Check that we can constrain the hidden tpye of a TAIT used in a const generic type

Author: oli-obk

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

@@ -0,0 +1,10 @@
+error: `Bar` is forbidden as the type of a const generic parameter
+  --> $DIR/const_generic_type.rs:7:24
+   |
+LL | async fn test<const N: crate::Bar>() {
+   |                        ^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+
+error: aborting due to 1 previous error
+

tests/ui/type-alias-impl-trait/const_generic_type.stderr renamed to tests/ui/type-alias-impl-trait/const_generic_type.no_infer.stderr

@@ -1,15 +1,15 @@
 error[E0283]: type annotations needed
-  --> $DIR/const_generic_type.rs:6:1
+  --> $DIR/const_generic_type.rs:7:1
    |
-LL | async fn test<const N: crate::Bar>() {}
+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
+  --> $DIR/const_generic_type.rs:7:24
    |
-LL | async fn test<const N: crate::Bar>() {}
+LL | async fn test<const N: crate::Bar>() {
    |                        ^^^^^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`

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

@@ -1,10 +1,14 @@
 //@edition: 2021
+//@revisions: infer no_infer
 
 #![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
+async fn test<const N: crate::Bar>() {
+    //[no_infer]~^ ERROR: type annotations needed
+    //~^^ ERROR: `Bar` is forbidden as the type of a const generic parameter
+    #[cfg(infer)]
+    let x: u32 = N;
+}
 
 fn main() {}