warn when generator function without any yield expression (microsoft#13847)

Author: Kingwl

src/compiler/checker.ts

@@ -18714,6 +18714,18 @@ namespace ts {
             }
         }
 
+        function checkGeneratorWithReturnStatementMustHaveYield (func: FunctionLikeDeclaration) {
+            let hasReturnStmt = false;
+            forEachReturnStatement(<Block>func.body, _ => hasReturnStmt = true);
+            if (hasReturnStmt) {
+                let hasYieldExpr = false;
+                forEachYieldExpression(<Block>func.body, _ => hasYieldExpr = true);
+                if (!hasYieldExpr) {
+                    error(func, Diagnostics.generator_with_return_statement_must_not_have_zero_yield_return_statements);
+                }
+            }
+        }
+
         function checkSignatureDeclaration(node: SignatureDeclaration) {
             // Grammar checking
             if (node.kind === SyntaxKind.IndexSignature) {
@@ -24968,7 +24980,10 @@ namespace ts {
                 if (isInAmbientContext(node)) {
                     return grammarErrorOnNode(node.asteriskToken, Diagnostics.Generators_are_not_allowed_in_an_ambient_context);
                 }
-                if (!node.body) {
+                if (node.body) {
+                    checkGeneratorWithReturnStatementMustHaveYield(node);
+                }
+                else {
                     return grammarErrorOnNode(node.asteriskToken, Diagnostics.An_overload_signature_cannot_be_declared_as_a_generator);
                 }
             }

src/compiler/diagnosticMessages.json

@@ -911,6 +911,10 @@
         "category": "Error",
         "code": 1329
     },
+    "generator with 'return statement' must not have zero 'yield return' statements.": {
+        "category": "Message",
+        "code": 1330
+    },
 
     "Duplicate identifier '{0}'.": {
         "category": "Error",

tests/baselines/reference/emitter.asyncGenerators.classMethods.es2015.js

@@ -38,7 +38,6 @@ class C6 {
 //// [C7.ts]
 class C7 {
     async * f() {
-        return 1;
     }
 }
 //// [C8.ts]
@@ -215,7 +214,6 @@ var __asyncGenerator = (this && this.__asyncGenerator) || function (thisArg, _ar
 class C7 {
     f() {
         return __asyncGenerator(this, arguments, function* f_1() {
-            return 1;
         });
     }
 }

tests/baselines/reference/emitter.asyncGenerators.classMethods.es2015.symbols

@@ -67,8 +67,6 @@ class C7 {
 
     async * f() {
 >f : Symbol(C7.f, Decl(C7.ts, 0, 10))
-
-        return 1;
     }
 }
 === tests/cases/conformance/emitter/es2015/asyncGenerators/C8.ts ===

tests/baselines/reference/emitter.asyncGenerators.classMethods.es2015.types

@@ -80,10 +80,7 @@ class C7 {
 >C7 : C7
 
     async * f() {
->f : () => AsyncIterableIterator<1>
-
-        return 1;
->1 : 1
+>f : () => AsyncIterableIterator<any>
     }
 }
 === tests/cases/conformance/emitter/es2015/asyncGenerators/C8.ts ===

tests/baselines/reference/emitter.asyncGenerators.classMethods.es5.js

@@ -38,7 +38,6 @@ class C6 {
 //// [C7.ts]
 class C7 {
     async * f() {
-        return 1;
     }
 }
 //// [C8.ts]
@@ -497,7 +496,7 @@ var C7 = /** @class */ (function () {
     C7.prototype.f = function () {
         return __asyncGenerator(this, arguments, function f_1() {
             return __generator(this, function (_a) {
-                return [2 /*return*/, 1];
+                return [2 /*return*/];
             });
         });
     };

tests/baselines/reference/emitter.asyncGenerators.classMethods.es5.symbols

@@ -67,8 +67,6 @@ class C7 {
 
     async * f() {
 >f : Symbol(C7.f, Decl(C7.ts, 0, 10))
-
-        return 1;
     }
 }
 === tests/cases/conformance/emitter/es5/asyncGenerators/C8.ts ===

tests/baselines/reference/emitter.asyncGenerators.classMethods.es5.types

@@ -80,10 +80,7 @@ class C7 {
 >C7 : C7
 
     async * f() {
->f : () => AsyncIterableIterator<1>
-
-        return 1;
->1 : 1
+>f : () => AsyncIterableIterator<any>
     }
 }
 === tests/cases/conformance/emitter/es5/asyncGenerators/C8.ts ===

tests/baselines/reference/emitter.asyncGenerators.classMethods.esnext.js

@@ -38,7 +38,6 @@ class C6 {
 //// [C7.ts]
 class C7 {
     async * f() {
-        return 1;
     }
 }
 //// [C8.ts]
@@ -98,7 +97,6 @@ class C6 {
 //// [C7.js]
 class C7 {
     async *f() {
-        return 1;
     }
 }
 //// [C8.js]

tests/baselines/reference/emitter.asyncGenerators.classMethods.esnext.symbols

@@ -67,8 +67,6 @@ class C7 {
 
     async * f() {
 >f : Symbol(C7.f, Decl(C7.ts, 0, 10))
-
-        return 1;
     }
 }
 === tests/cases/conformance/emitter/esnext/asyncGenerators/C8.ts ===