fix(compiler): error when $functions implementation has no corresponding extern fn declaration

[Copilot]

Defining a function in $functions without a matching extern fn declaration in TypeSpec silently had no effect — the implementation was simply ignored, leading to confusing behavior where calling the function would use a default no-op implementation.

Changes

• New diagnostic implementation-without-extern: Reported during checkProgram() for any function in a JS $functions map that has no corresponding extern fn declaration in TypeSpec.

  error implementation-without-extern: Function "testFn" is declared in `$functions`
  but does not have a corresponding `extern fn` declaration in TypeSpec.
  Add an `extern fn` declaration.
  

• Documentation: Added url and description fields to the diagnostic and created a dedicated markdown documentation page at website/src/content/docs/docs/standard-library/diags/implementation-without-extern.md (following the same pattern as other diagnostics like triple-quote).

• Checker: Added checkJsImplementations() (called from checkProgram()) that walks all JS source file symbol tables and checks each Function | Implementation symbol for a paired FunctionDeclarationStatement in the merged symbol's declarations.

• Test helpers: expectFunctionDiagnosticsEmpty / expectFunctionDiagnostics only filter the existing experimental-feature warning. Tests are now correctly structured so that every $functions entry has a corresponding extern fn declaration, eliminating the need to suppress implementation-without-extern in test helpers.

• Test fixes: Corrected semantic-walker.test.ts (removed shared $functions fixture, scoped it to the one test that needs it) and functions.test.ts (replaced shared multi-function testers with per-function testers so each test only registers the exact implementations it declares with extern fn).

• 4 new tests covering the error for both root-namespace and namespaced $functions entries, with and without a matching extern fn.

Example

// lib.js — BAD: implementation is silently ignored, no error previously
export const $functions = {
  "TestFn": {
    testFn: (context, target) => target,
  },
};

// lib.tsp — no `namespace TestFn { extern fn testFn(...); }` → now emits error

The fix surfaces implementation-without-extern pointing at the JS source file, making namespace mismatches and missing declarations immediately diagnosable.

[timotheeguerin]

@copilot lets add a documentation url and a markdown document for it like for triple-quote error

[timotheeguerin]

@copilot why do we need to filter this, shouldn't the test not testing for this error hjave an extern dec?

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/compiler@10765

commit: 181cebc