Add warning when @defaultResponse is used on model with status code or @error

[Copilot]

Using @defaultResponse on a model that already has @statusCode or @error produces duplicate $ref entries in OpenAPI output (e.g., anyOf with two identical refs). Rather than silently generating invalid output, emit a diagnostic warning at the source.

Changes

• packages/openapi/src/lib.ts — New default-response-with-status-code diagnostic with statusCode and error message variants
• packages/openapi/src/decorators.ts — $defaultResponse now checks the target model for existing @statusCode properties and @error decoration before setting the wildcard status code
• packages/openapi/test/decorators.test.ts — Tests for both warning cases and the clean no-warning path

Example

// This now emits a warning:
@error
@defaultResponse
model DefaultError is Error<500>;

// Recommended alternatives:
@error
model MyError { message: string; }
op read(): Data | MyError;

// Or split concerns:
@error model Error500 is Error<500>;
@error @defaultResponse model DefaultError { message: string; }
alias Result<T> = T | Error500 | DefaultError;

[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/openapi@10555

commit: 49c122f

[timotheeguerin]

@copilot fix lint

[Copilot]

@copilot fix lint

Fixed in c10f3ab — removed the unnecessary escape character \@ → @ in the regex, and added the missing changeset for @typespec/openapi.