From 694a48c445831efd3298136b245fce10d47155a9 Mon Sep 17 00:00:00 2001
From: Alexander <alexander@kuvaev.me>
Date: Wed, 2 Mar 2016 22:03:21 +0300
Subject: [PATCH] Added new diagnostics message to clarify error for type
 guards

New diagnostics message "A type guard's type must be assignable to its parameter's type." number 2677
is now using in chain report to clarify vague error message for type guards.
---
 src/compiler/checker.ts                       |  5 ++++-
 src/compiler/diagnosticMessages.json          |  4 ++++
 .../typeGuardFunctionErrors.errors.txt        | 22 ++++++++++++-------
 3 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index d5b024c57e977..4ca97793073e7 100644
--- a/src/compiler/checker.ts
+++ b/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) {
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index a8ccbbe0e3200..4ffbca3bb501e 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/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
diff --git a/tests/baselines/reference/typeGuardFunctionErrors.errors.txt b/tests/baselines/reference/typeGuardFunctionErrors.errors.txt
index 8e4bf6515188b..eac2660df8114 100644
--- a/tests/baselines/reference/typeGuardFunctionErrors.errors.txt
+++ b/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;
     }