Add regression test

Author: oli-obk

tests/ui/rfc-2632-const-trait-impl/function-pointer-does-not-require-const.rs

@@ -0,0 +1,8 @@
+// check-pass
+
+type I32Cmp = fn(&i32, &i32) -> core::cmp::Ordering;
+pub const fn min_by_i32() -> fn(i32, i32, I32Cmp) -> i32 {
+    core::cmp::min_by
+}
+
+fn main() {}