add regression test for #105009

Author: lcnr

src/test/ui/mir/issue-95978-validator-lifetime-comparison.rs renamed to src/test/ui/mir/validate/issue-95978-validator-lifetime-comparison.rs

@@ -0,0 +1,52 @@
+// Regression test for #105009. the issue here was that even after the `RevealAll` pass,
+// `validate` still used `Reveal::UserFacing`. This meant that it now ends up comparing
+// opaque types with their revealed version, resulting in an ICE.
+//
+// We're using these flags to run the `RevealAll` pass while making it less likely to
+// accidentally removing the assignment from `Foo<fn_ptr>` to `Foo<fn_def>`.
+
+// compile-flags: -Zinline_mir=yes -Zmir-opt-level=0 -Zvalidate-mir
+// run-pass
+
+use std::hint::black_box;
+
+trait Func {
+    type Ret: Id;
+}
+
+trait Id {
+    type Assoc;
+}
+impl Id for u32 {
+    type Assoc = u32;
+}
+impl Id for i32 {
+    type Assoc = i32;
+}
+
+impl<F: FnOnce() -> R, R: Id> Func for F {
+    type Ret = R;
+}
+
+fn bar() -> impl Copy + Id {
+    0u32
+}
+
+struct Foo<T: Func> {
+    _func: T,
+    value: Option<<<T as Func>::Ret as Id>::Assoc>,
+}
+
+fn main() {
+    let mut fn_def = black_box(Foo {
+        _func: bar,
+        value: None,
+    });
+    let fn_ptr = black_box(Foo {
+        _func: bar as fn() -> _,
+        value: None,
+    });
+
+    fn_def.value = fn_ptr.value;
+    black_box(fn_def);
+}