Add test for issue rust-lang#51892

Author: marmeladema

src/test/ui/specialization/issue-51892.rs

@@ -0,0 +1,19 @@
+#![allow(incomplete_features)]
+#![feature(const_generics)]
+#![feature(const_evaluatable_checked)]
+#![feature(specialization)]
+
+pub trait Trait {
+    type Type;
+}
+
+impl<T: ?Sized> Trait for T {
+    default type Type = [u8; 1];
+}
+
+impl<T: Trait> Trait for *const T {
+    type Type = [u8; std::mem::size_of::<<T as Trait>::Type>()];
+    //~^ ERROR: unconstrained generic constant
+}
+
+fn main() {}

src/test/ui/specialization/issue-51892.stderr

@@ -0,0 +1,10 @@
+error: unconstrained generic constant
+  --> $DIR/issue-51892.rs:15:5
+   |
+LL |     type Type = [u8; std::mem::size_of::<<T as Trait>::Type>()];
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: try adding a `where` bound using this expression: `where [(); std::mem::size_of::<<T as Trait>::Type>()]:`
+
+error: aborting due to previous error
+