Add regression test

Author: oli-obk

tests/ui/const-generics/generic_const_exprs/opaque_type.rs

@@ -0,0 +1,18 @@
+#![feature(generic_const_exprs, type_alias_impl_trait)]
+#![allow(incomplete_features)]
+
+type Foo = impl Sized;
+//~^ ERROR: cycle detected
+
+fn with_bound<const N: usize>() -> Foo
+where
+    [u8; (N / 2) as usize]: Sized,
+{
+    let _: [u8; (N / 2) as Foo] = [0; (N / 2) as usize];
+    //~^ ERROR: mismatched types
+    todo!()
+}
+
+fn main() {
+    with_bound::<4>();
+}

tests/ui/const-generics/generic_const_exprs/opaque_type.stderr

@@ -0,0 +1,22 @@
+error[E0308]: mismatched types
+  --> $DIR/opaque_type.rs:11:17
+   |
+LL | type Foo = impl Sized;
+   |            ---------- the found opaque type
+...
+LL |     let _: [u8; (N / 2) as Foo] = [0; (N / 2) as usize];
+   |                 ^^^^^^^^^^^^^^ expected `usize`, found opaque type
+   |
+   = note:     expected type `usize`
+           found opaque type `Foo`
+
+error[E0605]: non-primitive cast: `usize` as `Foo`
+  --> $DIR/opaque_type.rs:11:17
+   |
+LL |     let _: [u8; (N / 2) as Foo] = [0; (N / 2) as usize];
+   |                 ^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0308, E0605.
+For more information about an error, try `rustc --explain E0308`.