Also resolve the type of constants, even if we already turned it into an error constant

Author: oli-obk

compiler/rustc_hir_typeck/src/writeback.rs

@@ -865,6 +865,7 @@ impl<'cx, 'tcx> TypeFolder<TyCtxt<'tcx>> for Resolver<'cx, 'tcx> {
         self.handle_term(ct, ty::Const::outer_exclusive_binder, |tcx, guar| {
             ty::Const::new_error(tcx, guar, ct.ty())
         })
+        .super_fold_with(self)
     }
 
     fn fold_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {

tests/ui/type-alias-impl-trait/const_generic_type.rs

@@ -0,0 +1,10 @@
+//@edition: 2021
+
+#![feature(type_alias_impl_trait)]
+type Bar = impl std::fmt::Display;
+
+async fn test<const N: crate::Bar>() {}
+//~^ ERROR: type annotations needed
+//~| ERROR: `Bar` is forbidden as the type of a const generic parameter
+
+fn main() {}

tests/ui/type-alias-impl-trait/const_generic_type.stderr

@@ -0,0 +1,19 @@
+error[E0283]: type annotations needed
+  --> $DIR/const_generic_type.rs:6:1
+   |
+LL | async fn test<const N: crate::Bar>() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
+   |
+   = note: cannot satisfy `_: std::fmt::Display`
+
+error: `Bar` is forbidden as the type of a const generic parameter
+  --> $DIR/const_generic_type.rs:6:24
+   |
+LL | async fn test<const N: crate::Bar>() {}
+   |                        ^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0283`.