From b09afb913ed6ad4ef66fb8816990fe54653e8ac4 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Mon, 27 Jan 2020 11:12:55 -0800
Subject: [PATCH] Only parameterless arrow functions have early stage inference
 from return type

---
 src/compiler/checker.ts | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 572993e4560..ecc76e3250c 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -7136,7 +7136,7 @@ namespace ts {
                     }
                 }
                 // Use contextual parameter type if one is available
-                const type = declaration.symbol.escapedName === InternalSymbolName.This ? getContextualThisParameterType(func) : getContextuallyTypedParameterType(declaration, /*forCache*/ true);
+                const type = declaration.symbol.escapedName === InternalSymbolName.This ? getContextualThisParameterType(func) : getContextuallyTypedParameterType(declaration, /*forCache*/ false);
                 if (type) {
                     return addOptionality(type, isOptional);
                 }
@@ -26430,14 +26430,14 @@ namespace ts {
             }
         }
 
-        function checkFunctionExpressionOrObjectLiteralMethod(node: FunctionExpression | MethodDeclaration, checkMode?: CheckMode): Type {
+        function checkFunctionExpressionOrObjectLiteralMethod(node: FunctionExpression | ArrowFunction | MethodDeclaration, checkMode?: CheckMode): Type {
             Debug.assert(node.kind !== SyntaxKind.MethodDeclaration || isObjectLiteralMethod(node));
             checkNodeDeferred(node);
 
             // The identityMapper object is used to indicate that function expressions are wildcards
             if (checkMode && checkMode & CheckMode.SkipContextSensitive && isContextSensitive(node)) {
                 // Skip parameters, return signature with return type that retains noncontextual parts so inferences can still be drawn in an early stage
-                if (!getEffectiveReturnTypeNode(node) && hasContextSensitiveReturnExpression(node)) {
+                if (!getEffectiveReturnTypeNode(node) && node.kind === SyntaxKind.ArrowFunction && node.parameters.length === 0) {
                     // Return plain anyFunctionType if there is no possibility we'll make inferences from the return type
                     const contextualSignature = getContextualSignature(node);
                     if (contextualSignature && couldContainTypeVariables(getReturnTypeOfSignature(contextualSignature))) {