Port missing nil check in getContextualTypeForReturnExpression (#3755)

Author: DanielRosenwasser

internal/checker/checker.go

@@ -28938,6 +28938,9 @@ func (c *Checker) getContextualTypeForReturnExpression(node *ast.Node, contextFl
 			if functionFlags&ast.FunctionFlagsAsync != 0 {
 				// Get the awaited type without the `Awaited<T>` alias
 				contextualAwaitedType := c.mapType(contextualReturnType, c.getAwaitedTypeNoAlias)
+				if contextualAwaitedType == nil {
+					return nil
+				}
 				return c.getUnionType([]*Type{contextualAwaitedType, c.createPromiseLikeType(contextualAwaitedType)})
 			}
 			// Regular function or Generator function

testdata/baselines/reference/compiler/asyncFunctionReturnNonPromiseThenable.errors.txt

@@ -0,0 +1,20 @@
+asyncFunctionReturnNonPromiseThenable.ts(5,30): error TS1064: The return type of an async function or method must be the global Promise<T> type. Did you mean to write 'Promise<void>'?
+asyncFunctionReturnNonPromiseThenable.ts(6,5): error TS1058: The return type of an async function must either be a valid promise or must not contain a callable 'then' member.
+
+
+==== asyncFunctionReturnNonPromiseThenable.ts (2 errors) ====
+    export interface MyThenable {
+        then(): void;
+    }
+    
+    export async function foo(): MyThenable {
+                                 ~~~~~~~~~~
+!!! error TS1064: The return type of an async function or method must be the global Promise<T> type. Did you mean to write 'Promise<void>'?
+        return {
+        ~~~~~~
+!!! error TS1058: The return type of an async function must either be a valid promise or must not contain a callable 'then' member.
+            then() {
+            }
+        };
+    }
+    

testdata/baselines/reference/compiler/asyncFunctionReturnNonPromiseThenable.js

@@ -0,0 +1,22 @@
+//// [tests/cases/compiler/asyncFunctionReturnNonPromiseThenable.ts] ////
+
+//// [asyncFunctionReturnNonPromiseThenable.ts]
+export interface MyThenable {
+    then(): void;
+}
+
+export async function foo(): MyThenable {
+    return {
+        then() {
+        }
+    };
+}
+
+
+//// [asyncFunctionReturnNonPromiseThenable.js]
+export async function foo() {
+    return {
+        then() {
+        }
+    };
+}

testdata/baselines/reference/compiler/asyncFunctionReturnNonPromiseThenable.symbols

@@ -0,0 +1,21 @@
+//// [tests/cases/compiler/asyncFunctionReturnNonPromiseThenable.ts] ////
+
+=== asyncFunctionReturnNonPromiseThenable.ts ===
+export interface MyThenable {
+>MyThenable : Symbol(MyThenable, Decl(asyncFunctionReturnNonPromiseThenable.ts, 0, 0))
+
+    then(): void;
+>then : Symbol(MyThenable.then, Decl(asyncFunctionReturnNonPromiseThenable.ts, 0, 29))
+}
+
+export async function foo(): MyThenable {
+>foo : Symbol(foo, Decl(asyncFunctionReturnNonPromiseThenable.ts, 2, 1))
+>MyThenable : Symbol(MyThenable, Decl(asyncFunctionReturnNonPromiseThenable.ts, 0, 0))
+
+    return {
+        then() {
+>then : Symbol(then, Decl(asyncFunctionReturnNonPromiseThenable.ts, 5, 12))
+        }
+    };
+}
+

testdata/baselines/reference/compiler/asyncFunctionReturnNonPromiseThenable.types

@@ -0,0 +1,20 @@
+//// [tests/cases/compiler/asyncFunctionReturnNonPromiseThenable.ts] ////
+
+=== asyncFunctionReturnNonPromiseThenable.ts ===
+export interface MyThenable {
+    then(): void;
+>then : () => void
+}
+
+export async function foo(): MyThenable {
+>foo : () => MyThenable
+
+    return {
+>{        then() {        }    } : { then(): void; }
+
+        then() {
+>then : () => void
+        }
+    };
+}
+

testdata/tests/cases/compiler/asyncFunctionReturnNonPromiseThenable.ts

@@ -0,0 +1,13 @@
+// @strict: true
+// @target: esnext
+
+export interface MyThenable {
+    then(): void;
+}
+
+export async function foo(): MyThenable {
+    return {
+        then() {
+        }
+    };
+}