fix: throw a clear error when throwArg index equals the argument count (#2743)

* fix: throw a clear error when throwArg index equals the argument count

spyCall.throwArg(pos) guarded with pos > this.args.length, so calling it
with pos equal to the number of recorded arguments slipped past the guard
and reached throw this.args[pos], throwing undefined instead of the
intended TypeError. A thrown undefined cannot be inspected as an Error and
is reported by test frameworks as no exception thrown.

Use >= to match ensureArgs in behavior.js and the sibling callArg helpers,
which already reject an out-of-range index with a clear error.

Author: spokodev

src/sinon/proxy-call.js

@@ -140,7 +140,7 @@ const callProto = {
     },
 
     throwArg: function (pos) {
-        if (pos > this.args.length) {
+        if (pos >= this.args.length) {
             throw new TypeError(
                 `Not enough arguments: ${pos} required but only ${this.args.length} present`,
             );

test/src/proxy-call-test.js

@@ -613,6 +613,55 @@ describe("sinonSpy.call", function () {
         });
     });
 
+    describe("call.throwArg", function () {
+        beforeEach(spyCallCallSetup);
+
+        it("throws argument at specified index", function () {
+            const expectedError = new TypeError("catpants");
+            this.args.push(1, expectedError, 2);
+            const call = this.call;
+
+            assert.exception(
+                function () {
+                    call.throwArg(1);
+                },
+                { message: "catpants" },
+            );
+        });
+
+        it("throws if index equals the number of arguments", function () {
+            this.args.push(1, 2);
+            const call = this.call;
+
+            assert.exception(
+                function () {
+                    call.throwArg(2);
+                },
+                {
+                    name: "TypeError",
+                    message:
+                        "Not enough arguments: 2 required but only 2 present",
+                },
+            );
+        });
+
+        it("throws if there aren't enough arguments", function () {
+            this.args.push(1, 2);
+            const call = this.call;
+
+            assert.exception(
+                function () {
+                    call.throwArg(3);
+                },
+                {
+                    name: "TypeError",
+                    message:
+                        "Not enough arguments: 3 required but only 2 present",
+                },
+            );
+        });
+    });
+
     describe("call.yieldTest", function () {
         beforeEach(spyCallCallSetup);