Add regression tests

Author: SkiFire13

src/test/ui/impl-trait/explicit-generic-args-with-impl-trait/issue-87718.rs

@@ -0,0 +1,9 @@
+// check-pass
+
+#![feature(explicit_generic_args_with_impl_trait)]
+
+fn f<T: ?Sized>(_: impl AsRef<T>, _: impl AsRef<T>) {}
+
+fn main() {
+    f::<[u8]>("a", b"a");
+}

src/test/ui/impl-trait/explicit-generic-args-with-impl-trait/not-enough-args.rs

@@ -0,0 +1,8 @@
+#![feature(explicit_generic_args_with_impl_trait)]
+
+fn f<T: ?Sized, U: ?Sized>(_: impl AsRef<T>, _: impl AsRef<U>) {}
+
+fn main() {
+    f::<[u8]>("a", b"a");
+    //~^ ERROR: this function takes 2 generic arguments but 1 generic argument was supplied
+}

src/test/ui/impl-trait/explicit-generic-args-with-impl-trait/not-enough-args.stderr

@@ -0,0 +1,21 @@
+error[E0107]: this function takes 2 generic arguments but 1 generic argument was supplied
+  --> $DIR/not-enough-args.rs:6:5
+   |
+LL |     f::<[u8]>("a", b"a");
+   |     ^   ---- supplied 1 generic argument
+   |     |
+   |     expected 2 generic arguments
+   |
+note: function defined here, with 2 generic parameters: `T`, `U`
+  --> $DIR/not-enough-args.rs:3:4
+   |
+LL | fn f<T: ?Sized, U: ?Sized>(_: impl AsRef<T>, _: impl AsRef<U>) {}
+   |    ^ -          -
+help: add missing generic argument
+   |
+LL |     f::<[u8], U>("a", b"a");
+   |             ^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0107`.