Widen after sub-type-reduction took place

src/compiler/checker.ts

@@ -3378,7 +3378,7 @@ namespace ts {
         }
 
         // Return the inferred type for a variable, parameter, or property declaration
-        function getWidenedTypeForJSSpecialPropertyDeclaration(declaration: Declaration): Type {
+        function getTypeForJSSpecialPropertyDeclaration(declaration: Declaration): Type {
             const expression = declaration.kind === SyntaxKind.BinaryExpression ? <BinaryExpression>declaration :
                 declaration.kind === SyntaxKind.PropertyAccessExpression ? <BinaryExpression>getAncestor(declaration, SyntaxKind.BinaryExpression) :
                 undefined;
@@ -3395,7 +3395,7 @@ namespace ts {
                 }
             }
 
-            return getWidenedType(getWidenedLiteralType(checkExpressionCached(expression.right)));
+            return getWidenedLiteralType(checkExpressionCached(expression.right));
         }
 
         // Return the type implied by a binding pattern element. This is the type of the initializer of the element if
@@ -3552,7 +3552,7 @@ namespace ts {
                 // * className.prototype.method = expr
                 if (declaration.kind === SyntaxKind.BinaryExpression ||
                     declaration.kind === SyntaxKind.PropertyAccessExpression && declaration.parent.kind === SyntaxKind.BinaryExpression) {
-                    type = getUnionType(map(symbol.declarations, getWidenedTypeForJSSpecialPropertyDeclaration), /*subtypeReduction*/ true);
+                    type = getWidenedType(getUnionType(map(symbol.declarations, getTypeForJSSpecialPropertyDeclaration), /*subtypeReduction*/ true));
                 }
                 else {
                     type = getWidenedTypeForVariableLikeDeclaration(<VariableLikeDeclaration>declaration, /*reportErrors*/ true);

tests/baselines/reference/jsFileClassPropertyType3.errors.txt

@@ -0,0 +1,22 @@
+tests/cases/compiler/bar.ts(2,1): error TS2322: Type '"string"' is not assignable to type 'number'.
+
+
+==== tests/cases/compiler/foo.js (0 errors) ====
+    
+    class C {
+        constructor() {
+            if (cond) {
+                this.p = null;
+            }
+            else {
+                this.p = 0;
+            }
+        }
+    }
+    
+==== tests/cases/compiler/bar.ts (1 errors) ====
+    
+    (new C()).p = "string"; // Error
+    ~~~~~~~~~~~
+!!! error TS2322: Type '"string"' is not assignable to type 'number'.
+    

tests/baselines/reference/multipleDeclarations.types

@@ -21,7 +21,7 @@ C.prototype.m = function() {
     this.nothing();
 >this.nothing() : any
 >this.nothing : any
->this : { m: any; }
+>this : { m: () => void; }
 >nothing : any
 }
 class X {

tests/cases/compiler/jsFileClassPropertyType3.ts

@@ -0,0 +1,18 @@
+// @allowJs: true
+// @noEmit: true
+
+// @filename: foo.js
+class C {
+    constructor() {
+        if (cond) {
+            this.p = null;
+        }
+        else {
+            this.p = 0;
+        }
+    }
+}
+
+// @filename: bar.ts
+
+(new C()).p = "string"; // Error