Add diagnostic for valueof used in scalar initializer parameters

[Copilot]

Using valueof in a scalar initializer parameter constraint produced no diagnostic at the definition site. The error only surfaced at the call site as an unhelpful "unassignable to ErrorType" message, because ValueOfExpression fell through to errorType in checkNode.

Scalar initializer parameters implicitly accept values, so valueof is redundant and invalid. This adds an explicit diagnostic at the definition site.

Changes

• New diagnostic scalar-init-valueof in messages.ts — clear error telling the user to remove valueof
• Validation in checkScalarConstructor — detects ValueOfExpression nodes on parameter types before processing them
• Test in scalar.test.ts verifying the diagnostic fires

Example

scalar S {
  init i(s: valueof string);  // ← now emits: "Scalar initializer parameters implicitly accept values. Remove 'valueof' from the parameter type."
}

[markcowl]

/azp run typespec - PR Tools

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/compiler@10878

commit: 38966a6

[azure-sdk-automation]

You can try these changes here

🛝 Playground	🌐 Website	🛝 VSCode Extension