don't overwrite interface properties with template properties (#13720)

* test for all varieties of mistaken interface assignment
* fresh object for templates, do not mutate interfaces

CHANGELOG_BEGIN
- [daml codegen js] Some interface IDs were overwritten with template
  IDs, making exercise by interface ID impossible in those cases; this
  has been fixed.  Prior codegen output is invalid; codegen must be
  re-run to get the fix.
  See `issue #13720 <https://github.com/digital-asset/daml/pull/13720>`__.
CHANGELOG_END

language-support/ts/codegen/src/TsCodeGenMain.hs

@@ -391,8 +391,11 @@ renderTemplateDef TemplateDef {..} =
         T.unlines $
         concat
           [ ["exports." <> tplName <> " = Object.assign("]
+          -- defining the template directly here would overwrite the template ID
+          -- with an interface ID
+          , ["{},"]
           , [impl <> "," | (_, JsSerializerConRef impl) <- tplImplements']
-          -- we spread in the interface choices, the templateId field of the interface will be overwritten by the template object.
+          -- template ID and directly-defined choices are always top-priority
           , [ T.unlines $
               concat
                 [ ["{"]

language-support/ts/codegen/tests/ts/build-and-lint-test/src/__tests__/test.ts

@@ -547,6 +547,28 @@ test("create + fetch & exercise", async () => {
   expect(nonTopLevelContracts).toEqual([nonTopLevelContract]);
 });
 
+describe("interface definition", () => {
+  const tpl = buildAndLint.Main.Asset;
+  const if1 = buildAndLint.Main.Token;
+  const if2 = buildAndLint.Lib.Mod.Other;
+  test("separate object from template", () => {
+    expect(if1).not.toBe(tpl);
+    expect(if2).not.toBe(tpl);
+  });
+  test("template IDs not overwritten", () => {
+    expect(if1.templateId).not.toEqual(tpl.templateId);
+    expect(if2.templateId).not.toEqual(tpl.templateId);
+  });
+  test("choices not copied to interfaces", () => {
+    const key1 = "Transfer";
+    const key2 = "Something";
+    expect(if1).toHaveProperty(key1);
+    expect(if2).toHaveProperty(key2);
+    expect(if1).not.toHaveProperty(key2);
+    expect(if2).not.toHaveProperty(key1);
+  });
+});
+
 test("interfaces", async () => {
   const aliceLedger = new Ledger({
     token: ALICE_TOKEN,