Do not ICE on generics mismatch with non-local traits

Fixes #49841

src/test/compile-fail/impl-trait/impl-generic-mismatch.rs

@@ -28,4 +28,15 @@ impl Bar for () {
     //~^ Error method `bar` has incompatible signature for trait
 }
 
+// With non-local trait (#49841):
+
+use std::hash::{Hash, Hasher};
+
+struct X;
+
+impl Hash for X {
+    fn hash(&self, hasher: &mut impl Hasher) {}
+    //~^ Error method `hash` has incompatible signature for trait
+}
+
 fn main() {}