add regression test

Author: jdonszelmann

tests/ui/traits/next-solver/generalize/constrain-inference-during-normalize.rs

@@ -0,0 +1,25 @@
+// revisions: next old
+//[next] compile-flags: -Znext-solver
+//@ check-pass
+// Regression test for https://github.com/rust-lang/rust/issues/154173.
+// The ICE there was caused by a (flawed) attempt to eagerly normalize during generalization.
+// The normalize would constrain other inference variables, which we couldn't deal with.
+
+trait Trait<T> {
+    type Assoc;
+}
+
+impl Trait<u32> for () {
+    type Assoc = u32;
+}
+
+trait Eq {}
+impl<C: Trait<T>, T> Eq for (C, T, <C as Trait<T>>::Assoc) {}
+fn foo<A>()
+where
+    ((), A, A): Eq
+{}
+
+fn main() {
+    foo::<_>();
+}