fix(44123): forbid convert to async for generator callbacks (microsoft#44147)

Author: a-tarasyuk

src/services/suggestionDiagnostics.ts

@@ -174,6 +174,11 @@ namespace ts {
         switch (arg.kind) {
             case SyntaxKind.FunctionDeclaration:
             case SyntaxKind.FunctionExpression:
+                const functionFlags = getFunctionFlags(arg as FunctionDeclaration | FunctionExpression);
+                if (functionFlags & FunctionFlags.Generator) {
+                    return false;
+                }
+                // falls through
             case SyntaxKind.ArrowFunction:
                 visitedNestedConvertibleFunctions.set(getKeyFromNode(arg as FunctionLikeDeclaration), true);
                 // falls through

src/testRunner/unittests/services/convertToAsyncFunction.ts

@@ -1706,6 +1706,16 @@ class Foo {
         return Promise.resolve(1).then(n => n);
     }
 }
+`);
+
+        _testConvertToAsyncFunctionFailed("convertToAsyncFunction__NoSuggestionForGeneratorCallbacks", `
+function [#|foo|](p: Promise<string[]>) {
+    return p.then(function* (strings) {
+        for (const s of strings) {
+            yield s.toUpperCase();
+        }
+    });
+}
 `);
 
     });