Merge pull request #7348 from Vinatorul/issue6540

Added new diagnostics message to clarify error for type guards

Author: DanielRosenwasser

src/compiler/checker.ts

@@ -11815,9 +11815,12 @@ namespace ts {
                             Diagnostics.A_type_predicate_cannot_reference_a_rest_parameter);
                     }
                     else {
+                        const leadingError = chainDiagnosticMessages(undefined, Diagnostics.A_type_predicate_s_type_must_be_assignable_to_its_parameter_s_type);
                         checkTypeAssignableTo(typePredicate.type,
                             getTypeOfNode(parent.parameters[typePredicate.parameterIndex]),
-                            node.type);
+                            node.type,
+                            /*headMessage*/ undefined,
+                            leadingError);
                     }
                 }
                 else if (parameterName) {

src/compiler/diagnosticMessages.json

@@ -1847,6 +1847,10 @@
         "category": "Error",
         "code": 2676
     },
+    "A type predicate's type must be assignable to its parameter's type.": {
+        "category": "Error",
+        "code": 2677
+    },
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
         "code": 4000

tests/baselines/reference/typeGuardFunctionErrors.errors.txt

@@ -12,10 +12,13 @@ tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(31,5):
 tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(31,5): error TS7027: Unreachable code detected.
 tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(32,1): error TS1128: Declaration or statement expected.
 tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(34,38): error TS1225: Cannot find parameter 'x'.
-tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(38,51): error TS2322: Type 'B' is not assignable to type 'A'.
-  Property 'propA' is missing in type 'B'.
-tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(42,56): error TS2322: Type 'number' is not assignable to type 'string'.
-tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(46,56): error TS2322: Type 'T[]' is not assignable to type 'string'.
+tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(38,51): error TS2677: A type predicate's type must be assignable to its parameter's type.
+  Type 'B' is not assignable to type 'A'.
+    Property 'propA' is missing in type 'B'.
+tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(42,56): error TS2677: A type predicate's type must be assignable to its parameter's type.
+  Type 'number' is not assignable to type 'string'.
+tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(46,56): error TS2677: A type predicate's type must be assignable to its parameter's type.
+  Type 'T[]' is not assignable to type 'string'.
 tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(60,7): error TS2339: Property 'propB' does not exist on type 'A'.
 tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(65,7): error TS2339: Property 'propB' does not exist on type 'A'.
 tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(70,7): error TS2339: Property 'propB' does not exist on type 'A'.
@@ -129,20 +132,23 @@ tests/cases/conformance/expressions/typeGuards/typeGuardFunctionErrors.ts(137,39
 
     function hasNonMatchingParameterType1(x: A): x is B {
                                                       ~
-!!! error TS2322: Type 'B' is not assignable to type 'A'.
-!!! error TS2322:   Property 'propA' is missing in type 'B'.
+!!! error TS2677: A type predicate's type must be assignable to its parameter's type.
+!!! error TS2677:   Type 'B' is not assignable to type 'A'.
+!!! error TS2677:     Property 'propA' is missing in type 'B'.
         return true;
     }
 
     function hasNonMatchingParameterType2(x: string): x is number {
                                                            ~~~~~~
-!!! error TS2322: Type 'number' is not assignable to type 'string'.
+!!! error TS2677: A type predicate's type must be assignable to its parameter's type.
+!!! error TS2677:   Type 'number' is not assignable to type 'string'.
         return true;
     }
 
     function hasNonMathcingGenericType<T>(a: string): a is T[] {
                                                            ~~~
-!!! error TS2322: Type 'T[]' is not assignable to type 'string'.
+!!! error TS2677: A type predicate's type must be assignable to its parameter's type.
+!!! error TS2677:   Type 'T[]' is not assignable to type 'string'.
         return true;
     }