add tests for unsound subtype handling

tests/ui/codegen/subtyping-impacts-selection-1.rs

@@ -0,0 +1,44 @@
+// run-pass
+// revisions: mir codegen
+//[mir] compile-flags: -Zmir-opt-level=3
+//[codegen] compile-flags: -Zmir-opt-level=0
+
+// A regression test for #107205
+#![allow(coherence_leak_check)]
+struct Foo<T: 'static>(T);
+
+fn useful<'a>(_: &'a u8) {}
+
+trait GetInner {
+    type Assoc;
+    fn muahaha(&mut self) -> Self::Assoc;
+}
+
+impl GetInner for Foo<fn(&'static u8)> {
+    type Assoc = String;
+    fn muahaha(&mut self) -> String {
+        String::from("I am a string")
+    }
+}
+
+impl GetInner for Foo<for<'a> fn(&'a u8)> {
+    type Assoc = [usize; 3];
+    fn muahaha(&mut self) -> [usize; 3] {
+        [100; 3]
+    }
+}
+
+fn break_me(hr_fnptr: Box<Foo::<for<'a> fn(&'a u8)>>) -> Box<dyn GetInner<Assoc = String>> {
+    let lr_fnptr = hr_fnptr as Box<Foo<fn(&'static u8)>>;
+    lr_fnptr as Box<dyn GetInner<Assoc = String>>
+}
+
+fn main() {
+    drop(Box::new(Foo(useful as fn(&'static u8))) as Box<dyn GetInner<Assoc = String>>);
+    drop(Box::new(Foo(useful as fn(&u8))) as Box<dyn GetInner<Assoc = [usize; 3]>>);
+
+    let mut any = break_me(Box::new(Foo(useful)));
+
+    let evil_string = any.muahaha();
+    assert_eq!(evil_string, "I am a string");
+}

tests/ui/codegen/subtyping-impacts-selection-2.rs

@@ -0,0 +1,12 @@
+// run-pass
+// revisions: mir codegen
+//[mir] compile-flags: -Zmir-opt-level=3
+//[codegen] compile-flags: -Zmir-opt-level=0
+
+// A regression test for #107205
+
+const X: for<'b> fn(&'b ()) = |&()| ();
+fn main() {
+    let dyn_debug = Box::new(X) as Box<fn(&'static ())> as Box<dyn Send>;
+    drop(dyn_debug)
+}