fix(43939): forbid converting getters to async/await (microsoft#43944)

Author: a-tarasyuk

src/services/codefixes/convertToAsyncFunction.ts

@@ -54,7 +54,7 @@ namespace ts.codefix {
             functionToConvert = tokenAtPosition.parent.initializer;
         }
         else {
-            functionToConvert = tryCast(getContainingFunction(getTokenAtPosition(sourceFile, position)), isFunctionLikeDeclaration);
+            functionToConvert = tryCast(getContainingFunction(getTokenAtPosition(sourceFile, position)), canBeConvertedToAsync);
         }
 
         if (!functionToConvert) {

src/services/suggestionDiagnostics.ts

@@ -57,7 +57,7 @@ namespace ts {
                 }
             }
 
-            if (isFunctionLikeDeclaration(node)) {
+            if (canBeConvertedToAsync(node)) {
                 addConvertToAsyncFunctionDiagnostics(node, checker, diags);
             }
             node.forEachChild(check);
@@ -213,4 +213,16 @@ namespace ts {
 
         return false;
     }
+
+    export function canBeConvertedToAsync(node: Node): node is FunctionDeclaration | MethodDeclaration | FunctionExpression | ArrowFunction {
+        switch (node.kind) {
+            case SyntaxKind.FunctionDeclaration:
+            case SyntaxKind.MethodDeclaration:
+            case SyntaxKind.FunctionExpression:
+            case SyntaxKind.ArrowFunction:
+                return true;
+            default:
+                return false;
+        }
+    }
 }

src/testRunner/unittests/services/convertToAsyncFunction.ts

@@ -1698,6 +1698,14 @@ function [#|f|](x?: number): Promise<void> | number {
     if (x) return x;
     return fetch('').then(() => {});
 }
+`);
+
+        _testConvertToAsyncFunctionFailed("convertToAsyncFunction__NoSuggestionInGetters", `
+class Foo {
+    get [#|m|](): Promise<number> {
+        return Promise.resolve(1).then(n => n);
+    }
+}
 `);
 
     });