add regression test for rust-lang#50439

Author: TaKO8Ki

src/test/ui/consts/issue-50439.rs

@@ -0,0 +1,29 @@
+#![feature(specialization)]
+#![allow(incomplete_features)]
+
+pub trait ReflectDrop {
+    const REFLECT_DROP: bool = false;
+}
+
+impl<T> ReflectDrop for T where T: Clone {}
+
+pub trait PinDropInternal {
+    fn is_valid()
+    where
+        Self: ReflectDrop;
+}
+
+struct Bears<T>(T);
+
+default impl<T> ReflectDrop for Bears<T> {}
+
+impl<T: Sized> PinDropInternal for Bears<T> {
+    fn is_valid()
+    where
+        Self: ReflectDrop,
+    {
+        let _ = [(); 0 - !!(<Bears<T> as ReflectDrop>::REFLECT_DROP) as usize]; //~ ERROR constant expression depends on a generic parameter
+    }
+}
+
+fn main() {}

src/test/ui/consts/issue-50439.stderr

@@ -0,0 +1,10 @@
+error: constant expression depends on a generic parameter
+  --> $DIR/issue-50439.rs:25:22
+   |
+LL |         let _ = [(); 0 - !!(<Bears<T> as ReflectDrop>::REFLECT_DROP) as usize];
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error: aborting due to previous error
+