Add stub type test to mutator engine

.chronus/changes/fix-mutator-property-stub-type-ts-errors-2026-4-26-10-32-5.md

@@ -0,0 +1,7 @@
+---
+changeKind: internal
+packages:
+  - "@typespec/mutator-framework"
+---
+
+Add regression tests for alternate type dual-edge mutation (ARM + client paths) in mutator-framework

packages/http-canonicalization/src/model.ts

@@ -127,7 +127,12 @@ export class ModelHttpCanonicalization
       if (referenceTypes.length === 0) {
         return engine.mutate(union, options, halfEdge, { isSynthetic: true });
       } else {
-        return engine.replaceAndMutateReference(referenceTypes[0], union, options, halfEdge);
+        return engine.replaceAndMutateReference(
+          referenceTypes[0],
+          union,
+          options,
+          halfEdge,
+        ) as any as UnionHttpCanonicalization;
       }
     }
 
@@ -142,7 +147,7 @@ export class ModelHttpCanonicalization
           effectiveModel,
           options,
           halfEdge,
-        );
+        ) as any as ModelHttpCanonicalization;
       }
     }
 

packages/mutator-framework/src/mutation-node/mutation-node.test.ts

@@ -109,3 +109,18 @@ it("clones synthetic mutation nodes", async () => {
   expect(propNode.mutatedType.type === model).toBe(false);
   expect(propNode.mutatedType.type === typeNode.mutatedType).toBe(true);
 });
+
+it("creates a literal mutation node for StringTemplate types", async () => {
+  const { Foo, program } = await runner.compile(t.code`
+      model ${t.model("Foo")} {
+        prop: "Start \${123} end";
+      }
+    `);
+  const engine = getEngine(program);
+  const prop = Foo.properties.get("prop")!;
+
+  expect(prop.type.kind).toBe("StringTemplate");
+  const node = engine.getMutationNode(prop.type);
+  expect(node).toBeDefined();
+  expect(node.sourceType).toBe(prop.type);
+});