Convert string | URL unions to strings through the URL's href

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/compiler/src/frontend/lowering/class-to-primitive.ts

@@ -270,8 +270,8 @@ function refuse(lowerer: Lowerer, node: ts.Node, info: ClassInfo, what: string):
 }
 
 /** ToString(ToPrimitive(value, hint)) of a union whose arms are program
- * class instances and primitives (`Box | undefined`): each arm converts as
- * it would alone. Null for other unions. */
+ * class instances, URLs and primitives (`Box | undefined`, `string | URL`):
+ * each arm converts as it would alone. Null for other unions. */
 export function classUnionToString(
   lowerer: Lowerer,
   value: IrExpr,
@@ -281,9 +281,11 @@ export function classUnionToString(
   if (value.type.kind !== "union") return null;
   const unionId = value.type.unionId;
   const arms = lowerer.unions.get(unionId)?.arms;
-  if (!arms || !arms.some((arm) => arm.kind === "object")) return null;
+  if (!arms || !arms.some((arm) => arm.kind === "object" || arm.kind === "url")) return null;
   const convertible = (arm: IrType): boolean => {
     if (PRIMITIVE_KINDS.has(arm.kind)) return true;
+    // A URL arm converts through its href, like String(url).
+    if (arm.kind === "url") return true;
     if (arm.kind !== "object") return false;
     const info = lowerer.classes.get(arm.className);
     return info !== undefined && plainProgramClass(info);

packages/compiler/test/ts7/baselines/order-parity/tests/corpus/6250-string-of-string-or-url.ts.json

@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/6250-string-of-string-or-url.ts"
+ ],
+ "diags": []
+}

tests/corpus/6250-string-of-string-or-url.ts

@@ -0,0 +1,22 @@
+// String(), template literals and concatenation over a `string | URL`
+// union: the string arm passes through and the URL arm converts through its
+// href, exactly as each would alone (an origin URL keeps its trailing
+// slash, the origin string does not).
+function key(origin: string | URL): string {
+  return String(origin);
+}
+
+function label(origin: string | URL | undefined): string {
+  return `[${origin}]`;
+}
+
+const url = new URL("http://127.0.0.1:8080");
+console.log(key("http://127.0.0.1:8080"));
+console.log(key(url));
+console.log(key(new URL("https://example.com/a/b?x=1#h")));
+console.log(label(url), label("plain"), label(undefined));
+const both: (string | URL)[] = ["a", url];
+console.log(both.map((o) => "" + o).join(" "));
+const map = new Map<string, number>();
+map.set(key(url.origin), 1);
+console.log(map.get(String(url.origin)), map.get(String(url)));