add regression test for rust-lang#73120

Author: lcnr

src/test/ui/const-generics/issues/auxiliary/const_generic_issues_lib.rs

@@ -0,0 +1,14 @@
+#![feature(const_generics)]
+
+// All of these three items must be in `lib2` to reproduce the error
+
+pub trait TypeFn {
+    type Output;
+}
+
+pub struct GenericType<const B: i8>;
+
+// Removing the braces around `42` resolves the crash
+impl TypeFn for GenericType<{ 42 }> {
+    type Output = ();
+}

src/test/ui/const-generics/issues/issue-73120.rs

@@ -0,0 +1,8 @@
+// check-pass
+// aux-build:const_generic_issues_lib.rs
+extern crate const_generic_issues_lib as lib2;
+fn unused_function(
+    _: <lib2::GenericType<42> as lib2::TypeFn>::Output
+) {}
+
+fn main() {}