const-in-pattern: test that the PartialEq impl does not need to be const

tests/ui/traits/const-traits/pattern-custom-partial-eq

@@ -0,0 +1,53 @@
+//! Ensure that a `const fn` can match on constants of a type that is `PartialEq`
+//! but not `const PartialEq`.
+#![feature(const_trait_impl)]
+
+#[derive(Eq, PartialEq)]
+pub struct Y(u8);
+pub const GREEN: Y = Y(4);
+pub const fn is_green(x: Y) -> bool {
+    match x { GREEN => true, _ => false }
+}
+
+struct CustomEq;
+
+impl Eq for CustomEq {}
+impl PartialEq for CustomEq {
+    fn eq(&self, _: &Self) -> bool {
+        false
+    }
+}
+
+#[derive(PartialEq, Eq)]
+#[allow(unused)]
+enum Foo {
+    Bar,
+    Baz,
+    Qux(CustomEq),
+}
+
+const BAR_BAZ: Foo = if 42 == 42 {
+    Foo::Bar
+} else {
+    Foo::Qux(CustomEq) // dead arm
+};
+
+const EMPTY: &[CustomEq] = &[];
+
+const fn test() {
+    // BAR_BAZ itself is fine but the enum has other variants
+    // that are non-structural. Still, this should be accepted.
+    match Foo::Qux(CustomEq) {
+        BAR_BAZ => panic!(),
+        _ => {}
+    }
+
+    // Similarly, an empty slice of a type that is non-structural
+    // is accepted.
+    match &[CustomEq] as &[CustomEq] {
+        EMPTY => panic!(),
+        _ => {},
+    }
+}
+
+fn main() {}