Ignore awaited self tail calls when collecting the return type of an async function

Author: Andarist

src/compiler/checker.ts

@@ -36674,9 +36674,14 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         let hasReturnWithNoExpression = functionHasImplicitReturn(func);
         let hasReturnOfTypeNever = false;
         forEachReturnStatement(func.body as Block, returnStatement => {
-            const expr = returnStatement.expression;
+            let expr = returnStatement.expression;
             if (expr) {
+                expr = skipParentheses(expr);
                 // Bare calls to this same function don't contribute to inference
+                // and `return await` is also safe to unwrap here
+                if (functionFlags & FunctionFlags.Async && expr.kind === SyntaxKind.AwaitExpression) {
+                    expr = skipParentheses((expr as AwaitExpression).expression);
+                }
                 if (
                     expr.kind === SyntaxKind.CallExpression &&
                     (expr as CallExpression).expression.kind === SyntaxKind.Identifier &&

tests/baselines/reference/simpleRecursionWithBaseCase.symbols

@@ -1,11 +1,11 @@
-simpleRecursionWithBaseCase.ts(8,21): error TS2554: Expected 1 arguments, but got 0.
-simpleRecursionWithBaseCase.ts(13,20): error TS2554: Expected 1 arguments, but got 0.
-simpleRecursionWithBaseCase.ts(19,20): error TS2345: Argument of type 'string' is not assignable to parameter of type 'number'.
-simpleRecursionWithBaseCase.ts(27,16): error TS2304: Cannot find name 'notfoundsymbol'.
-simpleRecursionWithBaseCase.ts(31,10): error TS7023: 'fn5' implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
+simpleRecursionWithBaseCase1.ts(8,21): error TS2554: Expected 1 arguments, but got 0.
+simpleRecursionWithBaseCase1.ts(13,20): error TS2554: Expected 1 arguments, but got 0.
+simpleRecursionWithBaseCase1.ts(19,20): error TS2345: Argument of type 'string' is not assignable to parameter of type 'number'.
+simpleRecursionWithBaseCase1.ts(27,16): error TS2304: Cannot find name 'notfoundsymbol'.
+simpleRecursionWithBaseCase1.ts(31,10): error TS7023: 'fn5' implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
 
 
-==== simpleRecursionWithBaseCase.ts (5 errors) ====
+==== simpleRecursionWithBaseCase1.ts (5 errors) ====
     function fn1(n: number) {
         if (n === 0) {
             return 3;
@@ -16,15 +16,15 @@ simpleRecursionWithBaseCase.ts(31,10): error TS7023: 'fn5' implicitly has return
     const num: number = fn1();
                         ~~~~~
 !!! error TS2554: Expected 1 arguments, but got 0.
-!!! related TS6210 simpleRecursionWithBaseCase.ts:1:14: An argument for 'n' was not provided.
+!!! related TS6210 simpleRecursionWithBaseCase1.ts:1:14: An argument for 'n' was not provided.
 
     function fn2(n: number) {
         return fn2(n);
     }
     const nev: never = fn2();
                        ~~~~~
 !!! error TS2554: Expected 1 arguments, but got 0.
-!!! related TS6210 simpleRecursionWithBaseCase.ts:10:14: An argument for 'n' was not provided.
+!!! related TS6210 simpleRecursionWithBaseCase1.ts:10:14: An argument for 'n' was not provided.
 
     function fn3(n: number) {
         if (n === 0) {

tests/baselines/reference/simpleRecursionWithBaseCase.errors.txt renamed to tests/baselines/reference/simpleRecursionWithBaseCase1.errors.txt

@@ -1,6 +1,6 @@
-//// [tests/cases/compiler/simpleRecursionWithBaseCase.ts] ////
+//// [tests/cases/compiler/simpleRecursionWithBaseCase1.ts] ////
 
-//// [simpleRecursionWithBaseCase.ts]
+//// [simpleRecursionWithBaseCase1.ts]
 function fn1(n: number) {
     if (n === 0) {
         return 3;
@@ -36,7 +36,7 @@ function fn5() {
 }
 
 
-//// [simpleRecursionWithBaseCase.js]
+//// [simpleRecursionWithBaseCase1.js]
 "use strict";
 function fn1(n) {
     if (n === 0) {

tests/baselines/reference/simpleRecursionWithBaseCase.js renamed to tests/baselines/reference/simpleRecursionWithBaseCase1.js

@@ -0,0 +1,67 @@
+//// [tests/cases/compiler/simpleRecursionWithBaseCase1.ts] ////
+
+=== simpleRecursionWithBaseCase1.ts ===
+function fn1(n: number) {
+>fn1 : Symbol(fn1, Decl(simpleRecursionWithBaseCase1.ts, 0, 0))
+>n : Symbol(n, Decl(simpleRecursionWithBaseCase1.ts, 0, 13))
+
+    if (n === 0) {
+>n : Symbol(n, Decl(simpleRecursionWithBaseCase1.ts, 0, 13))
+
+        return 3;
+    } else {
+        return fn1(n - 1);
+>fn1 : Symbol(fn1, Decl(simpleRecursionWithBaseCase1.ts, 0, 0))
+>n : Symbol(n, Decl(simpleRecursionWithBaseCase1.ts, 0, 13))
+    }
+}
+const num: number = fn1();
+>num : Symbol(num, Decl(simpleRecursionWithBaseCase1.ts, 7, 5))
+>fn1 : Symbol(fn1, Decl(simpleRecursionWithBaseCase1.ts, 0, 0))
+
+function fn2(n: number) {
+>fn2 : Symbol(fn2, Decl(simpleRecursionWithBaseCase1.ts, 7, 26))
+>n : Symbol(n, Decl(simpleRecursionWithBaseCase1.ts, 9, 13))
+
+    return fn2(n);
+>fn2 : Symbol(fn2, Decl(simpleRecursionWithBaseCase1.ts, 7, 26))
+>n : Symbol(n, Decl(simpleRecursionWithBaseCase1.ts, 9, 13))
+}
+const nev: never = fn2();
+>nev : Symbol(nev, Decl(simpleRecursionWithBaseCase1.ts, 12, 5))
+>fn2 : Symbol(fn2, Decl(simpleRecursionWithBaseCase1.ts, 7, 26))
+
+function fn3(n: number) {
+>fn3 : Symbol(fn3, Decl(simpleRecursionWithBaseCase1.ts, 12, 25))
+>n : Symbol(n, Decl(simpleRecursionWithBaseCase1.ts, 14, 13))
+
+    if (n === 0) {
+>n : Symbol(n, Decl(simpleRecursionWithBaseCase1.ts, 14, 13))
+
+        return 3;
+    } else {
+        return fn1("hello world");
+>fn1 : Symbol(fn1, Decl(simpleRecursionWithBaseCase1.ts, 0, 0))
+    }
+}
+
+function fn4(n: number) {
+>fn4 : Symbol(fn4, Decl(simpleRecursionWithBaseCase1.ts, 20, 1))
+>n : Symbol(n, Decl(simpleRecursionWithBaseCase1.ts, 22, 13))
+
+    if (n === 0) {
+>n : Symbol(n, Decl(simpleRecursionWithBaseCase1.ts, 22, 13))
+
+        return 3;
+    } else {
+        return notfoundsymbol("hello world");
+    }
+}
+
+function fn5() {
+>fn5 : Symbol(fn5, Decl(simpleRecursionWithBaseCase1.ts, 28, 1))
+
+    return [fn5][0]();
+>fn5 : Symbol(fn5, Decl(simpleRecursionWithBaseCase1.ts, 28, 1))
+}
+

tests/baselines/reference/simpleRecursionWithBaseCase1.symbols

@@ -1,6 +1,6 @@
-//// [tests/cases/compiler/simpleRecursionWithBaseCase.ts] ////
+//// [tests/cases/compiler/simpleRecursionWithBaseCase1.ts] ////
 
-=== simpleRecursionWithBaseCase.ts ===
+=== simpleRecursionWithBaseCase1.ts ===
 function fn1(n: number) {
 >fn1 : (n: number) => number
 >n : number

tests/baselines/reference/simpleRecursionWithBaseCase.types renamed to tests/baselines/reference/simpleRecursionWithBaseCase1.types

@@ -0,0 +1,69 @@
+error TS2468: Cannot find global value 'Promise'.
+simpleRecursionWithBaseCase2.ts(17,16): error TS2705: An async function or method in ES5/ES3 requires the 'Promise' constructor.  Make sure you have a declaration for the 'Promise' constructor or include 'ES2015' in your '--lib' option.
+simpleRecursionWithBaseCase2.ts(21,16): error TS2705: An async function or method in ES5/ES3 requires the 'Promise' constructor.  Make sure you have a declaration for the 'Promise' constructor or include 'ES2015' in your '--lib' option.
+
+
+!!! error TS2468: Cannot find global value 'Promise'.
+==== simpleRecursionWithBaseCase2.ts (2 errors) ====
+    async function rec1() {
+      if (Math.random() < 0.5) {
+        return rec1();
+      } else {
+        return "hello";
+      }
+    }
+    
+    async function rec2() {
+      if (Math.random() < 0.5) {
+        return await rec2();
+      } else {
+        return "hello";
+      }
+    }
+    
+    async function rec3() {
+                   ~~~~
+!!! error TS2705: An async function or method in ES5/ES3 requires the 'Promise' constructor.  Make sure you have a declaration for the 'Promise' constructor or include 'ES2015' in your '--lib' option.
+      return rec3();
+    }
+    
+    async function rec4() {
+                   ~~~~
+!!! error TS2705: An async function or method in ES5/ES3 requires the 'Promise' constructor.  Make sure you have a declaration for the 'Promise' constructor or include 'ES2015' in your '--lib' option.
+      return await rec4();
+    }
+    
+    async function rec5() {
+      if (Math.random() < 0.5) {
+        return ((rec1()));
+      } else {
+        return "hello";
+      }
+    }
+    
+    async function rec6() {
+      if (Math.random() < 0.5) {
+        return await ((rec1()));
+      } else {
+        return "hello";
+      }
+    }
+    
+    declare const ps: Promise<string> | number;
+    
+    async function foo1() {
+      if (Math.random() > 0.5) {
+        return ps;
+      } else {
+        return await foo1();
+      }
+    }
+    
+    async function foo2() {
+      if (Math.random() > 0.5) {
+        return ps;
+      } else {
+        return foo2();
+      }
+    }
+