Ban inferred return type on async and generator functions.

src/compiler/checker.ts

@@ -48984,6 +48984,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
     function getSingleReturnExpression(declaration: SignatureDeclaration | undefined): Expression | undefined {
         let candidateExpr: Expression | undefined;
         if (declaration && !nodeIsMissing((declaration as FunctionLikeDeclaration).body)) {
+            if (getFunctionFlags(declaration) & FunctionFlags.AsyncGenerator) return undefined;
             const body = (declaration as FunctionLikeDeclaration).body;
             if (body && isBlock(body)) {
                 forEachReturnStatement(body, s => {

src/compiler/expressionToTypeNode.ts

@@ -14,10 +14,12 @@ import {
     Expression,
     forEachReturnStatement,
     FunctionExpression,
+    FunctionFlags,
     FunctionLikeDeclaration,
     GetAccessorDeclaration,
     getEffectiveReturnTypeNode,
     getEffectiveTypeAnnotationNode,
+    getFunctionFlags,
     getJSDocTypeAssertionType,
     getStrictOptionValue,
     HasInferredType,
@@ -469,6 +471,8 @@ export function createSyntacticTypeNodeBuilder(options: CompilerOptions, resolve
     function typeFromSingleReturnExpression(declaration: FunctionLikeDeclaration | undefined, context: SyntacticTypeNodeBuilderContext): boolean | undefined {
         let candidateExpr: Expression | undefined;
         if (declaration && !nodeIsMissing(declaration.body)) {
+            if (getFunctionFlags(declaration) & FunctionFlags.AsyncGenerator) return undefined;
+
             const body = declaration.body;
             if (body && isBlock(body)) {
                 forEachReturnStatement(body, s => {

tests/baselines/reference/transpile/declarationAsyncAndGeneratorFunctions.d.ts

@@ -0,0 +1,71 @@
+//// [declarationAsyncAndGeneratorFunctions.ts] ////
+export async function asyncFn() {
+    return {} as Promise<void>
+}
+
+export async function asyncFn2() {
+    return {} as number
+}
+
+export async function asyncFn3() {
+    return (await 42) as number;
+  }
+
+export function* generatorFn() {
+    return {} as number
+}
+
+export async function* asyncGeneratorFn() {
+    return {} as number
+}
+//// [declarationAsyncAndGeneratorFunctions.d.ts] ////
+export declare function asyncFn(): unknown;
+export declare function asyncFn2(): unknown;
+export declare function asyncFn3(): unknown;
+export declare function generatorFn(): {};
+export declare function asyncGeneratorFn(): {};
+
+
+//// [Diagnostics reported]
+declarationAsyncAndGeneratorFunctions.ts(1,23): error TS9007: Function must have an explicit return type annotation with --isolatedDeclarations.
+declarationAsyncAndGeneratorFunctions.ts(5,23): error TS9007: Function must have an explicit return type annotation with --isolatedDeclarations.
+declarationAsyncAndGeneratorFunctions.ts(9,23): error TS9007: Function must have an explicit return type annotation with --isolatedDeclarations.
+declarationAsyncAndGeneratorFunctions.ts(13,18): error TS9007: Function must have an explicit return type annotation with --isolatedDeclarations.
+declarationAsyncAndGeneratorFunctions.ts(17,24): error TS9007: Function must have an explicit return type annotation with --isolatedDeclarations.
+
+
+==== declarationAsyncAndGeneratorFunctions.ts (5 errors) ====
+    export async function asyncFn() {
+                          ~~~~~~~
+!!! error TS9007: Function must have an explicit return type annotation with --isolatedDeclarations.
+!!! related TS9031 declarationAsyncAndGeneratorFunctions.ts:1:23: Add a return type to the function declaration.
+        return {} as Promise<void>
+    }
+
+    export async function asyncFn2() {
+                          ~~~~~~~~
+!!! error TS9007: Function must have an explicit return type annotation with --isolatedDeclarations.
+!!! related TS9031 declarationAsyncAndGeneratorFunctions.ts:5:23: Add a return type to the function declaration.
+        return {} as number
+    }
+
+    export async function asyncFn3() {
+                          ~~~~~~~~
+!!! error TS9007: Function must have an explicit return type annotation with --isolatedDeclarations.
+!!! related TS9031 declarationAsyncAndGeneratorFunctions.ts:9:23: Add a return type to the function declaration.
+        return (await 42) as number;
+      }
+
+    export function* generatorFn() {
+                     ~~~~~~~~~~~
+!!! error TS9007: Function must have an explicit return type annotation with --isolatedDeclarations.
+!!! related TS9031 declarationAsyncAndGeneratorFunctions.ts:13:18: Add a return type to the function declaration.
+        return {} as number
+    }
+
+    export async function* asyncGeneratorFn() {
+                           ~~~~~~~~~~~~~~~~
+!!! error TS9007: Function must have an explicit return type annotation with --isolatedDeclarations.
+!!! related TS9031 declarationAsyncAndGeneratorFunctions.ts:17:24: Add a return type to the function declaration.
+        return {} as number
+    }

tests/baselines/reference/transpile/declarationAsyncAndGeneratorFunctions.js

@@ -0,0 +1,36 @@
+//// [declarationAsyncAndGeneratorFunctions.ts] ////
+export async function asyncFn() {
+    return {} as Promise<void>
+}
+
+export async function asyncFn2() {
+    return {} as number
+}
+
+export async function asyncFn3() {
+    return (await 42) as number;
+  }
+
+export function* generatorFn() {
+    return {} as number
+}
+
+export async function* asyncGeneratorFn() {
+    return {} as number
+}
+//// [declarationAsyncAndGeneratorFunctions.js] ////
+export async function asyncFn() {
+    return {};
+}
+export async function asyncFn2() {
+    return {};
+}
+export async function asyncFn3() {
+    return (await 42);
+}
+export function* generatorFn() {
+    return {};
+}
+export async function* asyncGeneratorFn() {
+    return {};
+}

tests/cases/transpile/declarationAsyncAndGeneratorFunctions.ts

@@ -0,0 +1,22 @@
+// @declaration: true
+// @target: esnext
+
+export async function asyncFn() {
+    return {} as Promise<void>
+}
+
+export async function asyncFn2() {
+    return {} as number
+}
+
+export async function asyncFn3() {
+    return (await 42) as number;
+  }
+
+export function* generatorFn() {
+    return {} as number
+}
+
+export async function* asyncGeneratorFn() {
+    return {} as number
+}