add const_evaluatable_checked test

src/test/ui/const-generics/const_evaluatable_checked/elaborate-trait-pred.rs

@@ -0,0 +1,24 @@
+// run-pass
+// Test that we use the elaborated predicates from traits
+// to satisfy const evaluatable predicates.
+#![feature(const_generics, const_evaluatable_checked)]
+#![allow(incomplete_features)]
+use std::mem::size_of;
+
+trait Foo: Sized
+where
+    [(); size_of::<Self>()]: Sized,
+{
+}
+
+impl Foo for u64 {}
+impl Foo for u32 {}
+
+fn foo<T: Foo>() -> [u8; size_of::<T>()] {
+    [0; size_of::<T>()]
+}
+
+fn main() {
+    assert_eq!(foo::<u32>(), [0; 4]);
+    assert_eq!(foo::<u64>(), [0; 8]);
+}