Add test for projection in const generics

Author: GuillaumeGomez

src/test/ui/const-generics/projection-as-arg-const.rs

@@ -0,0 +1,25 @@
+// run-pass
+
+#![allow(incomplete_features)] // This is needed for the `adt_const_params` feature.
+#![feature(associated_type_defaults)]
+
+pub trait Identity: Sized {
+    type Identity = Self;
+    // ^ This is needed because otherwise we get:
+    // error: internal compiler error: compiler/rustc_hir_analysis/src/collect/type_of.rs:271:17: \
+    // associated type missing default
+    //  --> test.rs:6:5
+    //   |
+    // 6 |     type Identity;
+    //   |     ^^^^^^^^^^^^^^
+}
+
+impl<T> Identity for T {}
+
+pub fn foo<const X: <i32 as Identity>::Identity>() {
+    assert!(X == 12);
+}
+
+fn main() {
+    foo::<12>();
+}