fix(50224): Intellisense for strings within a type's Union doesn't work properly for JSX (#50231)

* fix(50224): show string literal completions in JsxAttributeInitializer

* add feedback changes

Author: a-tarasyuk

src/services/stringCompletions.ts

@@ -241,7 +241,7 @@ namespace ts.Completions.StringCompletions {
                     // Get string literal completions from specialized signatures of the target
                     // i.e. declare function f(a: 'A');
                     // f("/*completion position*/")
-                    return argumentInfo ? getStringLiteralCompletionsFromSignature(argumentInfo.invocation, node, argumentInfo, typeChecker) : fromContextualType();
+                    return argumentInfo && getStringLiteralCompletionsFromSignature(argumentInfo.invocation, node, argumentInfo, typeChecker) || fromContextualType();
                 }
                 // falls through (is `require("")` or `require(""` or `import("")`)
 
@@ -283,7 +283,7 @@ namespace ts.Completions.StringCompletions {
             type !== current && isLiteralTypeNode(type) && isStringLiteral(type.literal) ? type.literal.text : undefined);
     }
 
-    function getStringLiteralCompletionsFromSignature(call: CallLikeExpression, arg: StringLiteralLike, argumentInfo: SignatureHelp.ArgumentInfoForCompletions, checker: TypeChecker): StringLiteralCompletionsFromTypes {
+    function getStringLiteralCompletionsFromSignature(call: CallLikeExpression, arg: StringLiteralLike, argumentInfo: SignatureHelp.ArgumentInfoForCompletions, checker: TypeChecker): StringLiteralCompletionsFromTypes | undefined {
         let isNewIdentifier = false;
         const uniques = new Map<string, true>();
         const candidates: Signature[] = [];
@@ -301,8 +301,7 @@ namespace ts.Completions.StringCompletions {
             isNewIdentifier = isNewIdentifier || !!(type.flags & TypeFlags.String);
             return getStringLiteralTypes(type, uniques);
         });
-
-        return { kind: StringLiteralCompletionKind.Types, types, isNewIdentifier };
+        return length(types) ? { kind: StringLiteralCompletionKind.Types, types, isNewIdentifier } : undefined;
     }
 
     function stringLiteralCompletionsFromProperties(type: Type | undefined): StringLiteralCompletionsFromProperties | undefined {

tests/cases/fourslash/completionForStringLiteralFromSignature2.ts

@@ -4,4 +4,4 @@
 ////declare function f(a: string, b: number): void;
 ////f("/**/", 0);
 
-verify.completions({ marker: "", exact: [], isNewIdentifierLocation: true });
+verify.completions({ marker: "", exact: [], isNewIdentifierLocation: false });

tests/cases/fourslash/stringLiteralCompletionsInJsxAttributeInitializer.ts

@@ -0,0 +1,11 @@
+/// <reference path="fourslash.ts" />
+
+// @jsx: preserve
+// @filename: /a.tsx
+////type Props = { a: number } | { b: "somethingelse" };
+////declare function Foo(args: Props): any
+////
+////const a1 = <Foo b={"/*1*/"} />
+////const a2 = <Foo b="/*2*/" />
+
+verify.completions({ marker: ["1", "2"], exact: ["somethingelse"] });