Skip expected_projection if proj_ty is forbidden

compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs

@@ -242,7 +242,9 @@ impl<T> Trait<T> for X {
                         }
                     }
                     (ty::Alias(ty::Projection | ty::Inherent, proj_ty), _)
-                        if !tcx.is_impl_trait_in_trait(proj_ty.def_id) =>
+                        if !tcx.is_impl_trait_in_trait(proj_ty.def_id)
+                            // proj ty is forbidden as the type of a const generic parameter, skip it
+                            && !matches!(cause.code(), ObligationCauseCode::WhereClause { .. }) =>
                     {
                         self.expected_projection(
                             diag,

tests/ui/specialization/skip-expect-proj-if-forbidden-issue-125757.rs

@@ -0,0 +1,19 @@
+#![feature(specialization)]
+#![allow(incomplete_features)]
+
+trait Trait {
+    type Type;
+}
+
+impl Trait for i32 {
+    default type Type = i32;
+}
+
+struct Wrapper<const C: <i32 as Trait>::Type> {}
+//~^ ERROR `<i32 as Trait>::Type` is forbidden as the type of a const generic parameter
+
+impl<const C: usize> Wrapper<C> {}
+//~^ ERROR the constant `C` is not of type `<i32 as Trait>::Type`
+//~^^ ERROR mismatched types
+
+fn main() {}

tests/ui/specialization/skip-expect-proj-if-forbidden-issue-125757.stderr

@@ -0,0 +1,34 @@
+error: `<i32 as Trait>::Type` is forbidden as the type of a const generic parameter
+  --> $DIR/skip-expect-proj-if-forbidden-issue-125757.rs:12:25
+   |
+LL | struct Wrapper<const C: <i32 as Trait>::Type> {}
+   |                         ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+
+error: the constant `C` is not of type `<i32 as Trait>::Type`
+  --> $DIR/skip-expect-proj-if-forbidden-issue-125757.rs:15:22
+   |
+LL | impl<const C: usize> Wrapper<C> {}
+   |                      ^^^^^^^^^^ expected associated type, found `usize`
+   |
+note: required by a bound in `Wrapper`
+  --> $DIR/skip-expect-proj-if-forbidden-issue-125757.rs:12:16
+   |
+LL | struct Wrapper<const C: <i32 as Trait>::Type> {}
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Wrapper`
+
+error[E0308]: mismatched types
+  --> $DIR/skip-expect-proj-if-forbidden-issue-125757.rs:15:30
+   |
+LL | impl<const C: usize> Wrapper<C> {}
+   |                              ^ expected associated type, found `usize`
+   |
+   = note: expected associated type `<i32 as Trait>::Type`
+                         found type `usize`
+   = help: consider constraining the associated type `<i32 as Trait>::Type` to `usize`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.