    function goo2(x: Item) {
+        if (x.foo !== undefined && x.kind === "A") {
+            x.foo.length;  // Error, intervening discriminant guard
+            ~~~~~
+!!! error TS2532: Object is possibly 'undefined'.
+        }
+    }
+    
+    function foo1(x: Item) {
+        if (x.bar && x.foo !== undefined) {
+            x.foo.length;
+        }
+    }
+    
+    function foo2(x: Item) {
+        if (x.foo !== undefined && x.bar) {
+            x.foo.length;
+        }
+    }
+    
+    function foo3(x: Item) {
+        if (x.baz && x.foo !== undefined) {
+            x.foo.length;
+        }
+    }
+    
+    function foo4(x: Item) {
+        if (x.foo !== undefined && x.baz) {
+            x.foo.length;
+        }
+    }
+    
+    function foo5(x: Item) {
+        if (x.qux && x.foo !== undefined) {
+            x.foo.length;
+        }
+    }
+    
+    function foo6(x: Item) {
+        if (x.foo !== undefined && x.qux) {
+            x.foo.length;  // Error, intervening discriminant guard
+            ~~~~~
+!!! error TS2532: Object is possibly 'undefined'.
+        }
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/discriminantPropertyCheck.js b/tests/baselines/reference/discriminantPropertyCheck.js
new file mode 100644
index 00000000000..b75a6277735
--- /dev/null
+++ b/tests/baselines/reference/discriminantPropertyCheck.js
@@ -0,0 +1,111 @@
+//// [discriminantPropertyCheck.ts]
+
+type Item = Item1 | Item2;
+
+interface Base {
+    bar: boolean;
+}
+
+interface Item1 extends Base {
+    kind: "A";
+    foo: string | undefined;
+    baz: boolean;
+    qux: true;
+}
+
+interface Item2 extends Base {
+    kind: "B";
+    foo: string | undefined;
+    baz: boolean;
+    qux: false;
+}
+
+function goo1(x: Item) {
+    if (x.kind === "A" && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function goo2(x: Item) {
+    if (x.foo !== undefined && x.kind === "A") {
+        x.foo.length;  // Error, intervening discriminant guard
+    }
+}
+
+function foo1(x: Item) {
+    if (x.bar && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function foo2(x: Item) {
+    if (x.foo !== undefined && x.bar) {
+        x.foo.length;
+    }
+}
+
+function foo3(x: Item) {
+    if (x.baz && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function foo4(x: Item) {
+    if (x.foo !== undefined && x.baz) {
+        x.foo.length;
+    }
+}
+
+function foo5(x: Item) {
+    if (x.qux && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function foo6(x: Item) {
+    if (x.foo !== undefined && x.qux) {
+        x.foo.length;  // Error, intervening discriminant guard
+    }
+}
+
+//// [discriminantPropertyCheck.js]
+function goo1(x) {
+    if (x.kind === "A" && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+function goo2(x) {
+    if (x.foo !== undefined && x.kind === "A") {
+        x.foo.length; // Error, intervening discriminant guard
+    }
+}
+function foo1(x) {
+    if (x.bar && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+function foo2(x) {
+    if (x.foo !== undefined && x.bar) {
+        x.foo.length;
+    }
+}
+function foo3(x) {
+    if (x.baz && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+function foo4(x) {
+    if (x.foo !== undefined && x.baz) {
+        x.foo.length;
+    }
+}
+function foo5(x) {
+    if (x.qux && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+function foo6(x) {
+    if (x.foo !== undefined && x.qux) {
+        x.foo.length; // Error, intervening discriminant guard
+    }
+}
diff --git a/tests/baselines/reference/discriminantsAndTypePredicates.js b/tests/baselines/reference/discriminantsAndTypePredicates.js
new file mode 100644
index 00000000000..66cfe056ab7
--- /dev/null
+++ b/tests/baselines/reference/discriminantsAndTypePredicates.js
@@ -0,0 +1,59 @@
+//// [discriminantsAndTypePredicates.ts]
+// Repro from #10145
+
+interface A { type: 'A' }
+interface B { type: 'B' }
+
+function isA(x: A | B): x is A { return x.type === 'A'; }
+function isB(x: A | B): x is B { return x.type === 'B'; }
+
+function foo1(x: A | B): any {
+    x;  // A | B
+    if (isA(x)) {
+        return x;  // A
+    }
+    x;  // B
+    if (isB(x)) {
+        return x;  // B
+    }
+    x;  // never
+}
+
+function foo2(x: A | B): any {
+    x;  // A | B
+    if (x.type === 'A') {
+        return x;  // A
+    }
+    x;  // B
+    if (x.type === 'B') {
+        return x;  // B
+    }
+    x;  // never
+}
+
+//// [discriminantsAndTypePredicates.js]
+// Repro from #10145
+function isA(x) { return x.type === 'A'; }
+function isB(x) { return x.type === 'B'; }
+function foo1(x) {
+    x; // A | B
+    if (isA(x)) {
+        return x; // A
+    }
+    x; // B
+    if (isB(x)) {
+        return x; // B
+    }
+    x; // never
+}
+function foo2(x) {
+    x; // A | B
+    if (x.type === 'A') {
+        return x; // A
+    }
+    x; // B
+    if (x.type === 'B') {
+        return x; // B
+    }
+    x; // never
+}
diff --git a/tests/baselines/reference/discriminantsAndTypePredicates.symbols b/tests/baselines/reference/discriminantsAndTypePredicates.symbols
new file mode 100644
index 00000000000..4f412823936
--- /dev/null
+++ b/tests/baselines/reference/discriminantsAndTypePredicates.symbols
@@ -0,0 +1,94 @@
+=== tests/cases/compiler/discriminantsAndTypePredicates.ts ===
+// Repro from #10145
+
+interface A { type: 'A' }
+>A : Symbol(A, Decl(discriminantsAndTypePredicates.ts, 0, 0))
+>type : Symbol(A.type, Decl(discriminantsAndTypePredicates.ts, 2, 13))
+
+interface B { type: 'B' }
+>B : Symbol(B, Decl(discriminantsAndTypePredicates.ts, 2, 25))
+>type : Symbol(B.type, Decl(discriminantsAndTypePredicates.ts, 3, 13))
+
+function isA(x: A | B): x is A { return x.type === 'A'; }
+>isA : Symbol(isA, Decl(discriminantsAndTypePredicates.ts, 3, 25))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 5, 13))
+>A : Symbol(A, Decl(discriminantsAndTypePredicates.ts, 0, 0))
+>B : Symbol(B, Decl(discriminantsAndTypePredicates.ts, 2, 25))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 5, 13))
+>A : Symbol(A, Decl(discriminantsAndTypePredicates.ts, 0, 0))
+>x.type : Symbol(type, Decl(discriminantsAndTypePredicates.ts, 2, 13), Decl(discriminantsAndTypePredicates.ts, 3, 13))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 5, 13))
+>type : Symbol(type, Decl(discriminantsAndTypePredicates.ts, 2, 13), Decl(discriminantsAndTypePredicates.ts, 3, 13))
+
+function isB(x: A | B): x is B { return x.type === 'B'; }
+>isB : Symbol(isB, Decl(discriminantsAndTypePredicates.ts, 5, 57))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 6, 13))
+>A : Symbol(A, Decl(discriminantsAndTypePredicates.ts, 0, 0))
+>B : Symbol(B, Decl(discriminantsAndTypePredicates.ts, 2, 25))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 6, 13))
+>B : Symbol(B, Decl(discriminantsAndTypePredicates.ts, 2, 25))
+>x.type : Symbol(type, Decl(discriminantsAndTypePredicates.ts, 2, 13), Decl(discriminantsAndTypePredicates.ts, 3, 13))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 6, 13))
+>type : Symbol(type, Decl(discriminantsAndTypePredicates.ts, 2, 13), Decl(discriminantsAndTypePredicates.ts, 3, 13))
+
+function foo1(x: A | B): any {
+>foo1 : Symbol(foo1, Decl(discriminantsAndTypePredicates.ts, 6, 57))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+>A : Symbol(A, Decl(discriminantsAndTypePredicates.ts, 0, 0))
+>B : Symbol(B, Decl(discriminantsAndTypePredicates.ts, 2, 25))
+
+    x;  // A | B
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+
+    if (isA(x)) {
+>isA : Symbol(isA, Decl(discriminantsAndTypePredicates.ts, 3, 25))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+
+        return x;  // A
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+    }
+    x;  // B
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+
+    if (isB(x)) {
+>isB : Symbol(isB, Decl(discriminantsAndTypePredicates.ts, 5, 57))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+
+        return x;  // B
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+    }
+    x;  // never
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+}
+
+function foo2(x: A | B): any {
+>foo2 : Symbol(foo2, Decl(discriminantsAndTypePredicates.ts, 18, 1))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+>A : Symbol(A, Decl(discriminantsAndTypePredicates.ts, 0, 0))
+>B : Symbol(B, Decl(discriminantsAndTypePredicates.ts, 2, 25))
+
+    x;  // A | B
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+
+    if (x.type === 'A') {
+>x.type : Symbol(type, Decl(discriminantsAndTypePredicates.ts, 2, 13), Decl(discriminantsAndTypePredicates.ts, 3, 13))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+>type : Symbol(type, Decl(discriminantsAndTypePredicates.ts, 2, 13), Decl(discriminantsAndTypePredicates.ts, 3, 13))
+
+        return x;  // A
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+    }
+    x;  // B
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+
+    if (x.type === 'B') {
+>x.type : Symbol(B.type, Decl(discriminantsAndTypePredicates.ts, 3, 13))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+>type : Symbol(B.type, Decl(discriminantsAndTypePredicates.ts, 3, 13))
+
+        return x;  // B
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+    }
+    x;  // never
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+}
diff --git a/tests/baselines/reference/discriminantsAndTypePredicates.types b/tests/baselines/reference/discriminantsAndTypePredicates.types
new file mode 100644
index 00000000000..f7675b14756
--- /dev/null
+++ b/tests/baselines/reference/discriminantsAndTypePredicates.types
@@ -0,0 +1,104 @@
+=== tests/cases/compiler/discriminantsAndTypePredicates.ts ===
+// Repro from #10145
+
+interface A { type: 'A' }
+>A : A
+>type : "A"
+
+interface B { type: 'B' }
+>B : B
+>type : "B"
+
+function isA(x: A | B): x is A { return x.type === 'A'; }
+>isA : (x: A | B) => x is A
+>x : A | B
+>A : A
+>B : B
+>x : any
+>A : A
+>x.type === 'A' : boolean
+>x.type : "A" | "B"
+>x : A | B
+>type : "A" | "B"
+>'A' : "A"
+
+function isB(x: A | B): x is B { return x.type === 'B'; }
+>isB : (x: A | B) => x is B
+>x : A | B
+>A : A
+>B : B
+>x : any
+>B : B
+>x.type === 'B' : boolean
+>x.type : "A" | "B"
+>x : A | B
+>type : "A" | "B"
+>'B' : "B"
+
+function foo1(x: A | B): any {
+>foo1 : (x: A | B) => any
+>x : A | B
+>A : A
+>B : B
+
+    x;  // A | B
+>x : A | B
+
+    if (isA(x)) {
+>isA(x) : boolean
+>isA : (x: A | B) => x is A
+>x : A | B
+
+        return x;  // A
+>x : A
+    }
+    x;  // B
+>x : B
+
+    if (isB(x)) {
+>isB(x) : boolean
+>isB : (x: A | B) => x is B
+>x : B
+
+        return x;  // B
+>x : B
+    }
+    x;  // never
+>x : never
+}
+
+function foo2(x: A | B): any {
+>foo2 : (x: A | B) => any
+>x : A | B
+>A : A
+>B : B
+
+    x;  // A | B
+>x : A | B
+
+    if (x.type === 'A') {
+>x.type === 'A' : boolean
+>x.type : "A" | "B"
+>x : A | B
+>type : "A" | "B"
+>'A' : "A"
+
+        return x;  // A
+>x : A
+    }
+    x;  // B
+>x : B
+
+    if (x.type === 'B') {
+>x.type === 'B' : boolean
+>x.type : "B"
+>x : B
+>type : "B"
+>'B' : "B"
+
+        return x;  // B
+>x : B
+    }
+    x;  // never
+>x : never
+}
diff --git a/tests/baselines/reference/stringLiteralTypesAsTags01.types b/tests/baselines/reference/stringLiteralTypesAsTags01.types
index e00fd18163d..e95a7364f93 100644
--- a/tests/baselines/reference/stringLiteralTypesAsTags01.types
+++ b/tests/baselines/reference/stringLiteralTypesAsTags01.types
@@ -99,8 +99,8 @@ if (hasKind(x, "A")) {
 }
 else {
     let b = x;
->b : A
->x : A
+>b : never
+>x : never
 }
 
 if (!hasKind(x, "B")) {
diff --git a/tests/baselines/reference/stringLiteralTypesAsTags02.types b/tests/baselines/reference/stringLiteralTypesAsTags02.types
index fb1632559ea..c984b8a78f9 100644
--- a/tests/baselines/reference/stringLiteralTypesAsTags02.types
+++ b/tests/baselines/reference/stringLiteralTypesAsTags02.types
@@ -93,8 +93,8 @@ if (hasKind(x, "A")) {
 }
 else {
     let b = x;
->b : A
->x : A
+>b : never
+>x : never
 }
 
 if (!hasKind(x, "B")) {
diff --git a/tests/baselines/reference/stringLiteralTypesAsTags03.types b/tests/baselines/reference/stringLiteralTypesAsTags03.types
index 05be633813b..fbe71ff07c1 100644
--- a/tests/baselines/reference/stringLiteralTypesAsTags03.types
+++ b/tests/baselines/reference/stringLiteralTypesAsTags03.types
@@ -96,8 +96,8 @@ if (hasKind(x, "A")) {
 }
 else {
     let b = x;
->b : A
->x : A
+>b : never
+>x : never
 }
 
 if (!hasKind(x, "B")) {
diff --git a/tests/cases/compiler/controlFlowInstanceof.ts b/tests/cases/compiler/controlFlowInstanceof.ts
new file mode 100644
index 00000000000..56f3ff97e4c
--- /dev/null
+++ b/tests/cases/compiler/controlFlowInstanceof.ts
@@ -0,0 +1,99 @@
+// @target: es6
+
+// Repros from #10167
+
+function f1(s: Set<string> | Set<number>) {
+    s = new Set<number>();
+    s;  // Set<number>
+    if (s instanceof Set) {
+        s;  // Set<number>
+    }
+    s;  // Set<number>
+    s.add(42);
+}
+
+function f2(s: Set<string> | Set<number>) {
+    s = new Set<number>();
+    s;  // Set<number>
+    if (s instanceof Promise) {
+        s;  // Set<number> & Promise<any>
+    }
+    s;  // Set<number>
+    s.add(42);
+}
+
+function f3(s: Set<string> | Set<number>) {
+    s;  // Set<string> | Set<number>
+    if (s instanceof Set) {
+        s;  // Set<string> | Set<number>
+    }
+    else {
+        s;  // never
+    }
+}
+
+function f4(s: Set<string> | Set<number>) {
+    s = new Set<number>();
+    s;  // Set<number>
+    if (s instanceof Set) {
+        s;  // Set<number>
+    }
+    else {
+        s;  // never
+    }
+}
+
+// More tests
+
+class A { a: string }
+class B extends A { b: string }
+class C extends A { c: string }
+
+function foo(x: A | undefined) {
+    x;  // A | undefined
+    if (x instanceof B || x instanceof C) {
+        x;  // B | C
+    }
+    x;  // A | undefined
+    if (x instanceof B && x instanceof C) {
+        x;  // B & C
+    }
+    x;  // A | undefined
+    if (!x) {
+        return;
+    }
+    x;  // A
+    if (x instanceof B) {
+        x;  // B
+        if (x instanceof C) {
+            x;  // B & C
+        }
+        else {
+            x;  // B
+        }
+        x;  // B
+    }
+    else {
+        x;  // A
+    }
+    x;  // A
+}
+
+// X is neither assignable to Y nor a subtype of Y
+// Y is assignable to X, but not a subtype of X
+
+interface X {
+    x?: string;
+}
+
+class Y {
+    y: string;
+}
+
+function goo(x: X) {
+    x;
+    if (x instanceof Y) {
+        x.y;
+    }
+    x;
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/discriminantPropertyCheck.ts b/tests/cases/compiler/discriminantPropertyCheck.ts
new file mode 100644
index 00000000000..e493f6bf770
--- /dev/null
+++ b/tests/cases/compiler/discriminantPropertyCheck.ts
@@ -0,0 +1,69 @@
+// @strictNullChecks: true
+
+type Item = Item1 | Item2;
+
+interface Base {
+    bar: boolean;
+}
+
+interface Item1 extends Base {
+    kind: "A";
+    foo: string | undefined;
+    baz: boolean;
+    qux: true;
+}
+
+interface Item2 extends Base {
+    kind: "B";
+    foo: string | undefined;
+    baz: boolean;
+    qux: false;
+}
+
+function goo1(x: Item) {
+    if (x.kind === "A" && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function goo2(x: Item) {
+    if (x.foo !== undefined && x.kind === "A") {
+        x.foo.length;  // Error, intervening discriminant guard
+    }
+}
+
+function foo1(x: Item) {
+    if (x.bar && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function foo2(x: Item) {
+    if (x.foo !== undefined && x.bar) {
+        x.foo.length;
+    }
+}
+
+function foo3(x: Item) {
+    if (x.baz && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function foo4(x: Item) {
+    if (x.foo !== undefined && x.baz) {
+        x.foo.length;
+    }
+}
+
+function foo5(x: Item) {
+    if (x.qux && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function foo6(x: Item) {
+    if (x.foo !== undefined && x.qux) {
+        x.foo.length;  // Error, intervening discriminant guard
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/discriminantsAndTypePredicates.ts b/tests/cases/compiler/discriminantsAndTypePredicates.ts
new file mode 100644
index 00000000000..c21ab7ec8f4
--- /dev/null
+++ b/tests/cases/compiler/discriminantsAndTypePredicates.ts
@@ -0,0 +1,31 @@
+// Repro from #10145
+
+interface A { type: 'A' }
+interface B { type: 'B' }
+
+function isA(x: A | B): x is A { return x.type === 'A'; }
+function isB(x: A | B): x is B { return x.type === 'B'; }
+
+function foo1(x: A | B): any {
+    x;  // A | B
+    if (isA(x)) {
+        return x;  // A
+    }
+    x;  // B
+    if (isB(x)) {
+        return x;  // B
+    }
+    x;  // never
+}
+
+function foo2(x: A | B): any {
+    x;  // A | B
+    if (x.type === 'A') {
+        return x;  // A
+    }
+    x;  // B
+    if (x.type === 'B') {
+        return x;  // B
+    }
+    x;  // never
+}
\ No newline at end of file