Add compile fail test for issue 27675

Author: 197g

src/test/compile-fail/issue-27675-unchecked-bounds.rs

@@ -0,0 +1,19 @@
+/// The compiler previously did not properly check the bound of `From` when it was used from type
+/// of the dyn trait object (use in `copy_any` below). Since the associated type is under user
+/// control in this usage, the compiler could be tricked to believe any type implemented any trait.
+/// This would ICE, except for pure marker traits like `Copy`. It did not require providing an
+/// instance of the dyn trait type, only name said type.
+trait Setup {
+    type From: Copy;
+}
+
+fn copy<U: Setup + ?Sized>(from: &U::From) -> U::From {
+    *from
+}
+
+pub fn copy_any<T>(t: &T) -> T {
+    copy::<dyn Setup<From=T>>(t)
+    //~^ ERROR the trait bound `T: Copy` is not satisfied
+}
+
+fn main() {}