Fix function call argument count errors reporting on declaration instead of call site

.chronus/changes/fix-function-error-location-2026-6-2-2-24-0.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Fixed `invalid-argument-count` diagnostic for function calls to report on the call expression instead of the function declaration.

packages/compiler/src/core/checker.ts

@@ -5812,7 +5812,7 @@ export function createChecker(program: Program, resolver: NameResolver): Checker
     node: CallExpressionNode,
     target: FunctionValue<unknown[]>,
   ): Type | Value | null {
-    const [satisfied, resolvedArgs] = checkFunctionCallArguments(ctx, node.arguments, target);
+    const [satisfied, resolvedArgs] = checkFunctionCallArguments(ctx, node.arguments, target, node);
 
     const canCall =
       satisfied &&
@@ -5914,6 +5914,7 @@ export function createChecker(program: Program, resolver: NameResolver): Checker
     ctx: CheckContext,
     args: Expression[],
     target: FunctionValue,
+    callNode: CallExpressionNode,
   ): [boolean, any[]] {
     let satisfied = true;
     const minArgs = target.parameters.filter((p) => !p.optional && !p.rest).length;
@@ -5927,7 +5928,7 @@ export function createChecker(program: Program, resolver: NameResolver): Checker
           code: "invalid-argument-count",
           messageId: "atLeast",
           format: { actual: args.length.toString(), expected: minArgs.toString() },
-          target: target.node!,
+          target: callNode,
         }),
       );
       return [false, []];
@@ -5936,7 +5937,7 @@ export function createChecker(program: Program, resolver: NameResolver): Checker
         createDiagnostic({
           code: "invalid-argument-count",
           format: { actual: args.length.toString(), expected: maxArgs.toString() },
-          target: target.node!,
+          target: callNode,
         }),
       );
       // This error doesn't actually prevent us from checking the arguments and evaluating the function.

packages/compiler/test/checker/functions.test.ts

@@ -6,6 +6,7 @@ import {
   IndeterminateEntity,
   Model,
   Namespace,
+  SyntaxKind,
   Type,
   Value,
 } from "../../src/core/types.js";
@@ -365,6 +366,23 @@ describe("usage", () => {
     strictEqual(v.value, "one");
   });
 
+  it("reports invalid-argument-count error at call site, not function declaration", async () => {
+    const fnTester = createTesterForFn("testFn", testFnImpl);
+    const [, diagnostics] = await fnTester.compileAndDiagnose(`
+      extern fn testFn(a: valueof string): valueof string;
+      const X = testFn("one", "two");
+    `);
+
+    const argCountDiag = diagnostics.find((d) => d.code === "invalid-argument-count");
+    ok(argCountDiag, "Expected invalid-argument-count diagnostic");
+    ok(argCountDiag.target !== undefined, "Expected diagnostic to have a target");
+    strictEqual(
+      (argCountDiag.target as any).kind,
+      SyntaxKind.CallExpression,
+      "Diagnostic should target the call expression, not the function declaration",
+    );
+  });
+
   it("errors if too few with rest", async () => {
     const t = await expectFunctionTypeUsage(
       "extern fn testFn(a: string, ...rest: string[])",