diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
index 71b5e1afbff..4eab97f28bb 100644
--- a/src/compiler/binder.ts
+++ b/src/compiler/binder.ts
@@ -749,6 +749,10 @@ namespace ts {
             return expr1.kind === SyntaxKind.TypeOfExpression && isNarrowableOperand((<TypeOfExpression>expr1).expression) && expr2.kind === SyntaxKind.StringLiteral;
         }
 
+        function isNarrowableInOperands(left: Expression, right: Expression) {
+            return left.kind === SyntaxKind.StringLiteral && isNarrowingExpression(right);
+        }
+
         function isNarrowingBinaryExpression(expr: BinaryExpression) {
             switch (expr.operatorToken.kind) {
                 case SyntaxKind.EqualsToken:
@@ -761,6 +765,8 @@ namespace ts {
                         isNarrowingTypeofOperands(expr.right, expr.left) || isNarrowingTypeofOperands(expr.left, expr.right);
                 case SyntaxKind.InstanceOfKeyword:
                     return isNarrowableOperand(expr.left);
+                case SyntaxKind.InKeyword:
+                    return isNarrowableInOperands(expr.left, expr.right);
                 case SyntaxKind.CommaToken:
                     return isNarrowingExpression(expr.right);
             }
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 25d967795a7..7f7a75a33f9 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -12722,6 +12722,14 @@ namespace ts {
                 return type;
             }
 
+            function narrowByInKeyword(type: Type, literal: LiteralExpression, assumeTrue: boolean) {
+                if ((type.flags & (TypeFlags.Union | TypeFlags.Object)) || (type.flags & TypeFlags.TypeParameter && (type as TypeParameter).isThisType)) {
+                    const propName = literal.text;
+                    return  filterType(type, t => !!getPropertyOfType(t, propName) === assumeTrue);
+                }
+                return type;
+            }
+
             function narrowTypeByBinaryExpression(type: Type, expr: BinaryExpression, assumeTrue: boolean): Type {
                 switch (expr.operatorToken.kind) {
                     case SyntaxKind.EqualsToken:
@@ -12757,6 +12765,12 @@ namespace ts {
                         break;
                     case SyntaxKind.InstanceOfKeyword:
                         return narrowTypeByInstanceof(type, expr, assumeTrue);
+                    case SyntaxKind.InKeyword:
+                        const target = getReferenceCandidate(expr.right);
+                        if (expr.left.kind === SyntaxKind.StringLiteral && isMatchingReference(reference, target)) {
+                            return narrowByInKeyword(type, <LiteralExpression>expr.left, assumeTrue);
+                        }
+                        break;
                     case SyntaxKind.CommaToken:
                         return narrowType(type, expr.right, assumeTrue);
                 }
diff --git a/tests/baselines/reference/inKeywordTypeguard.errors.txt b/tests/baselines/reference/inKeywordTypeguard.errors.txt
new file mode 100644
index 00000000000..4ee09e71b60
--- /dev/null
+++ b/tests/baselines/reference/inKeywordTypeguard.errors.txt
@@ -0,0 +1,154 @@
+tests/cases/compiler/inKeywordTypeguard.ts(6,11): error TS2339: Property 'b' does not exist on type 'A'.
+tests/cases/compiler/inKeywordTypeguard.ts(8,11): error TS2339: Property 'a' does not exist on type 'B'.
+tests/cases/compiler/inKeywordTypeguard.ts(14,11): error TS2339: Property 'b' does not exist on type 'A'.
+tests/cases/compiler/inKeywordTypeguard.ts(16,11): error TS2339: Property 'a' does not exist on type 'B'.
+tests/cases/compiler/inKeywordTypeguard.ts(42,11): error TS2339: Property 'b' does not exist on type 'AWithMethod'.
+tests/cases/compiler/inKeywordTypeguard.ts(49,11): error TS2339: Property 'a' does not exist on type 'never'.
+tests/cases/compiler/inKeywordTypeguard.ts(50,11): error TS2339: Property 'b' does not exist on type 'never'.
+tests/cases/compiler/inKeywordTypeguard.ts(52,11): error TS2339: Property 'a' does not exist on type 'AWithMethod | BWithMethod'.
+  Property 'a' does not exist on type 'BWithMethod'.
+tests/cases/compiler/inKeywordTypeguard.ts(53,11): error TS2339: Property 'b' does not exist on type 'AWithMethod | BWithMethod'.
+  Property 'b' does not exist on type 'AWithMethod'.
+tests/cases/compiler/inKeywordTypeguard.ts(62,11): error TS2339: Property 'b' does not exist on type 'A | C | D'.
+  Property 'b' does not exist on type 'A'.
+tests/cases/compiler/inKeywordTypeguard.ts(64,11): error TS2339: Property 'a' does not exist on type 'B'.
+tests/cases/compiler/inKeywordTypeguard.ts(72,32): error TS2339: Property 'b' does not exist on type 'A'.
+tests/cases/compiler/inKeywordTypeguard.ts(74,32): error TS2339: Property 'a' does not exist on type 'B'.
+tests/cases/compiler/inKeywordTypeguard.ts(82,39): error TS2339: Property 'b' does not exist on type 'A'.
+tests/cases/compiler/inKeywordTypeguard.ts(84,39): error TS2339: Property 'a' does not exist on type 'B'.
+tests/cases/compiler/inKeywordTypeguard.ts(94,26): error TS2339: Property 'a' does not exist on type 'never'.
+
+
+==== tests/cases/compiler/inKeywordTypeguard.ts (16 errors) ====
+    class A { a: string; }
+    class B { b: string; }
+    
+    function negativeClassesTest(x: A | B) {
+        if ("a" in x) {
+            x.b = "1";
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'A'.
+        } else {
+            x.a = "1";
+              ~
+!!! error TS2339: Property 'a' does not exist on type 'B'.
+        }
+    }
+    
+    function positiveClassesTest(x: A | B) {
+        if ("a" in x) {
+            x.b = "1";
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'A'.
+        } else {
+            x.a = "1";
+              ~
+!!! error TS2339: Property 'a' does not exist on type 'B'.
+        }
+    }
+    
+    class AOpt { a?: string }
+    class BOpn { b?: string }
+    
+    function positiveTestClassesWithOptionalProperties(x: AOpt | BOpn) {
+        if ("a" in x) {
+            x.a = "1";
+        } else {
+            x.b = "1";
+        }
+    }
+    
+    class AWithMethod {
+        a(): string { return "" }
+    }
+    
+    class BWithMethod {
+        b(): string { return "" }
+    }
+    
+    function negativeTestClassesWithMembers(x: AWithMethod | BWithMethod) {
+        if ("a" in x) {
+            x.a();
+            x.b();
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'AWithMethod'.
+        } else {
+        }
+    }
+    
+    function negativeTestClassesWithMemberMissingInBothClasses(x: AWithMethod | BWithMethod) {
+        if ("c" in x) {
+            x.a();
+              ~
+!!! error TS2339: Property 'a' does not exist on type 'never'.
+            x.b();
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'never'.
+        } else {
+            x.a();
+              ~
+!!! error TS2339: Property 'a' does not exist on type 'AWithMethod | BWithMethod'.
+!!! error TS2339:   Property 'a' does not exist on type 'BWithMethod'.
+            x.b();
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'AWithMethod | BWithMethod'.
+!!! error TS2339:   Property 'b' does not exist on type 'AWithMethod'.
+        }
+    }
+    
+    class C { a: string }
+    class D { a: string }
+    
+    function negativeMultipleClassesTest(x: A | B | C | D) {
+        if ("a" in x) {
+            x.b = "1";
+              ~
+!!! error TS2339: Property 'b' does not exist on type 'A | C | D'.
+!!! error TS2339:   Property 'b' does not exist on type 'A'.
+        } else {
+            x.a = "1";
+              ~
+!!! error TS2339: Property 'a' does not exist on type 'B'.
+        }
+    }
+    
+    class ClassWithProp { prop: A | B }
+    
+    function negativePropTest(x: ClassWithProp) {
+        if ("a" in x.prop) {
+            let y: string = x.prop.b;
+                                   ~
+!!! error TS2339: Property 'b' does not exist on type 'A'.
+        } else {
+            let z: string = x.prop.a;
+                                   ~
+!!! error TS2339: Property 'a' does not exist on type 'B'.
+        }
+    }
+    
+    class NegativeClassTest {
+        protected prop: A | B;
+        inThis() {
+            if ('a' in this.prop) {
+                let z: number = this.prop.b;
+                                          ~
+!!! error TS2339: Property 'b' does not exist on type 'A'.
+            } else {
+                let y: string = this.prop.a;
+                                          ~
+!!! error TS2339: Property 'a' does not exist on type 'B'.
+            }
+        }
+    }
+    
+    class UnreachableCodeDetection {
+        a: string;
+        inThis() {
+            if ('a' in this) {
+            } else {
+                let y = this.a;
+                             ~
+!!! error TS2339: Property 'a' does not exist on type 'never'.
+            }
+        }
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/inKeywordTypeguard.js b/tests/baselines/reference/inKeywordTypeguard.js
new file mode 100644
index 00000000000..33e8eefa146
--- /dev/null
+++ b/tests/baselines/reference/inKeywordTypeguard.js
@@ -0,0 +1,230 @@
+//// [inKeywordTypeguard.ts]
+class A { a: string; }
+class B { b: string; }
+
+function negativeClassesTest(x: A | B) {
+    if ("a" in x) {
+        x.b = "1";
+    } else {
+        x.a = "1";
+    }
+}
+
+function positiveClassesTest(x: A | B) {
+    if ("a" in x) {
+        x.b = "1";
+    } else {
+        x.a = "1";
+    }
+}
+
+class AOpt { a?: string }
+class BOpn { b?: string }
+
+function positiveTestClassesWithOptionalProperties(x: AOpt | BOpn) {
+    if ("a" in x) {
+        x.a = "1";
+    } else {
+        x.b = "1";
+    }
+}
+
+class AWithMethod {
+    a(): string { return "" }
+}
+
+class BWithMethod {
+    b(): string { return "" }
+}
+
+function negativeTestClassesWithMembers(x: AWithMethod | BWithMethod) {
+    if ("a" in x) {
+        x.a();
+        x.b();
+    } else {
+    }
+}
+
+function negativeTestClassesWithMemberMissingInBothClasses(x: AWithMethod | BWithMethod) {
+    if ("c" in x) {
+        x.a();
+        x.b();
+    } else {
+        x.a();
+        x.b();
+    }
+}
+
+class C { a: string }
+class D { a: string }
+
+function negativeMultipleClassesTest(x: A | B | C | D) {
+    if ("a" in x) {
+        x.b = "1";
+    } else {
+        x.a = "1";
+    }
+}
+
+class ClassWithProp { prop: A | B }
+
+function negativePropTest(x: ClassWithProp) {
+    if ("a" in x.prop) {
+        let y: string = x.prop.b;
+    } else {
+        let z: string = x.prop.a;
+    }
+}
+
+class NegativeClassTest {
+    protected prop: A | B;
+    inThis() {
+        if ('a' in this.prop) {
+            let z: number = this.prop.b;
+        } else {
+            let y: string = this.prop.a;
+        }
+    }
+}
+
+class UnreachableCodeDetection {
+    a: string;
+    inThis() {
+        if ('a' in this) {
+        } else {
+            let y = this.a;
+        }
+    }
+}
+
+//// [inKeywordTypeguard.js]
+var A = (function () {
+    function A() {
+    }
+    return A;
+}());
+var B = (function () {
+    function B() {
+    }
+    return B;
+}());
+function negativeClassesTest(x) {
+    if ("a" in x) {
+        x.b = "1";
+    }
+    else {
+        x.a = "1";
+    }
+}
+function positiveClassesTest(x) {
+    if ("a" in x) {
+        x.b = "1";
+    }
+    else {
+        x.a = "1";
+    }
+}
+var AOpt = (function () {
+    function AOpt() {
+    }
+    return AOpt;
+}());
+var BOpn = (function () {
+    function BOpn() {
+    }
+    return BOpn;
+}());
+function positiveTestClassesWithOptionalProperties(x) {
+    if ("a" in x) {
+        x.a = "1";
+    }
+    else {
+        x.b = "1";
+    }
+}
+var AWithMethod = (function () {
+    function AWithMethod() {
+    }
+    AWithMethod.prototype.a = function () { return ""; };
+    return AWithMethod;
+}());
+var BWithMethod = (function () {
+    function BWithMethod() {
+    }
+    BWithMethod.prototype.b = function () { return ""; };
+    return BWithMethod;
+}());
+function negativeTestClassesWithMembers(x) {
+    if ("a" in x) {
+        x.a();
+        x.b();
+    }
+    else {
+    }
+}
+function negativeTestClassesWithMemberMissingInBothClasses(x) {
+    if ("c" in x) {
+        x.a();
+        x.b();
+    }
+    else {
+        x.a();
+        x.b();
+    }
+}
+var C = (function () {
+    function C() {
+    }
+    return C;
+}());
+var D = (function () {
+    function D() {
+    }
+    return D;
+}());
+function negativeMultipleClassesTest(x) {
+    if ("a" in x) {
+        x.b = "1";
+    }
+    else {
+        x.a = "1";
+    }
+}
+var ClassWithProp = (function () {
+    function ClassWithProp() {
+    }
+    return ClassWithProp;
+}());
+function negativePropTest(x) {
+    if ("a" in x.prop) {
+        var y = x.prop.b;
+    }
+    else {
+        var z = x.prop.a;
+    }
+}
+var NegativeClassTest = (function () {
+    function NegativeClassTest() {
+    }
+    NegativeClassTest.prototype.inThis = function () {
+        if ('a' in this.prop) {
+            var z = this.prop.b;
+        }
+        else {
+            var y = this.prop.a;
+        }
+    };
+    return NegativeClassTest;
+}());
+var UnreachableCodeDetection = (function () {
+    function UnreachableCodeDetection() {
+    }
+    UnreachableCodeDetection.prototype.inThis = function () {
+        if ('a' in this) {
+        }
+        else {
+            var y = this.a;
+        }
+    };
+    return UnreachableCodeDetection;
+}());
diff --git a/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.js b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.js
new file mode 100644
index 00000000000..644abfaf33f
--- /dev/null
+++ b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.js
@@ -0,0 +1,184 @@
+//// [typeGuardOfFromPropNameInUnionType.ts]
+class A { a: string; }
+class B { b: number; }
+class C { b: Object; }
+class D { a: Date; }
+class ClassWithProp { prop: A | B }
+class NestedClassWithProp { outer: ClassWithProp }
+
+function namedClasses(x: A | B) {
+    if ("a" in x) {
+        x.a = "1";
+    } else {
+        x.b = 1;
+    }
+}
+
+function multipleClasses(x: A | B | C | D) {
+    if ("a" in x) {
+        let y: string | Date = x.a;
+    } else {
+        let z: number | Object = x.b;
+    }
+}
+
+function anonymousClasses(x: { a: string; } | { b: number; }) {
+    if ("a" in x) {
+        let y: string = x.a;
+    } else {
+        let z: number = x.b;
+    }
+}
+function inParenthesizedExpression(x: A | B) {
+    if ("a" in (x)) {
+        let y: string = x.a;
+    } else {
+        let z: number = x.b;
+    }
+}
+
+
+function inProperty(x: ClassWithProp) {
+    if ("a" in x.prop) {
+        let y: string = x.prop.a;
+    } else {
+        let z: number = x.prop.b;
+    }
+}
+
+
+function innestedProperty(x: NestedClassWithProp) {
+    if ("a" in x.outer.prop) {
+        let y: string = x.outer.prop.a;
+    } else {
+        let z: number = x.outer.prop.b;
+    }
+}
+
+class InMemberOfClass {
+    protected prop: A | B;
+    inThis() {
+        if ('a' in this.prop) {
+            let y: string = this.prop.a;
+        } else {
+            let z: number = this.prop.b;
+        }
+    }
+}
+
+//added for completeness
+class SelfAssert {
+    a: string;
+    inThis() {
+        if ('a' in this) {
+            let y: string = this.a;
+        } else {
+        }
+    }
+}
+
+//// [typeGuardOfFromPropNameInUnionType.js]
+var A = (function () {
+    function A() {
+    }
+    return A;
+}());
+var B = (function () {
+    function B() {
+    }
+    return B;
+}());
+var C = (function () {
+    function C() {
+    }
+    return C;
+}());
+var D = (function () {
+    function D() {
+    }
+    return D;
+}());
+var ClassWithProp = (function () {
+    function ClassWithProp() {
+    }
+    return ClassWithProp;
+}());
+var NestedClassWithProp = (function () {
+    function NestedClassWithProp() {
+    }
+    return NestedClassWithProp;
+}());
+function namedClasses(x) {
+    if ("a" in x) {
+        x.a = "1";
+    }
+    else {
+        x.b = 1;
+    }
+}
+function multipleClasses(x) {
+    if ("a" in x) {
+        var y = x.a;
+    }
+    else {
+        var z = x.b;
+    }
+}
+function anonymousClasses(x) {
+    if ("a" in x) {
+        var y = x.a;
+    }
+    else {
+        var z = x.b;
+    }
+}
+function inParenthesizedExpression(x) {
+    if ("a" in (x)) {
+        var y = x.a;
+    }
+    else {
+        var z = x.b;
+    }
+}
+function inProperty(x) {
+    if ("a" in x.prop) {
+        var y = x.prop.a;
+    }
+    else {
+        var z = x.prop.b;
+    }
+}
+function innestedProperty(x) {
+    if ("a" in x.outer.prop) {
+        var y = x.outer.prop.a;
+    }
+    else {
+        var z = x.outer.prop.b;
+    }
+}
+var InMemberOfClass = (function () {
+    function InMemberOfClass() {
+    }
+    InMemberOfClass.prototype.inThis = function () {
+        if ('a' in this.prop) {
+            var y = this.prop.a;
+        }
+        else {
+            var z = this.prop.b;
+        }
+    };
+    return InMemberOfClass;
+}());
+//added for completeness
+var SelfAssert = (function () {
+    function SelfAssert() {
+    }
+    SelfAssert.prototype.inThis = function () {
+        if ('a' in this) {
+            var y = this.a;
+        }
+        else {
+        }
+    };
+    return SelfAssert;
+}());
diff --git a/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.symbols b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.symbols
new file mode 100644
index 00000000000..0302635b558
--- /dev/null
+++ b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.symbols
@@ -0,0 +1,252 @@
+=== tests/cases/conformance/expressions/typeGuards/typeGuardOfFromPropNameInUnionType.ts ===
+class A { a: string; }
+>A : Symbol(A, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 0))
+>a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+
+class B { b: number; }
+>B : Symbol(B, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 22))
+>b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+
+class C { b: Object; }
+>C : Symbol(C, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 22))
+>b : Symbol(C.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 2, 9))
+>Object : Symbol(Object, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+
+class D { a: Date; }
+>D : Symbol(D, Decl(typeGuardOfFromPropNameInUnionType.ts, 2, 22))
+>a : Symbol(D.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 3, 9))
+>Date : Symbol(Date, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+
+class ClassWithProp { prop: A | B }
+>ClassWithProp : Symbol(ClassWithProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 3, 20))
+>prop : Symbol(ClassWithProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 21))
+>A : Symbol(A, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 0))
+>B : Symbol(B, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 22))
+
+class NestedClassWithProp { outer: ClassWithProp }
+>NestedClassWithProp : Symbol(NestedClassWithProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 35))
+>outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 5, 27))
+>ClassWithProp : Symbol(ClassWithProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 3, 20))
+
+function namedClasses(x: A | B) {
+>namedClasses : Symbol(namedClasses, Decl(typeGuardOfFromPropNameInUnionType.ts, 5, 50))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 7, 22))
+>A : Symbol(A, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 0))
+>B : Symbol(B, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 22))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 7, 22))
+
+        x.a = "1";
+>x.a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 7, 22))
+>a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+
+    } else {
+        x.b = 1;
+>x.b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 7, 22))
+>b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+    }
+}
+
+function multipleClasses(x: A | B | C | D) {
+>multipleClasses : Symbol(multipleClasses, Decl(typeGuardOfFromPropNameInUnionType.ts, 13, 1))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 15, 25))
+>A : Symbol(A, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 0))
+>B : Symbol(B, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 22))
+>C : Symbol(C, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 22))
+>D : Symbol(D, Decl(typeGuardOfFromPropNameInUnionType.ts, 2, 22))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 15, 25))
+
+        let y: string | Date = x.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 17, 11))
+>Date : Symbol(Date, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+>x.a : Symbol(a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9), Decl(typeGuardOfFromPropNameInUnionType.ts, 3, 9))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 15, 25))
+>a : Symbol(a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9), Decl(typeGuardOfFromPropNameInUnionType.ts, 3, 9))
+
+    } else {
+        let z: number | Object = x.b;
+>z : Symbol(z, Decl(typeGuardOfFromPropNameInUnionType.ts, 19, 11))
+>Object : Symbol(Object, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+>x.b : Symbol(b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9), Decl(typeGuardOfFromPropNameInUnionType.ts, 2, 9))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 15, 25))
+>b : Symbol(b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9), Decl(typeGuardOfFromPropNameInUnionType.ts, 2, 9))
+    }
+}
+
+function anonymousClasses(x: { a: string; } | { b: number; }) {
+>anonymousClasses : Symbol(anonymousClasses, Decl(typeGuardOfFromPropNameInUnionType.ts, 21, 1))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 23, 26))
+>a : Symbol(a, Decl(typeGuardOfFromPropNameInUnionType.ts, 23, 30))
+>b : Symbol(b, Decl(typeGuardOfFromPropNameInUnionType.ts, 23, 47))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 23, 26))
+
+        let y: string = x.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 25, 11))
+>x.a : Symbol(a, Decl(typeGuardOfFromPropNameInUnionType.ts, 23, 30))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 23, 26))
+>a : Symbol(a, Decl(typeGuardOfFromPropNameInUnionType.ts, 23, 30))
+
+    } else {
+        let z: number = x.b;
+>z : Symbol(z, Decl(typeGuardOfFromPropNameInUnionType.ts, 27, 11))
+>x.b : Symbol(b, Decl(typeGuardOfFromPropNameInUnionType.ts, 23, 47))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 23, 26))
+>b : Symbol(b, Decl(typeGuardOfFromPropNameInUnionType.ts, 23, 47))
+    }
+}
+function inParenthesizedExpression(x: A | B) {
+>inParenthesizedExpression : Symbol(inParenthesizedExpression, Decl(typeGuardOfFromPropNameInUnionType.ts, 29, 1))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 30, 35))
+>A : Symbol(A, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 0))
+>B : Symbol(B, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 22))
+
+    if ("a" in (x)) {
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 30, 35))
+
+        let y: string = x.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 32, 11))
+>x.a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 30, 35))
+>a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+
+    } else {
+        let z: number = x.b;
+>z : Symbol(z, Decl(typeGuardOfFromPropNameInUnionType.ts, 34, 11))
+>x.b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 30, 35))
+>b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+    }
+}
+
+
+function inProperty(x: ClassWithProp) {
+>inProperty : Symbol(inProperty, Decl(typeGuardOfFromPropNameInUnionType.ts, 36, 1))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 39, 20))
+>ClassWithProp : Symbol(ClassWithProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 3, 20))
+
+    if ("a" in x.prop) {
+>x.prop : Symbol(ClassWithProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 21))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 39, 20))
+>prop : Symbol(ClassWithProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 21))
+
+        let y: string = x.prop.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 41, 11))
+>x.prop.a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+>x.prop : Symbol(ClassWithProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 21))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 39, 20))
+>prop : Symbol(ClassWithProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 21))
+>a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+
+    } else {
+        let z: number = x.prop.b;
+>z : Symbol(z, Decl(typeGuardOfFromPropNameInUnionType.ts, 43, 11))
+>x.prop.b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+>x.prop : Symbol(ClassWithProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 21))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 39, 20))
+>prop : Symbol(ClassWithProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 21))
+>b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+    }
+}
+
+
+function innestedProperty(x: NestedClassWithProp) {
+>innestedProperty : Symbol(innestedProperty, Decl(typeGuardOfFromPropNameInUnionType.ts, 45, 1))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 48, 26))
+>NestedClassWithProp : Symbol(NestedClassWithProp, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 35))
+
+    if ("a" in x.outer.prop) {
+>x.outer.prop : Symbol(ClassWithProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 21))
+>x.outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 5, 27))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 48, 26))
+>outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 5, 27))
+>prop : Symbol(ClassWithProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 21))
+
+        let y: string = x.outer.prop.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 50, 11))
+>x.outer.prop.a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+>x.outer.prop : Symbol(ClassWithProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 21))
+>x.outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 5, 27))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 48, 26))
+>outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 5, 27))
+>prop : Symbol(ClassWithProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 21))
+>a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+
+    } else {
+        let z: number = x.outer.prop.b;
+>z : Symbol(z, Decl(typeGuardOfFromPropNameInUnionType.ts, 52, 11))
+>x.outer.prop.b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+>x.outer.prop : Symbol(ClassWithProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 21))
+>x.outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 5, 27))
+>x : Symbol(x, Decl(typeGuardOfFromPropNameInUnionType.ts, 48, 26))
+>outer : Symbol(NestedClassWithProp.outer, Decl(typeGuardOfFromPropNameInUnionType.ts, 5, 27))
+>prop : Symbol(ClassWithProp.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 4, 21))
+>b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+    }
+}
+
+class InMemberOfClass {
+>InMemberOfClass : Symbol(InMemberOfClass, Decl(typeGuardOfFromPropNameInUnionType.ts, 54, 1))
+
+    protected prop: A | B;
+>prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 56, 23))
+>A : Symbol(A, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 0))
+>B : Symbol(B, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 22))
+
+    inThis() {
+>inThis : Symbol(InMemberOfClass.inThis, Decl(typeGuardOfFromPropNameInUnionType.ts, 57, 26))
+
+        if ('a' in this.prop) {
+>this.prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 56, 23))
+>this : Symbol(InMemberOfClass, Decl(typeGuardOfFromPropNameInUnionType.ts, 54, 1))
+>prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 56, 23))
+
+            let y: string = this.prop.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 60, 15))
+>this.prop.a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+>this.prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 56, 23))
+>this : Symbol(InMemberOfClass, Decl(typeGuardOfFromPropNameInUnionType.ts, 54, 1))
+>prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 56, 23))
+>a : Symbol(A.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 0, 9))
+
+        } else {
+            let z: number = this.prop.b;
+>z : Symbol(z, Decl(typeGuardOfFromPropNameInUnionType.ts, 62, 15))
+>this.prop.b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+>this.prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 56, 23))
+>this : Symbol(InMemberOfClass, Decl(typeGuardOfFromPropNameInUnionType.ts, 54, 1))
+>prop : Symbol(InMemberOfClass.prop, Decl(typeGuardOfFromPropNameInUnionType.ts, 56, 23))
+>b : Symbol(B.b, Decl(typeGuardOfFromPropNameInUnionType.ts, 1, 9))
+        }
+    }
+}
+
+//added for completeness
+class SelfAssert {
+>SelfAssert : Symbol(SelfAssert, Decl(typeGuardOfFromPropNameInUnionType.ts, 65, 1))
+
+    a: string;
+>a : Symbol(SelfAssert.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 68, 18))
+
+    inThis() {
+>inThis : Symbol(SelfAssert.inThis, Decl(typeGuardOfFromPropNameInUnionType.ts, 69, 14))
+
+        if ('a' in this) {
+>this : Symbol(SelfAssert, Decl(typeGuardOfFromPropNameInUnionType.ts, 65, 1))
+
+            let y: string = this.a;
+>y : Symbol(y, Decl(typeGuardOfFromPropNameInUnionType.ts, 72, 15))
+>this.a : Symbol(SelfAssert.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 68, 18))
+>this : Symbol(SelfAssert, Decl(typeGuardOfFromPropNameInUnionType.ts, 65, 1))
+>a : Symbol(SelfAssert.a, Decl(typeGuardOfFromPropNameInUnionType.ts, 68, 18))
+
+        } else {
+        }
+    }
+}
diff --git a/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.types b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.types
new file mode 100644
index 00000000000..d9984840fb3
--- /dev/null
+++ b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.types
@@ -0,0 +1,273 @@
+=== tests/cases/conformance/expressions/typeGuards/typeGuardOfFromPropNameInUnionType.ts ===
+class A { a: string; }
+>A : A
+>a : string
+
+class B { b: number; }
+>B : B
+>b : number
+
+class C { b: Object; }
+>C : C
+>b : Object
+>Object : Object
+
+class D { a: Date; }
+>D : D
+>a : Date
+>Date : Date
+
+class ClassWithProp { prop: A | B }
+>ClassWithProp : ClassWithProp
+>prop : A | B
+>A : A
+>B : B
+
+class NestedClassWithProp { outer: ClassWithProp }
+>NestedClassWithProp : NestedClassWithProp
+>outer : ClassWithProp
+>ClassWithProp : ClassWithProp
+
+function namedClasses(x: A | B) {
+>namedClasses : (x: A | B) => void
+>x : A | B
+>A : A
+>B : B
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : A | B
+
+        x.a = "1";
+>x.a = "1" : "1"
+>x.a : string
+>x : A
+>a : string
+>"1" : "1"
+
+    } else {
+        x.b = 1;
+>x.b = 1 : 1
+>x.b : number
+>x : B
+>b : number
+>1 : 1
+    }
+}
+
+function multipleClasses(x: A | B | C | D) {
+>multipleClasses : (x: A | B | C | D) => void
+>x : A | B | C | D
+>A : A
+>B : B
+>C : C
+>D : D
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : A | B | C | D
+
+        let y: string | Date = x.a;
+>y : string | Date
+>Date : Date
+>x.a : string | Date
+>x : A | D
+>a : string | Date
+
+    } else {
+        let z: number | Object = x.b;
+>z : number | Object
+>Object : Object
+>x.b : number | Object
+>x : B | C
+>b : number | Object
+    }
+}
+
+function anonymousClasses(x: { a: string; } | { b: number; }) {
+>anonymousClasses : (x: { a: string; } | { b: number; }) => void
+>x : { a: string; } | { b: number; }
+>a : string
+>b : number
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : { a: string; } | { b: number; }
+
+        let y: string = x.a;
+>y : string
+>x.a : string
+>x : { a: string; }
+>a : string
+
+    } else {
+        let z: number = x.b;
+>z : number
+>x.b : number
+>x : { b: number; }
+>b : number
+    }
+}
+function inParenthesizedExpression(x: A | B) {
+>inParenthesizedExpression : (x: A | B) => void
+>x : A | B
+>A : A
+>B : B
+
+    if ("a" in (x)) {
+>"a" in (x) : boolean
+>"a" : "a"
+>(x) : A | B
+>x : A | B
+
+        let y: string = x.a;
+>y : string
+>x.a : string
+>x : A
+>a : string
+
+    } else {
+        let z: number = x.b;
+>z : number
+>x.b : number
+>x : B
+>b : number
+    }
+}
+
+
+function inProperty(x: ClassWithProp) {
+>inProperty : (x: ClassWithProp) => void
+>x : ClassWithProp
+>ClassWithProp : ClassWithProp
+
+    if ("a" in x.prop) {
+>"a" in x.prop : boolean
+>"a" : "a"
+>x.prop : A | B
+>x : ClassWithProp
+>prop : A | B
+
+        let y: string = x.prop.a;
+>y : string
+>x.prop.a : string
+>x.prop : A
+>x : ClassWithProp
+>prop : A
+>a : string
+
+    } else {
+        let z: number = x.prop.b;
+>z : number
+>x.prop.b : number
+>x.prop : B
+>x : ClassWithProp
+>prop : B
+>b : number
+    }
+}
+
+
+function innestedProperty(x: NestedClassWithProp) {
+>innestedProperty : (x: NestedClassWithProp) => void
+>x : NestedClassWithProp
+>NestedClassWithProp : NestedClassWithProp
+
+    if ("a" in x.outer.prop) {
+>"a" in x.outer.prop : boolean
+>"a" : "a"
+>x.outer.prop : A | B
+>x.outer : ClassWithProp
+>x : NestedClassWithProp
+>outer : ClassWithProp
+>prop : A | B
+
+        let y: string = x.outer.prop.a;
+>y : string
+>x.outer.prop.a : string
+>x.outer.prop : A
+>x.outer : ClassWithProp
+>x : NestedClassWithProp
+>outer : ClassWithProp
+>prop : A
+>a : string
+
+    } else {
+        let z: number = x.outer.prop.b;
+>z : number
+>x.outer.prop.b : number
+>x.outer.prop : B
+>x.outer : ClassWithProp
+>x : NestedClassWithProp
+>outer : ClassWithProp
+>prop : B
+>b : number
+    }
+}
+
+class InMemberOfClass {
+>InMemberOfClass : InMemberOfClass
+
+    protected prop: A | B;
+>prop : A | B
+>A : A
+>B : B
+
+    inThis() {
+>inThis : () => void
+
+        if ('a' in this.prop) {
+>'a' in this.prop : boolean
+>'a' : "a"
+>this.prop : A | B
+>this : this
+>prop : A | B
+
+            let y: string = this.prop.a;
+>y : string
+>this.prop.a : string
+>this.prop : A
+>this : this
+>prop : A
+>a : string
+
+        } else {
+            let z: number = this.prop.b;
+>z : number
+>this.prop.b : number
+>this.prop : B
+>this : this
+>prop : B
+>b : number
+        }
+    }
+}
+
+//added for completeness
+class SelfAssert {
+>SelfAssert : SelfAssert
+
+    a: string;
+>a : string
+
+    inThis() {
+>inThis : () => void
+
+        if ('a' in this) {
+>'a' in this : boolean
+>'a' : "a"
+>this : this
+
+            let y: string = this.a;
+>y : string
+>this.a : string
+>this : this
+>a : string
+
+        } else {
+        }
+    }
+}
diff --git a/tests/cases/compiler/inKeywordTypeguard.ts b/tests/cases/compiler/inKeywordTypeguard.ts
new file mode 100644
index 00000000000..90d1942e7c5
--- /dev/null
+++ b/tests/cases/compiler/inKeywordTypeguard.ts
@@ -0,0 +1,97 @@
+class A { a: string; }
+class B { b: string; }
+
+function negativeClassesTest(x: A | B) {
+    if ("a" in x) {
+        x.b = "1";
+    } else {
+        x.a = "1";
+    }
+}
+
+function positiveClassesTest(x: A | B) {
+    if ("a" in x) {
+        x.b = "1";
+    } else {
+        x.a = "1";
+    }
+}
+
+class AOpt { a?: string }
+class BOpn { b?: string }
+
+function positiveTestClassesWithOptionalProperties(x: AOpt | BOpn) {
+    if ("a" in x) {
+        x.a = "1";
+    } else {
+        x.b = "1";
+    }
+}
+
+class AWithMethod {
+    a(): string { return "" }
+}
+
+class BWithMethod {
+    b(): string { return "" }
+}
+
+function negativeTestClassesWithMembers(x: AWithMethod | BWithMethod) {
+    if ("a" in x) {
+        x.a();
+        x.b();
+    } else {
+    }
+}
+
+function negativeTestClassesWithMemberMissingInBothClasses(x: AWithMethod | BWithMethod) {
+    if ("c" in x) {
+        x.a();
+        x.b();
+    } else {
+        x.a();
+        x.b();
+    }
+}
+
+class C { a: string }
+class D { a: string }
+
+function negativeMultipleClassesTest(x: A | B | C | D) {
+    if ("a" in x) {
+        x.b = "1";
+    } else {
+        x.a = "1";
+    }
+}
+
+class ClassWithProp { prop: A | B }
+
+function negativePropTest(x: ClassWithProp) {
+    if ("a" in x.prop) {
+        let y: string = x.prop.b;
+    } else {
+        let z: string = x.prop.a;
+    }
+}
+
+class NegativeClassTest {
+    protected prop: A | B;
+    inThis() {
+        if ('a' in this.prop) {
+            let z: number = this.prop.b;
+        } else {
+            let y: string = this.prop.a;
+        }
+    }
+}
+
+class UnreachableCodeDetection {
+    a: string;
+    inThis() {
+        if ('a' in this) {
+        } else {
+            let y = this.a;
+        }
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/conformance/expressions/typeGuards/typeGuardOfFromPropNameInUnionType.ts b/tests/cases/conformance/expressions/typeGuards/typeGuardOfFromPropNameInUnionType.ts
new file mode 100644
index 00000000000..2a14fd2aea8
--- /dev/null
+++ b/tests/cases/conformance/expressions/typeGuards/typeGuardOfFromPropNameInUnionType.ts
@@ -0,0 +1,77 @@
+class A { a: string; }
+class B { b: number; }
+class C { b: Object; }
+class D { a: Date; }
+class ClassWithProp { prop: A | B }
+class NestedClassWithProp { outer: ClassWithProp }
+
+function namedClasses(x: A | B) {
+    if ("a" in x) {
+        x.a = "1";
+    } else {
+        x.b = 1;
+    }
+}
+
+function multipleClasses(x: A | B | C | D) {
+    if ("a" in x) {
+        let y: string | Date = x.a;
+    } else {
+        let z: number | Object = x.b;
+    }
+}
+
+function anonymousClasses(x: { a: string; } | { b: number; }) {
+    if ("a" in x) {
+        let y: string = x.a;
+    } else {
+        let z: number = x.b;
+    }
+}
+function inParenthesizedExpression(x: A | B) {
+    if ("a" in (x)) {
+        let y: string = x.a;
+    } else {
+        let z: number = x.b;
+    }
+}
+
+
+function inProperty(x: ClassWithProp) {
+    if ("a" in x.prop) {
+        let y: string = x.prop.a;
+    } else {
+        let z: number = x.prop.b;
+    }
+}
+
+
+function innestedProperty(x: NestedClassWithProp) {
+    if ("a" in x.outer.prop) {
+        let y: string = x.outer.prop.a;
+    } else {
+        let z: number = x.outer.prop.b;
+    }
+}
+
+class InMemberOfClass {
+    protected prop: A | B;
+    inThis() {
+        if ('a' in this.prop) {
+            let y: string = this.prop.a;
+        } else {
+            let z: number = this.prop.b;
+        }
+    }
+}
+
+//added for completeness
+class SelfAssert {
+    a: string;
+    inThis() {
+        if ('a' in this) {
+            let y: string = this.a;
+        } else {
+        }
+    }
+}
\ No newline at end of file