add test

Author: lcnr

tests/ui/traits/next-solver/coherence/ambiguity-causes-visitor-hang.rs

@@ -0,0 +1,56 @@
+// Computing the ambiguity causes for the overlap ended up
+// causing an exponential blowup when recursing into the normalization
+// goals for `<Box<?t> as RecursiveSuper>::Assoc`. This test
+// takes multiple minutes when doing so and less than a second
+// otherwise.
+
+//@ compile-flags: -Znext-solver=coherence
+
+trait RecursiveSuper:
+    Super<
+        A0 = Self::Assoc,
+        A1 = Self::Assoc,
+        A2 = Self::Assoc,
+        A3 = Self::Assoc,
+        A4 = Self::Assoc,
+        A5 = Self::Assoc,
+        A6 = Self::Assoc,
+        A7 = Self::Assoc,
+        A8 = Self::Assoc,
+        A9 = Self::Assoc,
+        A10 = Self::Assoc,
+        A11 = Self::Assoc,
+        A12 = Self::Assoc,
+        A13 = Self::Assoc,
+        A14 = Self::Assoc,
+        A15 = Self::Assoc,
+    >
+{
+    type Assoc;
+}
+
+trait Super {
+    type A0;
+    type A1;
+    type A2;
+    type A3;
+    type A4;
+    type A5;
+    type A6;
+    type A7;
+    type A8;
+    type A9;
+    type A10;
+    type A11;
+    type A12;
+    type A13;
+    type A14;
+    type A15;
+}
+
+trait Overlap {}
+impl<T: RecursiveSuper> Overlap for T {}
+impl<T> Overlap for Box<T> {}
+//~^ ERROR conflicting implementations of trait `Overlap` for type `Box<_>`
+
+fn main() {}

tests/ui/traits/next-solver/coherence/ambiguity-causes-visitor-hang.stderr

@@ -0,0 +1,14 @@
+error[E0119]: conflicting implementations of trait `Overlap` for type `Box<_>`
+  --> $DIR/ambiguity-causes-visitor-hang.rs:53:1
+   |
+LL | impl<T: RecursiveSuper> Overlap for T {}
+   | ------------------------------------- first implementation here
+LL | impl<T> Overlap for Box<T> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Box<_>`
+   |
+   = note: downstream crates may implement trait `Super` for type `std::boxed::Box<_>`
+   = note: downstream crates may implement trait `RecursiveSuper` for type `std::boxed::Box<_>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.