trait solver: Resolve region vars in max universe

compiler/rustc_type_ir/src/region_constraint.rs

@@ -1013,6 +1013,10 @@ impl<'a, Infcx: InferCtxtLike<Interner = I>, I: Interner> TypeVisitor<I>
         match t.kind() {
             TyKind::Placeholder(p) => self.max_universe = self.max_universe.max(p.universe),
             TyKind::Infer(InferTy::TyVar(inf)) => {
+                // Unlike `visit_region`, we don't resolve the variable first: callers
+                // computing assumptions bail on any non-region inference variable
+                // before reaching here, so a type infer var is always unresolved and
+                // has a universe.
                 let u = self.infcx.universe_of_ty(inf).unwrap();
                 debug!("var {inf:?} in universe {u:?}");
                 self.max_universe = self.max_universe.max(u);
@@ -1025,6 +1029,8 @@ impl<'a, Infcx: InferCtxtLike<Interner = I>, I: Interner> TypeVisitor<I>
         match c.kind() {
             ConstKind::Placeholder(p) => self.max_universe = self.max_universe.max(p.universe),
             ConstKind::Infer(rustc_type_ir::InferConst::Var(inf)) => {
+                // See the comment in `visit_ty`: a const infer var is always
+                // unresolved here, so unlike a region it needs no resolving first.
                 let u = self.infcx.universe_of_ct(inf).unwrap();
                 debug!("var {inf:?} in universe {u:?}");
                 self.max_universe = self.max_universe.max(u);
@@ -1037,9 +1043,20 @@ impl<'a, Infcx: InferCtxtLike<Interner = I>, I: Interner> TypeVisitor<I>
         match r.kind() {
             RegionKind::RePlaceholder(p) => self.max_universe = self.max_universe.max(p.universe),
             RegionKind::ReVar(var) => {
-                let u = self.infcx.universe_of_lt(var).unwrap();
-                debug!("var {var:?} in universe {u:?}");
-                self.max_universe = self.max_universe.max(u);
+                // The variable may already have been unified with another region.
+                // `universe_of_lt` returns `None` for a resolved variable, so resolve
+                // it first and inspect whatever it points at.
+                match self.infcx.opportunistic_resolve_lt_var(var).kind() {
+                    RegionKind::RePlaceholder(p) => {
+                        self.max_universe = self.max_universe.max(p.universe)
+                    }
+                    RegionKind::ReVar(var) => {
+                        let u = self.infcx.universe_of_lt(var).unwrap();
+                        debug!("var {var:?} in universe {u:?}");
+                        self.max_universe = self.max_universe.max(u);
+                    }
+                    _ => (),
+                }
             }
             _ => (),
         }

tests/ui/assumptions_on_binders/resolved-region-var-max-universe.rs

@@ -0,0 +1,26 @@
+//@ compile-flags: -Zassumptions-on-binders -Znext-solver=globally
+
+// Regression test for an ICE in the `MaxUniverse` region visitor. When computing
+// the max universe of a region constraint, a `ReVar` term could already have been
+// unified with another region. `universe_of_lt` returns `None` for such a resolved
+// variable, so the visitor used to `unwrap()` `None` and panic. It now resolves the
+// variable before inspecting its universe.
+
+#![feature(min_generic_const_args, inherent_associated_types, generic_const_items)]
+
+struct Parent<'a> {
+    a: &'a str,
+}
+
+impl<'a> Parent<'a> {
+    type const CT<T: 'a>: usize = 0;
+}
+
+fn check/*<T>*/()
+where
+    [(); Parent::CT::<T>]:,
+    //~^ ERROR cannot find type `T` in this scope
+{
+}
+
+fn main() {}

tests/ui/assumptions_on_binders/resolved-region-var-max-universe.stderr

@@ -0,0 +1,14 @@
+error[E0425]: cannot find type `T` in this scope
+  --> $DIR/resolved-region-var-max-universe.rs:21:23
+   |
+LL |     [(); Parent::CT::<T>]:,
+   |                       ^ not found in this scope
+   |
+help: you might be missing a type parameter
+   |
+LL | fn check<T>/*<T>*/()
+   |         +++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0425`.