Don't ICE when non-self part of trait goal is constrained in new solver

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -274,7 +274,9 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
 
         let goal =
             goal.with(self.tcx(), goal.predicate.with_self_ty(self.tcx(), normalized_self_ty));
-        debug_assert_eq!(goal, self.resolve_vars_if_possible(goal));
+        // Vars that show up in the rest of the goal substs may have been constrained by
+        // normalizing the self type as well, since type variables are not uniquified.
+        let goal = self.resolve_vars_if_possible(goal);
 
         let mut candidates = vec![];
 

tests/ui/traits/next-solver/normalize-self-type-constrains-trait-args.rs

@@ -0,0 +1,24 @@
+//@ check-pass
+
+// This goal is also possible w/ a GAT, but lazy_type_alias
+// makes the behavior a bit more readable.
+#![feature(lazy_type_alias)]
+//~^ WARN the feature `lazy_type_alias` is incomplete
+
+struct Wr<T>(T);
+trait Foo {}
+impl Foo for Wr<i32> {}
+
+type Alias<T> = (T,)
+    where Wr<T>: Foo;
+
+fn hello<T>() where Alias<T>: Into<(T,)>, Wr<T>: Foo {}
+
+fn main() {
+    // When calling `hello`, proving `Alias<?0>: Into<(?0,)>` will require
+    // normalizing the self type of the goal. This will emit the where
+    // clause `Wr<?0>: Foo`, which constrains `?0` in both the self type
+    // *and* the non-self part of the goal. That used to trigger a debug
+    // assertion.
+    hello::<_>();
+}

tests/ui/traits/next-solver/normalize-self-type-constrains-trait-args.stderr

@@ -0,0 +1,11 @@
+warning: the feature `lazy_type_alias` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/normalize-self-type-constrains-trait-args.rs:5:12
+   |
+LL | #![feature(lazy_type_alias)]
+   |            ^^^^^^^^^^^^^^^
+   |
+   = note: see issue #112792 <https://github.com/rust-lang/rust/issues/112792> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+