Don't allow implementing trait directly on type-alias-impl-trait

This is specifically disallowed by the RFC, but we never added a check
for it.

Fixes #76202

src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.rs

@@ -0,0 +1,23 @@
+// Regression test for issue #76202
+// Tests that we don't ICE when we have a trait impl on a TAIT.
+
+#![feature(type_alias_impl_trait)]
+
+trait Dummy {}
+impl Dummy for () {}
+
+type F = impl Dummy;
+fn f() -> F {}
+
+trait Test {
+    fn test(self);
+}
+
+impl Test for F { //~ ERROR cannot implement trait
+    fn test(self) {}
+}
+
+fn main() {
+    let x: F = f();
+    x.test();
+}

src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.stderr

@@ -0,0 +1,14 @@
+error: cannot implement trait on type alias impl trait
+  --> $DIR/issue-76202-trait-impl-for-tait.rs:16:1
+   |
+LL | impl Test for F {
+   | ^^^^^^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/issue-76202-trait-impl-for-tait.rs:9:10
+   |
+LL | type F = impl Dummy;
+   |          ^^^^^^^^^^
+
+error: aborting due to previous error
+