add a regression test

tests/ui/traits/next-solver/normalize/ambig-goal-infer-in-type-oulives.rs

@@ -0,0 +1,29 @@
+//@ check-pass
+//@ compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicitly enabled)
+
+// Regression test for an ICE when trying to bootstrap rustc
+// with #125343. An ambiguous goal returned a `TypeOutlives`
+// constraint referencing an inference variable. This inference
+// variable was created inside of the goal, causing it to be
+// unconstrained in the caller. This then caused an ICE in MIR
+// borrowck.
+
+struct Foo<T>(T);
+trait Extend<T> {
+    fn extend<I: IntoIterator<Item = T>>(iter: I);
+}
+
+impl<T> Extend<T> for Foo<T> {
+    fn extend<I: IntoIterator<Item = T>>(_: I) {
+        todo!()
+    }
+}
+
+impl<'a, T: 'a + Copy> Extend<&'a T> for Foo<T> {
+    fn extend<I: IntoIterator<Item = &'a T>>(iter: I) {
+        <Self as Extend<T>>::extend(iter.into_iter().copied())
+    }
+}
+
+fn main() {}