Fix ICE in label_fn_like when splatted arg index is out of bounds

Author: Ajay-singh1

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1715,7 +1715,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             spans.push_span_label(expected_param.span(), "");
                         }
                         None => {
-                            if !tuple_arguments.is_splatted() {
+                            if tuple_arguments.is_splatted() {
                                 // FIXME(splat): when the arg is splatted, adjust its index
                                 use_splat_fallback = true;
                             } else {

tests/ui/splat/splat-overload-at-home-fail.rs

@@ -0,0 +1,45 @@
+// tests/ui/splat/splat-overload-at-home-fail.rs
+
+// ignore-tidy-linelength
+//! Test error cases for `#[splat]` "overloading at home" example code.
+//! Splatted calls that don't match any registered MethodArgs impl should fail.
+#![allow(incomplete_features)]
+#![feature(splat)]
+#![feature(tuple_trait)]
+
+struct Foo;
+
+trait MethodArgs: std::marker::Tuple {
+    fn call_method(self, _this: &Foo);
+}
+
+impl MethodArgs for () {
+    fn call_method(self, _this: &Foo) {}
+}
+
+impl MethodArgs for (i32,) {
+    fn call_method(self, _this: &Foo) {}
+}
+
+impl MethodArgs for (i32, String) {
+    fn call_method(self, _this: &Foo) {}
+}
+
+impl Foo {
+    fn method<T: MethodArgs>(&self, #[splat] args: T) {
+        args.call_method(self)
+    }
+}
+
+fn main() {
+    let foo = Foo;
+
+    // No impl for (f32,) — wrong type
+    foo.method(42f32);
+    //~^ ERROR mismatched types
+
+    // No impl for (i32,i32) - wrong type
+    foo.method(42i32, 42i32);
+    //~^ ERROR mismatched types
+
+ }

tests/ui/splat/splat-overload-at-home-fail.stderr

@@ -0,0 +1,40 @@
+error[E0308]: mismatched types
+  --> $DIR/splat-overload-at-home-fail.rs:38:16
+   |
+LL |     foo.method(42f32);
+   |         ------ ^^^^^ expected `i32`, found `f32`
+   |         |
+   |         arguments to this method are incorrect
+   |
+note: method defined here
+  --> $DIR/splat-overload-at-home-fail.rs:29:8
+   |
+LL |     fn method<T: MethodArgs>(&self, #[splat] args: T) {
+   |        ^^^^^^                       ----------------
+help: change the type of the numeric literal from `f32` to `i32`
+   |
+LL -     foo.method(42f32);
+LL +     foo.method(42i32);
+   |
+
+error[E0308]: mismatched types
+  --> $DIR/splat-overload-at-home-fail.rs:42:23
+   |
+LL |     foo.method(42i32, 42i32);
+   |         ------        ^^^^^ expected `String`, found `i32`
+   |         |
+   |         arguments to this method are incorrect
+   |
+note: method defined here
+  --> $DIR/splat-overload-at-home-fail.rs:29:8
+   |
+LL |     fn method<T: MethodArgs>(&self, #[splat] args: T) {
+   |        ^^^^^^
+help: try using a conversion method
+   |
+LL |     foo.method(42i32, 42i32.to_string());
+   |                            ++++++++++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.