Allow case comparison to undefined and null in strict null checking mode

src/compiler/checker.ts

@@ -15106,7 +15106,7 @@ namespace ts {
                     // In a 'switch' statement, each 'case' expression must be of a type that is comparable
                     // to or from the type of the 'switch' expression.
                     const caseType = checkExpression(caseClause.expression);
-                    if (!isTypeComparableTo(expressionType, caseType)) {
+                    if (!isTypeEqualityComparableTo(expressionType, caseType)) {
                         // expressionType is not comparable to caseType, try the reversed check and report errors if it fails
                         checkTypeComparableTo(caseType, expressionType, caseClause.expression, /*headMessage*/ undefined);
                     }

tests/baselines/reference/contextuallyTypedObjectLiteralReturnType.errors.txt

@@ -0,0 +1,18 @@
+tests/cases/conformance/types/contextualTypes/methodDeclarations/contextuallyTypedObjectLiteralReturnType.ts(6,24): error TS7024: Function implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
+tests/cases/conformance/types/contextualTypes/methodDeclarations/contextuallyTypedObjectLiteralReturnType.ts(7,38): error TS2339: Property 'length' does not exist on type 'number'.
+
+
+==== tests/cases/conformance/types/contextualTypes/methodDeclarations/contextuallyTypedObjectLiteralReturnType.ts (2 errors) ====
+    function id<T>(x: T): T { return x };
+    // Correct: type of fnWrapper is (y: number) => { y: number }
+    var fn = function(y: number) { return { y } };
+    var fnWrapper = id(fn);
+    // Incorrect: type of inlineWrapper is (z: number) => any 
+    var inlineWrapper = id(function(z: number) { return { z } });
+                           ~~~~~~~~
+!!! error TS7024: Function implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
+    let error1: number = fnWrapper(12).y.length; // should error
+                                         ~~~~~~
+!!! error TS2339: Property 'length' does not exist on type 'number'.
+    let error2: number = inlineWrapper(12).z.length; // should error
+    

tests/baselines/reference/contextuallyTypedObjectLiteralReturnType.js

@@ -0,0 +1,21 @@
+//// [contextuallyTypedObjectLiteralReturnType.ts]
+function id<T>(x: T): T { return x };
+// Correct: type of fnWrapper is (y: number) => { y: number }
+var fn = function(y: number) { return { y } };
+var fnWrapper = id(fn);
+// Incorrect: type of inlineWrapper is (z: number) => any 
+var inlineWrapper = id(function(z: number) { return { z } });
+let error1: number = fnWrapper(12).y.length; // should error
+let error2: number = inlineWrapper(12).z.length; // should error
+
+
+//// [contextuallyTypedObjectLiteralReturnType.js]
+function id(x) { return x; }
+;
+// Correct: type of fnWrapper is (y: number) => { y: number }
+var fn = function (y) { return { y: y }; };
+var fnWrapper = id(fn);
+// Incorrect: type of inlineWrapper is (z: number) => any 
+var inlineWrapper = id(function (z) { return { z: z }; });
+var error1 = fnWrapper(12).y.length; // should error
+var error2 = inlineWrapper(12).z.length; // should error

tests/baselines/reference/equalityStrictNulls.errors.txt

@@ -81,4 +81,14 @@ tests/cases/conformance/types/typeRelationships/comparable/equalityStrictNulls.t
 !!! error TS2365: Operator '<=' cannot be applied to types 'number' and 'undefined'.
         }
     }
+    function f5(x: string) {
+        switch(x) {
+            case null:
+                break;
+            case undefined:
+                break;
+            default:
+                return;
+        }
+    }
     

tests/baselines/reference/equalityStrictNulls.js

@@ -67,6 +67,16 @@ function f4(x: number) {
     if (x <= undefined) {
     }
 }
+function f5(x: string) {
+    switch(x) {
+        case null:
+            break;
+        case undefined:
+            break;
+        default:
+            return;
+    }
+}
 
 
 //// [equalityStrictNulls.js]
@@ -134,3 +144,13 @@ function f4(x) {
     if (x <= undefined) {
     }
 }
+function f5(x) {
+    switch (x) {
+        case null:
+            break;
+        case undefined:
+            break;
+        default:
+            return;
+    }
+}

tests/cases/compiler/inferFromConstraints.ts

@@ -0,0 +1,9 @@
+interface Ctor<T> {
+    new(): T;
+}
+// declare function create1<T>(ctor: Ctor<T>): T;
+declare function create2<T, C extends Ctor<T>>(ctor: C): T;
+
+class A { a: number }
+// let a1 = create1(A).a; // a: A --> OK
+let a2 = create2(A).a; // a: {} --> Should be A

tests/cases/conformance/types/contextualTypes/methodDeclarations/contextuallyTypedObjectLiteralReturnType.ts

@@ -0,0 +1,9 @@
+// @noImplicitAny: true
+function id<T>(x: T): T { return x };
+// Correct: type of fnWrapper is (y: number) => { y: number }
+var fn = function(y: number) { return { y } };
+var fnWrapper = id(fn);
+// Incorrect: type of inlineWrapper is (z: number) => any 
+var inlineWrapper = id(function(z: number) { return { z } });
+let error1: number = fnWrapper(12).y.length; // should error
+let error2: number = inlineWrapper(12).z.length; // should error

tests/cases/conformance/types/typeRelationships/comparable/equalityStrictNulls.ts

@@ -67,3 +67,13 @@ function f4(x: number) {
     if (x <= undefined) {
     }
 }
+function f5(x: string) {
+    switch(x) {
+        case null:
+            break;
+        case undefined:
+            break;
+        default:
+            return;
+    }
+}