preserve some old behavior at @DanielRosenwassers request

weswigham · weswigham · commit 0228ec3ced6e · 2015-12-08T17:45:11.000-08:00
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -5633,6 +5633,16 @@ namespace ts {
                 if (targetReturnType === voidType) return result;
                 const sourceReturnType = getReturnTypeOfSignature(source);
 
+                // The follow block preserves old behavior forbidding boolean returning functions from being assignable to type guard returning functions
+                if (targetReturnType.flags & TypeFlags.PredicateType && (targetReturnType as PredicateType).predicate.kind === TypePredicateKind.Identifier) {
+                    if (!(sourceReturnType.flags & TypeFlags.PredicateType)) {
+                        if (reportErrors) {
+                            reportError(Diagnostics.Signature_0_must_have_a_type_predicate, signatureToString(source));
+                        }
+                        return Ternary.False;
+                    }
+                }
+
                 return result & isRelatedTo(sourceReturnType, targetReturnType, reportErrors);
             }
 
diff --git a/tests/baselines/reference/stringLiteralTypesAsTags01.errors.txt b/tests/baselines/reference/stringLiteralTypesAsTags01.errors.txt
@@ -1,7 +1,8 @@
+tests/cases/conformance/types/stringLiteral/stringLiteralTypesAsTags01.ts(20,10): error TS2394: Overload signature is not compatible with function implementation.
 tests/cases/conformance/types/stringLiteral/stringLiteralTypesAsTags01.ts(22,21): error TS2304: Cannot find name 'is'.
 
 
-==== tests/cases/conformance/types/stringLiteral/stringLiteralTypesAsTags01.ts (1 errors) ====
+==== tests/cases/conformance/types/stringLiteral/stringLiteralTypesAsTags01.ts (2 errors) ====
     
     type Kind = "A" | "B"
     
@@ -22,6 +23,8 @@ tests/cases/conformance/types/stringLiteral/stringLiteralTypesAsTags01.ts(22,21)
     function hasKind(entity: Entity, kind: "A"): entity is A;
     function hasKind(entity: Entity, kind: "B"): entity is B;
     function hasKind(entity: Entity, kind: Kind): entity is Entity;
+             ~~~~~~~
+!!! error TS2394: Overload signature is not compatible with function implementation.
     function hasKind(entity: Entity, kind: Kind): boolean {
         return kind === is;
                         ~~
diff --git a/tests/baselines/reference/typeGuardFunctionErrors.errors.txt b/tests/baselines/reference/typeGuardFunctionErrors.errors.txt
@@ -17,6 +17,8 @@ tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(75,46)
   Type predicate 'p1 is C' is not assignable to 'p1 is B'.
     Type 'C' is not assignable to type 'B'.
       Property 'propB' is missing in type 'C'.
+tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(79,1): error TS2322: Type '(p1: any, p2: any) => boolean' is not assignable to type '(p1: any, p2: any) => p1 is A'.
+  Signature '(p1: any, p2: any): boolean' must have a type predicate.
 tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(85,1): error TS2322: Type '(p1: any, p2: any) => p2 is A' is not assignable to type '(p1: any, p2: any) => p1 is A'.
   Type predicate 'p2 is A' is not assignable to 'p1 is A'.
     Parameter 'p2' is not in the same position as parameter 'p1'.
@@ -39,7 +41,7 @@ tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(133,34
 tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(137,39): error TS1230: A type predicate cannot reference element 'p1' in a binding pattern.
 
 
-==== tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts (32 errors) ====
+==== tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts (33 errors) ====
     
     class A {
         propA: number;
@@ -153,6 +155,9 @@ tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(137,39
     // Boolean not assignable to type guard
     var assign1: (p1, p2) => p1 is A;
     assign1 = function(p1, p2): boolean {
+    ~~~~~~~
+!!! error TS2322: Type '(p1: any, p2: any) => boolean' is not assignable to type '(p1: any, p2: any) => p1 is A'.
+!!! error TS2322:   Signature '(p1: any, p2: any): boolean' must have a type predicate.
         return true;
     };
     
