Fix mutation regression with numeric arguments in decorators (#9319)

fix #9318

Author: timotheeguerin

.chronus/changes/fix-numeric-regression-2026-0-9-14-5-13.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: internal
+packages:
+  - "@typespec/compiler"
+---
+
+Fix mutation regression with numeric arguments in decorators

packages/compiler/src/experimental/mutators.ts

@@ -1,5 +1,6 @@
 import { compilerAssert } from "../core/diagnostics.js";
 import { getLocationContext } from "../core/helpers/location-context.js";
+import { isNumeric } from "../core/numeric.js";
 import { Program } from "../core/program.js";
 import { isTemplateInstance, isType, isValue } from "../core/type-utils.js";
 import {
@@ -738,10 +739,9 @@ function createMutatorEngine(
     }
   }
 
-  function mutateDecoratorArgumentValue<T extends DecoratorArgument["jsValue"]>(
-    value: T,
-    newMutators: Set<MutatorAll>,
-  ): T {
+  function mutateDecoratorArgumentValue<
+    T extends DecoratorArgument["jsValue"] | DecoratorArgument["value"],
+  >(value: T, newMutators: Set<MutatorAll>): T {
     if (typeof value === "object" && value !== null) {
       if (isType(value as any)) {
         return isMutableTypeWithNamespace(value as any)
@@ -751,6 +751,9 @@ function createMutatorEngine(
       if (isValue(value as any)) {
         return mutateValue(value as any, newMutators) as T;
       }
+      if (isNumeric(value)) {
+        return value;
+      }
       if (Array.isArray(value)) {
         return value.map((item) => mutateDecoratorArgumentValue(item as any, newMutators)) as T;
       }

packages/compiler/test/experimental/mutator.test.ts

@@ -8,9 +8,11 @@ import {
   MutatorWithNamespace,
 } from "../../src/experimental/mutators.js";
 import { Model, ModelProperty, Namespace, Operation } from "../../src/index.js";
+import { t } from "../../src/testing/index.js";
 import { createTestHost } from "../../src/testing/test-host.js";
 import { createTestWrapper, expectTypeEquals } from "../../src/testing/test-utils.js";
 import { BasicTestRunner, TestHost } from "../../src/testing/types.js";
+import { Tester } from "../tester.js";
 
 let host: TestHost;
 let runner: BasicTestRunner;
@@ -481,6 +483,21 @@ describe("decorators", () => {
     expect(visited).toStrictEqual(["Foo", "a", "E"]);
   });
 
+  // Regression test for https://github.com/microsoft/typespec/issues/9318
+  it("doesn't crash when mutating numeric value", async () => {
+    const { prop, program } = await Tester.compile(t.code`
+      model Test {
+        @minValue(123)
+        ${t.modelProperty("prop")}: int32;
+      }
+    `);
+    const mutator: Mutator = {
+      name: "test",
+      ModelProperty: { mutate: (_, clone) => {} },
+    };
+    expect(() => mutateSubgraph(program, [mutator], prop)).not.toThrow();
+  });
+
   // Regression test for https://github.com/microsoft/typespec/issues/6655
   it("doesn't crash when mutating null value", async () => {
     const host = await createTestHost();