Simply do not ICE

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -109,7 +109,10 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
         let mut errors = Vec::new();
         for i in 0.. {
             if !infcx.tcx.recursion_limit().value_within_limit(i) {
-                unimplemented!("overflowed on pending obligations: {:?}", self.obligations);
+                // Only return true errors that we have accumulated while processing;
+                // keep ambiguities around, *including overflows*, because they shouldn't
+                // be considered true errors.
+                return errors;
             }
 
             let mut has_changed = false;

tests/ui/traits/next-solver/coherence-fulfill-overflow.rs

@@ -0,0 +1,15 @@
+//@ compile-flags: -Znext-solver=coherence
+
+#![recursion_limit = "10"]
+
+trait Trait {}
+
+struct W<T: ?Sized>(*const T);
+trait TwoW {}
+impl<T: ?Sized + TwoW> TwoW for W<W<T>> {}
+
+impl<T: ?Sized + TwoW> Trait for W<T> {}
+impl<T: ?Sized + TwoW> Trait for T {}
+//~^ ERROR conflicting implementations of trait `Trait` for type `W
+
+fn main() {}

tests/ui/traits/next-solver/coherence-fulfill-overflow.stderr

@@ -0,0 +1,11 @@
+error[E0119]: conflicting implementations of trait `Trait` for type `W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<_>>>>>>>>>>>>>>>>>>>>>`
+  --> $DIR/coherence-fulfill-overflow.rs:12:1
+   |
+LL | impl<T: ?Sized + TwoW> Trait for W<T> {}
+   | ------------------------------------- first implementation here
+LL | impl<T: ?Sized + TwoW> Trait for T {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<W<_>>>>>>>>>>>>>>>>>>>>>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.