diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 7f2db45bab2..5404a54975d 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -12722,30 +12722,18 @@ namespace ts {
                 return type;
             }
 
-            function isTypePresencePossible(type: Type, propName: string, shouldHaveProperty: boolean) {
+            function isTypePresencePossible(type: Type, propName: __String, assumeTrue: boolean) {
                 const prop = getPropertyOfType(type, propName);
-                if (!prop) {
-                    // if there is NO property:
-                    // - but we assume type SHOULD have it then presence of object of following type IS NOT possible
-                    // - and we assume type SHOULD NOT have it then presence of object of following type IS possible
-                    return !shouldHaveProperty;
-                } else if (prop.flags & SymbolFlags.Optional) {
-                    // if there is an optional property:
-                    // - and we assume type SHOULD have it then presence of object of following type IS possible
-                    // - but assume type SHOULD NOT have it then presence of object of following type IS still possible
-                    return true;
-                } else /* if (prop.flags & SymbolFlags.Required) */ {
-                    // if there is a required property:
-                    // - and we assume type SHOULD have it then presence of object of following type IS possible
-                    // - but we assume type SHOULD NOT have it then presence of object of following type IS NOT possible
-                    return shouldHaveProperty;
+                if (prop) {
+                    return (prop.flags & SymbolFlags.Optional) ? true : assumeTrue;
                 }
+                return !assumeTrue;
             }
 
             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 => isTypePresencePossible(t, propName, /* shouldHaveProperty */ assumeTrue));
+                    const propName = escapeLeadingUnderscores(literal.text);
+                    return filterType(type, t => isTypePresencePossible(t, propName, /* assumeTrue */ assumeTrue));
                 }
                 return type;
             }
diff --git a/tests/baselines/reference/fixSignatureCaching.errors.txt b/tests/baselines/reference/fixSignatureCaching.errors.txt
index 07d706a846f..e991908c850 100644
--- a/tests/baselines/reference/fixSignatureCaching.errors.txt
+++ b/tests/baselines/reference/fixSignatureCaching.errors.txt
@@ -3,6 +3,7 @@ tests/cases/conformance/fixSignatureCaching.ts(284,10): error TS2339: Property '
 tests/cases/conformance/fixSignatureCaching.ts(293,10): error TS2339: Property 'FALLBACK_PHONE' does not exist on type '{}'.
 tests/cases/conformance/fixSignatureCaching.ts(294,10): error TS2339: Property 'FALLBACK_TABLET' does not exist on type '{}'.
 tests/cases/conformance/fixSignatureCaching.ts(295,10): error TS2339: Property 'FALLBACK_MOBILE' does not exist on type '{}'.
+tests/cases/conformance/fixSignatureCaching.ts(301,17): error TS2339: Property 'isArray' does not exist on type 'never'.
 tests/cases/conformance/fixSignatureCaching.ts(330,74): error TS2339: Property 'mobileDetectRules' does not exist on type '{}'.
 tests/cases/conformance/fixSignatureCaching.ts(369,10): error TS2339: Property 'findMatch' does not exist on type '{}'.
 tests/cases/conformance/fixSignatureCaching.ts(387,10): error TS2339: Property 'findMatches' does not exist on type '{}'.
@@ -71,7 +72,7 @@ tests/cases/conformance/fixSignatureCaching.ts(982,23): error TS2304: Cannot fin
 tests/cases/conformance/fixSignatureCaching.ts(983,37): error TS2304: Cannot find name 'window'.
 
 
-==== tests/cases/conformance/fixSignatureCaching.ts (71 errors) ====
+==== tests/cases/conformance/fixSignatureCaching.ts (72 errors) ====
     // Repro from #10697
     
     (function (define, undefined) {
@@ -383,6 +384,8 @@ tests/cases/conformance/fixSignatureCaching.ts(983,37): error TS2304: Cannot fin
         isArray = 'isArray' in Array
             ? function (value) { return Object.prototype.toString.call(value) === '[object Array]'; }
             : Array.isArray;
+                    ~~~~~~~
+!!! error TS2339: Property 'isArray' does not exist on type 'never'.
     
         function equalIC(a, b) {
             return a != null && b != null && a.toLowerCase() === b.toLowerCase();
diff --git a/tests/baselines/reference/fixSignatureCaching.symbols b/tests/baselines/reference/fixSignatureCaching.symbols
index 9bae31520fd..87cf8d0c44e 100644
--- a/tests/baselines/reference/fixSignatureCaching.symbols
+++ b/tests/baselines/reference/fixSignatureCaching.symbols
@@ -358,9 +358,7 @@ define(function () {
 >value : Symbol(value, Decl(fixSignatureCaching.ts, 299, 20))
 
         : Array.isArray;
->Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.d.ts, --, --))
 >Array : Symbol(Array, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
->isArray : Symbol(ArrayConstructor.isArray, Decl(lib.d.ts, --, --))
 
     function equalIC(a, b) {
 >equalIC : Symbol(equalIC, Decl(fixSignatureCaching.ts, 300, 24))
diff --git a/tests/baselines/reference/fixSignatureCaching.types b/tests/baselines/reference/fixSignatureCaching.types
index 61d195e2155..60502cae583 100644
--- a/tests/baselines/reference/fixSignatureCaching.types
+++ b/tests/baselines/reference/fixSignatureCaching.types
@@ -893,9 +893,9 @@ define(function () {
 >'[object Array]' : "[object Array]"
 
     isArray = 'isArray' in Array
->isArray = 'isArray' in Array        ? function (value) { return Object.prototype.toString.call(value) === '[object Array]'; }        : Array.isArray : (value: any) => boolean
+>isArray = 'isArray' in Array        ? function (value) { return Object.prototype.toString.call(value) === '[object Array]'; }        : Array.isArray : any
 >isArray : any
->'isArray' in Array        ? function (value) { return Object.prototype.toString.call(value) === '[object Array]'; }        : Array.isArray : (value: any) => boolean
+>'isArray' in Array        ? function (value) { return Object.prototype.toString.call(value) === '[object Array]'; }        : Array.isArray : any
 >'isArray' in Array : boolean
 >'isArray' : "isArray"
 >Array : ArrayConstructor
@@ -916,9 +916,9 @@ define(function () {
 >'[object Array]' : "[object Array]"
 
         : Array.isArray;
->Array.isArray : (arg: any) => arg is any[]
->Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>Array.isArray : any
+>Array : never
+>isArray : any
 
     function equalIC(a, b) {
 >equalIC : (a: any, b: any) => boolean
diff --git a/tests/baselines/reference/inKeywordTypeguard.js b/tests/baselines/reference/inKeywordTypeguard.js
index 643c73feb50..41ad27f96df 100644
--- a/tests/baselines/reference/inKeywordTypeguard.js
+++ b/tests/baselines/reference/inKeywordTypeguard.js
@@ -98,12 +98,12 @@ class UnreachableCodeDetection {
 }
 
 //// [inKeywordTypeguard.js]
-var A = (function () {
+var A = /** @class */ (function () {
     function A() {
     }
     return A;
 }());
-var B = (function () {
+var B = /** @class */ (function () {
     function B() {
     }
     return B;
@@ -124,12 +124,12 @@ function positiveClassesTest(x) {
         x.a = "1";
     }
 }
-var AWithOptionalProp = (function () {
+var AWithOptionalProp = /** @class */ (function () {
     function AWithOptionalProp() {
     }
     return AWithOptionalProp;
 }());
-var BWithOptionalProp = (function () {
+var BWithOptionalProp = /** @class */ (function () {
     function BWithOptionalProp() {
     }
     return BWithOptionalProp;
@@ -142,13 +142,13 @@ function positiveTestClassesWithOptionalProperties(x) {
         x.b = "1";
     }
 }
-var AWithMethod = (function () {
+var AWithMethod = /** @class */ (function () {
     function AWithMethod() {
     }
     AWithMethod.prototype.a = function () { return ""; };
     return AWithMethod;
 }());
-var BWithMethod = (function () {
+var BWithMethod = /** @class */ (function () {
     function BWithMethod() {
     }
     BWithMethod.prototype.b = function () { return ""; };
@@ -172,12 +172,12 @@ function negativeTestClassesWithMemberMissingInBothClasses(x) {
         x.b();
     }
 }
-var C = (function () {
+var C = /** @class */ (function () {
     function C() {
     }
     return C;
 }());
-var D = (function () {
+var D = /** @class */ (function () {
     function D() {
     }
     return D;
@@ -190,7 +190,7 @@ function negativeMultipleClassesTest(x) {
         x.a = "1";
     }
 }
-var ClassWithUnionProp = (function () {
+var ClassWithUnionProp = /** @class */ (function () {
     function ClassWithUnionProp() {
     }
     return ClassWithUnionProp;
@@ -203,7 +203,7 @@ function negativePropTest(x) {
         var z = x.prop.a;
     }
 }
-var NegativeClassTest = (function () {
+var NegativeClassTest = /** @class */ (function () {
     function NegativeClassTest() {
     }
     NegativeClassTest.prototype.inThis = function () {
@@ -216,7 +216,7 @@ var NegativeClassTest = (function () {
     };
     return NegativeClassTest;
 }());
-var UnreachableCodeDetection = (function () {
+var UnreachableCodeDetection = /** @class */ (function () {
     function UnreachableCodeDetection() {
     }
     UnreachableCodeDetection.prototype.inThis = function () {
diff --git a/tests/baselines/reference/inKeywordTypeguard.symbols b/tests/baselines/reference/inKeywordTypeguard.symbols
new file mode 100644
index 00000000000..c73f6f4901d
--- /dev/null
+++ b/tests/baselines/reference/inKeywordTypeguard.symbols
@@ -0,0 +1,241 @@
+=== tests/cases/compiler/inKeywordTypeguard.ts ===
+class A { a: string; }
+>A : Symbol(A, Decl(inKeywordTypeguard.ts, 0, 0))
+>a : Symbol(A.a, Decl(inKeywordTypeguard.ts, 0, 9))
+
+class B { b: string; }
+>B : Symbol(B, Decl(inKeywordTypeguard.ts, 0, 22))
+>b : Symbol(B.b, Decl(inKeywordTypeguard.ts, 1, 9))
+
+function negativeClassesTest(x: A | B) {
+>negativeClassesTest : Symbol(negativeClassesTest, Decl(inKeywordTypeguard.ts, 1, 22))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 3, 29))
+>A : Symbol(A, Decl(inKeywordTypeguard.ts, 0, 0))
+>B : Symbol(B, Decl(inKeywordTypeguard.ts, 0, 22))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 3, 29))
+
+        x.b = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 3, 29))
+
+    } else {
+        x.a = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 3, 29))
+    }
+}
+
+function positiveClassesTest(x: A | B) {
+>positiveClassesTest : Symbol(positiveClassesTest, Decl(inKeywordTypeguard.ts, 9, 1))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 11, 29))
+>A : Symbol(A, Decl(inKeywordTypeguard.ts, 0, 0))
+>B : Symbol(B, Decl(inKeywordTypeguard.ts, 0, 22))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 11, 29))
+
+        x.b = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 11, 29))
+
+    } else {
+        x.a = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 11, 29))
+    }
+}
+
+class AWithOptionalProp { a?: string; }
+>AWithOptionalProp : Symbol(AWithOptionalProp, Decl(inKeywordTypeguard.ts, 17, 1))
+>a : Symbol(AWithOptionalProp.a, Decl(inKeywordTypeguard.ts, 19, 25))
+
+class BWithOptionalProp { b?: string; }
+>BWithOptionalProp : Symbol(BWithOptionalProp, Decl(inKeywordTypeguard.ts, 19, 39))
+>b : Symbol(BWithOptionalProp.b, Decl(inKeywordTypeguard.ts, 20, 25))
+
+function positiveTestClassesWithOptionalProperties(x: AWithOptionalProp | BWithOptionalProp) {
+>positiveTestClassesWithOptionalProperties : Symbol(positiveTestClassesWithOptionalProperties, Decl(inKeywordTypeguard.ts, 20, 39))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 22, 51))
+>AWithOptionalProp : Symbol(AWithOptionalProp, Decl(inKeywordTypeguard.ts, 17, 1))
+>BWithOptionalProp : Symbol(BWithOptionalProp, Decl(inKeywordTypeguard.ts, 19, 39))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 22, 51))
+
+        x.a = "1";
+>x.a : Symbol(AWithOptionalProp.a, Decl(inKeywordTypeguard.ts, 19, 25))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 22, 51))
+>a : Symbol(AWithOptionalProp.a, Decl(inKeywordTypeguard.ts, 19, 25))
+
+    } else {
+        x.b = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 22, 51))
+    }
+}
+
+class AWithMethod {
+>AWithMethod : Symbol(AWithMethod, Decl(inKeywordTypeguard.ts, 28, 1))
+
+    a(): string { return ""; }
+>a : Symbol(AWithMethod.a, Decl(inKeywordTypeguard.ts, 30, 19))
+}
+
+class BWithMethod {
+>BWithMethod : Symbol(BWithMethod, Decl(inKeywordTypeguard.ts, 32, 1))
+
+    b(): string { return ""; }
+>b : Symbol(BWithMethod.b, Decl(inKeywordTypeguard.ts, 34, 19))
+}
+
+function negativeTestClassesWithMembers(x: AWithMethod | BWithMethod) {
+>negativeTestClassesWithMembers : Symbol(negativeTestClassesWithMembers, Decl(inKeywordTypeguard.ts, 36, 1))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 38, 40))
+>AWithMethod : Symbol(AWithMethod, Decl(inKeywordTypeguard.ts, 28, 1))
+>BWithMethod : Symbol(BWithMethod, Decl(inKeywordTypeguard.ts, 32, 1))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 38, 40))
+
+        x.a();
+>x.a : Symbol(AWithMethod.a, Decl(inKeywordTypeguard.ts, 30, 19))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 38, 40))
+>a : Symbol(AWithMethod.a, Decl(inKeywordTypeguard.ts, 30, 19))
+
+        x.b();
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 38, 40))
+
+    } else {
+    }
+}
+
+function negativeTestClassesWithMemberMissingInBothClasses(x: AWithMethod | BWithMethod) {
+>negativeTestClassesWithMemberMissingInBothClasses : Symbol(negativeTestClassesWithMemberMissingInBothClasses, Decl(inKeywordTypeguard.ts, 44, 1))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 46, 59))
+>AWithMethod : Symbol(AWithMethod, Decl(inKeywordTypeguard.ts, 28, 1))
+>BWithMethod : Symbol(BWithMethod, Decl(inKeywordTypeguard.ts, 32, 1))
+
+    if ("c" in x) {
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 46, 59))
+
+        x.a();
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 46, 59))
+
+        x.b();
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 46, 59))
+
+    } else {
+        x.a();
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 46, 59))
+
+        x.b();
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 46, 59))
+    }
+}
+
+class C { a: string; }
+>C : Symbol(C, Decl(inKeywordTypeguard.ts, 54, 1))
+>a : Symbol(C.a, Decl(inKeywordTypeguard.ts, 56, 9))
+
+class D { a: string; }
+>D : Symbol(D, Decl(inKeywordTypeguard.ts, 56, 22))
+>a : Symbol(D.a, Decl(inKeywordTypeguard.ts, 57, 9))
+
+function negativeMultipleClassesTest(x: A | B | C | D) {
+>negativeMultipleClassesTest : Symbol(negativeMultipleClassesTest, Decl(inKeywordTypeguard.ts, 57, 22))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 59, 37))
+>A : Symbol(A, Decl(inKeywordTypeguard.ts, 0, 0))
+>B : Symbol(B, Decl(inKeywordTypeguard.ts, 0, 22))
+>C : Symbol(C, Decl(inKeywordTypeguard.ts, 54, 1))
+>D : Symbol(D, Decl(inKeywordTypeguard.ts, 56, 22))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 59, 37))
+
+        x.b = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 59, 37))
+
+    } else {
+        x.a = "1";
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 59, 37))
+    }
+}
+
+class ClassWithUnionProp { prop: A | B }
+>ClassWithUnionProp : Symbol(ClassWithUnionProp, Decl(inKeywordTypeguard.ts, 65, 1))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+>A : Symbol(A, Decl(inKeywordTypeguard.ts, 0, 0))
+>B : Symbol(B, Decl(inKeywordTypeguard.ts, 0, 22))
+
+function negativePropTest(x: ClassWithUnionProp) {
+>negativePropTest : Symbol(negativePropTest, Decl(inKeywordTypeguard.ts, 67, 40))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 69, 26))
+>ClassWithUnionProp : Symbol(ClassWithUnionProp, Decl(inKeywordTypeguard.ts, 65, 1))
+
+    if ("a" in x.prop) {
+>x.prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 69, 26))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+
+        let y: string = x.prop.b;
+>y : Symbol(y, Decl(inKeywordTypeguard.ts, 71, 11))
+>x.prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 69, 26))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+
+    } else {
+        let z: string = x.prop.a;
+>z : Symbol(z, Decl(inKeywordTypeguard.ts, 73, 11))
+>x.prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 69, 26))
+>prop : Symbol(ClassWithUnionProp.prop, Decl(inKeywordTypeguard.ts, 67, 26))
+    }
+}
+
+class NegativeClassTest {
+>NegativeClassTest : Symbol(NegativeClassTest, Decl(inKeywordTypeguard.ts, 75, 1))
+
+    protected prop: A | B;
+>prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+>A : Symbol(A, Decl(inKeywordTypeguard.ts, 0, 0))
+>B : Symbol(B, Decl(inKeywordTypeguard.ts, 0, 22))
+
+    inThis() {
+>inThis : Symbol(NegativeClassTest.inThis, Decl(inKeywordTypeguard.ts, 78, 26))
+
+        if ("a" in this.prop) {
+>this.prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+>this : Symbol(NegativeClassTest, Decl(inKeywordTypeguard.ts, 75, 1))
+>prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+
+            let z: number = this.prop.b;
+>z : Symbol(z, Decl(inKeywordTypeguard.ts, 81, 15))
+>this.prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+>this : Symbol(NegativeClassTest, Decl(inKeywordTypeguard.ts, 75, 1))
+>prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+
+        } else {
+            let y: string = this.prop.a;
+>y : Symbol(y, Decl(inKeywordTypeguard.ts, 83, 15))
+>this.prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+>this : Symbol(NegativeClassTest, Decl(inKeywordTypeguard.ts, 75, 1))
+>prop : Symbol(NegativeClassTest.prop, Decl(inKeywordTypeguard.ts, 77, 25))
+        }
+    }
+}
+
+class UnreachableCodeDetection {
+>UnreachableCodeDetection : Symbol(UnreachableCodeDetection, Decl(inKeywordTypeguard.ts, 86, 1))
+
+    a: string;
+>a : Symbol(UnreachableCodeDetection.a, Decl(inKeywordTypeguard.ts, 88, 32))
+
+    inThis() {
+>inThis : Symbol(UnreachableCodeDetection.inThis, Decl(inKeywordTypeguard.ts, 89, 14))
+
+        if ("a" in this) {
+>this : Symbol(UnreachableCodeDetection, Decl(inKeywordTypeguard.ts, 86, 1))
+
+        } else {
+            let y = this.a;
+>y : Symbol(y, Decl(inKeywordTypeguard.ts, 93, 15))
+        }
+    }
+}
diff --git a/tests/baselines/reference/inKeywordTypeguard.types b/tests/baselines/reference/inKeywordTypeguard.types
new file mode 100644
index 00000000000..3f9e8fd0638
--- /dev/null
+++ b/tests/baselines/reference/inKeywordTypeguard.types
@@ -0,0 +1,318 @@
+=== tests/cases/compiler/inKeywordTypeguard.ts ===
+class A { a: string; }
+>A : A
+>a : string
+
+class B { b: string; }
+>B : B
+>b : string
+
+function negativeClassesTest(x: A | B) {
+>negativeClassesTest : (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.b = "1";
+>x.b = "1" : "1"
+>x.b : any
+>x : A
+>b : any
+>"1" : "1"
+
+    } else {
+        x.a = "1";
+>x.a = "1" : "1"
+>x.a : any
+>x : B
+>a : any
+>"1" : "1"
+    }
+}
+
+function positiveClassesTest(x: A | B) {
+>positiveClassesTest : (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.b = "1";
+>x.b = "1" : "1"
+>x.b : any
+>x : A
+>b : any
+>"1" : "1"
+
+    } else {
+        x.a = "1";
+>x.a = "1" : "1"
+>x.a : any
+>x : B
+>a : any
+>"1" : "1"
+    }
+}
+
+class AWithOptionalProp { a?: string; }
+>AWithOptionalProp : AWithOptionalProp
+>a : string
+
+class BWithOptionalProp { b?: string; }
+>BWithOptionalProp : BWithOptionalProp
+>b : string
+
+function positiveTestClassesWithOptionalProperties(x: AWithOptionalProp | BWithOptionalProp) {
+>positiveTestClassesWithOptionalProperties : (x: AWithOptionalProp | BWithOptionalProp) => void
+>x : AWithOptionalProp | BWithOptionalProp
+>AWithOptionalProp : AWithOptionalProp
+>BWithOptionalProp : BWithOptionalProp
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : AWithOptionalProp | BWithOptionalProp
+
+        x.a = "1";
+>x.a = "1" : "1"
+>x.a : string
+>x : AWithOptionalProp
+>a : string
+>"1" : "1"
+
+    } else {
+        x.b = "1";
+>x.b = "1" : "1"
+>x.b : any
+>x : AWithOptionalProp | BWithOptionalProp
+>b : any
+>"1" : "1"
+    }
+}
+
+class AWithMethod {
+>AWithMethod : AWithMethod
+
+    a(): string { return ""; }
+>a : () => string
+>"" : ""
+}
+
+class BWithMethod {
+>BWithMethod : BWithMethod
+
+    b(): string { return ""; }
+>b : () => string
+>"" : ""
+}
+
+function negativeTestClassesWithMembers(x: AWithMethod | BWithMethod) {
+>negativeTestClassesWithMembers : (x: AWithMethod | BWithMethod) => void
+>x : AWithMethod | BWithMethod
+>AWithMethod : AWithMethod
+>BWithMethod : BWithMethod
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : AWithMethod | BWithMethod
+
+        x.a();
+>x.a() : string
+>x.a : () => string
+>x : AWithMethod
+>a : () => string
+
+        x.b();
+>x.b() : any
+>x.b : any
+>x : AWithMethod
+>b : any
+
+    } else {
+    }
+}
+
+function negativeTestClassesWithMemberMissingInBothClasses(x: AWithMethod | BWithMethod) {
+>negativeTestClassesWithMemberMissingInBothClasses : (x: AWithMethod | BWithMethod) => void
+>x : AWithMethod | BWithMethod
+>AWithMethod : AWithMethod
+>BWithMethod : BWithMethod
+
+    if ("c" in x) {
+>"c" in x : boolean
+>"c" : "c"
+>x : AWithMethod | BWithMethod
+
+        x.a();
+>x.a() : any
+>x.a : any
+>x : never
+>a : any
+
+        x.b();
+>x.b() : any
+>x.b : any
+>x : never
+>b : any
+
+    } else {
+        x.a();
+>x.a() : any
+>x.a : any
+>x : AWithMethod | BWithMethod
+>a : any
+
+        x.b();
+>x.b() : any
+>x.b : any
+>x : AWithMethod | BWithMethod
+>b : any
+    }
+}
+
+class C { a: string; }
+>C : C
+>a : string
+
+class D { a: string; }
+>D : D
+>a : string
+
+function negativeMultipleClassesTest(x: A | B | C | D) {
+>negativeMultipleClassesTest : (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
+
+        x.b = "1";
+>x.b = "1" : "1"
+>x.b : any
+>x : A | C | D
+>b : any
+>"1" : "1"
+
+    } else {
+        x.a = "1";
+>x.a = "1" : "1"
+>x.a : any
+>x : B
+>a : any
+>"1" : "1"
+    }
+}
+
+class ClassWithUnionProp { prop: A | B }
+>ClassWithUnionProp : ClassWithUnionProp
+>prop : A | B
+>A : A
+>B : B
+
+function negativePropTest(x: ClassWithUnionProp) {
+>negativePropTest : (x: ClassWithUnionProp) => void
+>x : ClassWithUnionProp
+>ClassWithUnionProp : ClassWithUnionProp
+
+    if ("a" in x.prop) {
+>"a" in x.prop : boolean
+>"a" : "a"
+>x.prop : A | B
+>x : ClassWithUnionProp
+>prop : A | B
+
+        let y: string = x.prop.b;
+>y : string
+>x.prop.b : any
+>x.prop : A
+>x : ClassWithUnionProp
+>prop : A
+>b : any
+
+    } else {
+        let z: string = x.prop.a;
+>z : string
+>x.prop.a : any
+>x.prop : B
+>x : ClassWithUnionProp
+>prop : B
+>a : any
+    }
+}
+
+class NegativeClassTest {
+>NegativeClassTest : NegativeClassTest
+
+    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 z: number = this.prop.b;
+>z : number
+>this.prop.b : any
+>this.prop : A
+>this : this
+>prop : A
+>b : any
+
+        } else {
+            let y: string = this.prop.a;
+>y : string
+>this.prop.a : any
+>this.prop : B
+>this : this
+>prop : B
+>a : any
+        }
+    }
+}
+
+class UnreachableCodeDetection {
+>UnreachableCodeDetection : UnreachableCodeDetection
+
+    a: string;
+>a : string
+
+    inThis() {
+>inThis : () => void
+
+        if ("a" in this) {
+>"a" in this : boolean
+>"a" : "a"
+>this : this
+
+        } else {
+            let y = this.a;
+>y : any
+>this.a : any
+>this : never
+>a : any
+        }
+    }
+}
diff --git a/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.js b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.js
index 76b10066fdd..a2579328646 100644
--- a/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.js
+++ b/tests/baselines/reference/typeGuardOfFromPropNameInUnionType.js
@@ -92,22 +92,22 @@ class SelfAssert {
 }
 
 //// [typeGuardOfFromPropNameInUnionType.js]
-var A = (function () {
+var A = /** @class */ (function () {
     function A() {
     }
     return A;
 }());
-var B = (function () {
+var B = /** @class */ (function () {
     function B() {
     }
     return B;
 }());
-var C = (function () {
+var C = /** @class */ (function () {
     function C() {
     }
     return C;
 }());
-var D = (function () {
+var D = /** @class */ (function () {
     function D() {
     }
     return D;
@@ -136,12 +136,12 @@ function anonymousClasses(x) {
         var z = x.b;
     }
 }
-var AWithOptionalProp = (function () {
+var AWithOptionalProp = /** @class */ (function () {
     function AWithOptionalProp() {
     }
     return AWithOptionalProp;
 }());
-var BWithOptionalProp = (function () {
+var BWithOptionalProp = /** @class */ (function () {
     function BWithOptionalProp() {
     }
     return BWithOptionalProp;
@@ -164,7 +164,7 @@ function inParenthesizedExpression(x) {
         var z = x.b;
     }
 }
-var ClassWithUnionProp = (function () {
+var ClassWithUnionProp = /** @class */ (function () {
     function ClassWithUnionProp() {
     }
     return ClassWithUnionProp;
@@ -177,7 +177,7 @@ function inProperty(x) {
         var z = x.prop.b;
     }
 }
-var NestedClassWithProp = (function () {
+var NestedClassWithProp = /** @class */ (function () {
     function NestedClassWithProp() {
     }
     return NestedClassWithProp;
@@ -190,7 +190,7 @@ function innestedProperty(x) {
         var z = x.outer.prop.b;
     }
 }
-var InMemberOfClass = (function () {
+var InMemberOfClass = /** @class */ (function () {
     function InMemberOfClass() {
     }
     InMemberOfClass.prototype.inThis = function () {
@@ -204,7 +204,7 @@ var InMemberOfClass = (function () {
     return InMemberOfClass;
 }());
 // added for completeness
-var SelfAssert = (function () {
+var SelfAssert = /** @class */ (function () {
     function SelfAssert() {
     }
     SelfAssert.prototype.inThis = function () {