fix(26141): show completions for string parenthesized types

Author: a-tarasyuk

src/services/stringCompletions.ts

@@ -114,6 +114,10 @@ namespace ts.Completions.StringCompletions {
                 switch (parent.parent.kind) {
                     case SyntaxKind.TypeReference:
                         return { kind: StringLiteralCompletionKind.Types, types: getStringLiteralTypes(typeChecker.getTypeArgumentConstraint(parent as LiteralTypeNode)), isNewIdentifier: false };
+                    case SyntaxKind.ParenthesizedType: {
+                        const typeNode = findAncestor(parent.parent, n => !isParenthesizedTypeNode(n.parent)) as ParenthesizedTypeNode;
+                        return { kind: StringLiteralCompletionKind.Types, types: getStringLiteralTypes(typeChecker.getTypeArgumentConstraint(typeNode)), isNewIdentifier: false };
+                    }
                     case SyntaxKind.IndexedAccessType:
                         // Get all apparent property names
                         // i.e. interface Foo {

tests/cases/fourslash/completionListStringParenthesizedType.ts

@@ -0,0 +1,45 @@
+/// <reference path='fourslash.ts'/>
+
+////type T1 = "a" | "b" | "c";
+////type T2<T extends T1> = {};
+////
+////type T3 = T2<"[|/*1*/|]">;
+////type T4 = T2<("[|/*2*/|]")>;
+////type T5 = T2<(("[|/*3*/|]"))>;
+////type T6 = T2<((("[|/*4*/|]")))>;
+
+const [r1, r2, r3, r4] = test.ranges();
+verify.completions(
+    {
+        marker: "1",
+        exact: [
+            { name: "a", replacementSpan: r1 },
+            { name: "b", replacementSpan: r1 },
+            { name: "c", replacementSpan: r1 },
+        ]
+    },
+    {
+        marker: "2",
+        exact: [
+            { name: "a", replacementSpan: r2 },
+            { name: "b", replacementSpan: r2 },
+            { name: "c", replacementSpan: r2 },
+        ]
+    },
+    {
+        marker: "3",
+        exact: [
+            { name: "a", replacementSpan: r3 },
+            { name: "b", replacementSpan: r3 },
+            { name: "c", replacementSpan: r3 },
+        ]
+    },
+    {
+        marker: "4",
+        exact: [
+            { name: "a", replacementSpan: r4 },
+            { name: "b", replacementSpan: r4 },
+            { name: "c", replacementSpan: r4 },
+        ]
+    }
+);