add known-bug test for unsound issue 105782

tests/ui/specialization/specialization-default-items-drop-coherence.rs

@@ -0,0 +1,30 @@
+// check-pass
+// known-bug: #105782
+
+// Should fail. Default items completely drop candidates instead of ambiguity,
+// which is unsound during coherence, since coherence requires completeness.
+
+#![feature(specialization)]
+#![allow(incomplete_features)]
+
+trait Default {
+   type Id;
+}
+
+impl<T> Default for T {
+   default type Id = T;
+}
+
+trait Overlap {
+   type Assoc;
+}
+
+impl Overlap for u32 {
+   type Assoc = usize;
+}
+
+impl Overlap for <u32 as Default>::Id {
+   type Assoc = Box<usize>;
+}
+
+fn main() {}