Add regression test

src/test/ui/const-generics/issues/issue-64494.rs

@@ -0,0 +1,19 @@
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+trait Foo {
+    const VAL: usize;
+}
+
+trait MyTrait {}
+
+trait True {}
+struct Is<const T: bool>;
+impl True for Is<{true}> {}
+
+impl<T: Foo> MyTrait for T where Is<{T::VAL == 5}>: True {}
+//~^ ERROR constant expression depends on a generic parameter
+impl<T: Foo> MyTrait for T where Is<{T::VAL == 6}>: True {}
+//~^ ERROR constant expression depends on a generic parameter
+
+fn main() {}

src/test/ui/const-generics/issues/issue-64494.stderr

@@ -0,0 +1,18 @@
+error: constant expression depends on a generic parameter
+  --> $DIR/issue-64494.rs:14:53
+   |
+LL | impl<T: Foo> MyTrait for T where Is<{T::VAL == 5}>: True {}
+   |                                                     ^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error: constant expression depends on a generic parameter
+  --> $DIR/issue-64494.rs:16:53
+   |
+LL | impl<T: Foo> MyTrait for T where Is<{T::VAL == 6}>: True {}
+   |                                                     ^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error: aborting due to 2 previous errors
+