Add diagnostic for valueof used in scalar initializer parameters

.chronus/changes/scalar-init-valueof-diagnostic-2026-6-3-18-59-0.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Emit a clear error diagnostic when `valueof` is used in a scalar initializer parameter constraint. Scalar initializer parameters implicitly accept values, so `valueof` is redundant and was previously causing confusing errors.

packages/compiler/src/core/checker.ts

@@ -7382,6 +7382,17 @@ export function createChecker(program: Program, resolver: NameResolver): Checker
       return links.declaredType as ScalarConstructor;
     }
 
+    for (const param of node.parameters) {
+      if (param.type && param.type.kind === SyntaxKind.ValueOfExpression) {
+        reportCheckerDiagnostic(
+          createDiagnostic({
+            code: "scalar-init-valueof",
+            target: param.type,
+          }),
+        );
+      }
+    }
+
     const member: ScalarConstructor = createType({
       kind: "ScalarConstructor",
       scalar: parentScalar,

packages/compiler/src/core/messages.ts

@@ -541,6 +541,12 @@ const diagnostics = {
       default: paramMessage`A constructor already exists with name ${"name"}`,
     },
   },
+  "scalar-init-valueof": {
+    severity: "error",
+    messages: {
+      default: `Scalar initializer parameters implicitly accept values. Remove 'valueof' from the parameter type.`,
+    },
+  },
   "spread-enum": {
     severity: "error",
     messages: {

packages/compiler/test/checker/scalar.test.ts

@@ -157,4 +157,19 @@ describe("compiler: scalars", () => {
       });
     });
   });
+
+  describe("scalar initializer valueof constraint", () => {
+    it("emit error when scalar init parameter uses valueof", async () => {
+      const diagnostics = await Tester.diagnose(`
+        scalar S {
+          init i(s: valueof string);
+        }
+      `);
+      expectDiagnostics(diagnostics, {
+        code: "scalar-init-valueof",
+        message:
+          "Scalar initializer parameters implicitly accept values. Remove 'valueof' from the parameter type.",
+      });
+    });
+  });
 });