From 8f45d2367ae859ce8267f8c85e2a3e719a4e31e2 Mon Sep 17 00:00:00 2001
From: Daniel Rosenwasser <DanielRosenwasser@users.noreply.github.com>
Date: Fri, 28 Aug 2020 23:56:43 +0000
Subject: [PATCH] Only widen return types of function expressions in the
 absence of a contextual signature.

---
 src/compiler/checker.ts | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 0b1804c926a..46485f24d36 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -28295,14 +28295,16 @@ namespace ts {
                 returnType = getUnionType(types, UnionReduction.Subtype);
             }
 
+            const contextualSignature = getContextualSignatureForFunctionLikeDeclaration(func);
             if (returnType || yieldType || nextType) {
-                if (yieldType) reportErrorsFromWidening(func, yieldType, WideningKind.GeneratorYield);
-                if (returnType) reportErrorsFromWidening(func, returnType, WideningKind.FunctionReturn);
-                if (nextType) reportErrorsFromWidening(func, nextType, WideningKind.GeneratorNext);
+                if (!contextualSignature) {
+                    if (yieldType) reportErrorsFromWidening(func, yieldType, WideningKind.GeneratorYield);
+                    if (returnType) reportErrorsFromWidening(func, returnType, WideningKind.FunctionReturn);
+                    if (nextType) reportErrorsFromWidening(func, nextType, WideningKind.GeneratorNext);
+                }
                 if (returnType && isUnitType(returnType) ||
                     yieldType && isUnitType(yieldType) ||
                     nextType && isUnitType(nextType)) {
-                    const contextualSignature = getContextualSignatureForFunctionLikeDeclaration(func);
                     const contextualType = !contextualSignature ? undefined :
                         contextualSignature === getSignatureFromDeclaration(func) ? isGenerator ? undefined : returnType :
                         instantiateContextualType(getReturnTypeOfSignature(contextualSignature), func);
@@ -28316,9 +28318,11 @@ namespace ts {
                     }
                 }
 
-                if (yieldType) yieldType = getWidenedType(yieldType);
-                if (returnType) returnType = getWidenedType(returnType);
-                if (nextType) nextType = getWidenedType(nextType);
+                if (!contextualSignature) {
+                    if (yieldType) yieldType = getWidenedType(yieldType);
+                    if (returnType) returnType = getWidenedType(returnType);
+                    if (nextType) nextType = getWidenedType(nextType);
+                }
             }
 
             if (isGenerator) {