Rollup merge of #88592 - b-naber:region_substs, r=oli-obk

Fix ICE in const check

Fixes https://github.com/rust-lang/rust/issues/88433

src/test/ui/const-generics/const_trait_fn-issue-88433.rs

@@ -0,0 +1,26 @@
+// build-pass
+
+#![feature(const_trait_impl)]
+
+trait Func<T> {
+    type Output;
+
+    fn call_once(self, arg: T) -> Self::Output;
+}
+
+
+struct Closure;
+
+impl const Func<&usize> for Closure {
+    type Output = usize;
+
+    fn call_once(self, arg: &usize) -> Self::Output {
+        *arg
+    }
+}
+
+enum Bug<T = [(); Closure.call_once(&0) ]> {
+    V(T),
+}
+
+fn main() {}