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

.chronus/changes/copilot-add-error-check-function-implementation-2026-5-21.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Report an error when a function is declared in the `$functions` map in a JS file but has no corresponding `extern fn` declaration in TypeSpec. Previously this would silently have no effect.

packages/compiler/src/core/checker.ts

@@ -4848,6 +4848,7 @@ export function createChecker(program: Program, resolver: NameResolver): Checker
       checkSourceFile(file);
     }
 
+    checkJsImplementations();
     internalDecoratorValidation();
     assertNoPendingResolutions();
     runPostValidators(postCheckValidators);
@@ -4924,6 +4925,41 @@ export function createChecker(program: Program, resolver: NameResolver): Checker
     validateInheritanceDiscriminatedUnions(program);
   }
 
+  /**
+   * Validate that every function implementation in a JS `$functions` map has a corresponding
+   * `extern fn` declaration in TypeSpec. Without a declaration the function implementation is
+   * silently ignored. Report an error to help the user diagnose the problem.
+   */
+  function checkJsImplementations() {
+    for (const jsFile of program.jsSourceFiles.values()) {
+      checkJsSymbolTableForUnboundFunctions(jsFile.symbol.exports!);
+    }
+  }
+
+  function checkJsSymbolTableForUnboundFunctions(table: SymbolTable) {
+    for (const sym of table.values()) {
+      if (sym.flags & SymbolFlags.Namespace) {
+        if (sym.exports) {
+          checkJsSymbolTableForUnboundFunctions(sym.exports);
+        }
+      } else if (sym.flags & SymbolFlags.Function && sym.flags & SymbolFlags.Implementation) {
+        const mergedSym = getMergedSymbol(sym);
+        const hasTypeSpecDeclaration = mergedSym.declarations.some(
+          (decl) => decl.kind === SyntaxKind.FunctionDeclarationStatement,
+        );
+        if (!hasTypeSpecDeclaration) {
+          reportCheckerDiagnostic(
+            createDiagnostic({
+              code: "implementation-without-extern",
+              format: { name: sym.name },
+              target: sym.declarations[0],
+            }),
+          );
+        }
+      }
+    }
+  }
+
   function checkSourceFile(file: TypeSpecScriptNode) {
     for (const statement of file.statements) {
       checkNode(CheckContext.DEFAULT, statement, undefined);

packages/compiler/src/core/messages.ts

@@ -588,6 +588,15 @@ const diagnostics = {
       default: "Extern declaration must have an implementation in JS file.",
     },
   },
+  "implementation-without-extern": {
+    severity: "error",
+    description:
+      "Report when a function is registered in $functions in a JS file but has no corresponding `extern fn` declaration in TypeSpec.",
+    url: "https://typespec.io/docs/standard-library/diags/implementation-without-extern",
+    messages: {
+      default: paramMessage`Function "${"name"}" is declared in \`$functions\` but does not have a corresponding \`extern fn\` declaration in TypeSpec. Add an \`extern fn\` declaration.`,
+    },
+  },
   "overload-same-parent": {
     severity: "error",
     messages: {