Rollup merge of rust-lang#90578 - lcnr:add-test, r=Mark-Simulacrum

add const generics test

cc rust-lang#89829 (comment)

r? rust-lang/project-const-generics

src/test/ui/const-generics/invariant.rs

@@ -0,0 +1,33 @@
+#![feature(generic_const_exprs)]
+#![allow(incomplete_features)]
+use std::marker::PhantomData;
+
+trait SadBee {
+    const ASSOC: usize;
+}
+// fn(&'static ())` is a supertype of `for<'a> fn(&'a ())` while
+// we allow two different impls for these types, leading
+// to different const eval results.
+impl SadBee for for<'a> fn(&'a ()) {
+    const ASSOC: usize = 0;
+}
+impl SadBee for fn(&'static ()) {
+    //~^ WARNING conflicting implementations of trait
+    //~| WARNING this was previously accepted
+    const ASSOC: usize = 100;
+}
+
+struct Foo<T: SadBee>([u8; <T as SadBee>::ASSOC], PhantomData<T>)
+where
+    [(); <T as SadBee>::ASSOC]: ;
+
+fn covariant(
+    v: &'static Foo<for<'a> fn(&'a ())>
+) -> &'static Foo<fn(&'static ())> {
+    v //~ ERROR mismatched types
+}
+
+fn main() {
+    let y = covariant(&Foo([], PhantomData));
+    println!("{:?}", y.0);
+}

src/test/ui/const-generics/invariant.stderr

@@ -0,0 +1,26 @@
+warning: conflicting implementations of trait `SadBee` for type `for<'a> fn(&'a ())`
+  --> $DIR/invariant.rs:14:1
+   |
+LL | impl SadBee for for<'a> fn(&'a ()) {
+   | ---------------------------------- first implementation here
+...
+LL | impl SadBee for fn(&'static ()) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `for<'a> fn(&'a ())`
+   |
+   = note: `#[warn(coherence_leak_check)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #56105 <https://github.com/rust-lang/rust/issues/56105>
+   = note: this behavior recently changed as a result of a bug fix; see rust-lang/rust#56105 for details
+
+error[E0308]: mismatched types
+  --> $DIR/invariant.rs:27:5
+   |
+LL |     v
+   |     ^ one type is more general than the other
+   |
+   = note: expected reference `&'static Foo<fn(&'static ())>`
+              found reference `&'static Foo<for<'a> fn(&'a ())>`
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0308`.