Make sure to register imported variables when importing specializations

Closes #4671

crates/compiler/solve/tests/solve_expr.rs

@@ -8431,4 +8431,29 @@ mod solve_expr {
         @"n : Dec"
         );
     }
+
+    #[test]
+    fn resolve_set_eq_issue_4671() {
+        infer_queries!(
+            indoc!(
+                r#"
+                app "test" provides [main] to "./platform"
+
+                main =
+                    s1 : Set U8
+                    s1 = Set.empty
+
+                    s2 : Set Str
+                    s2 = Set.empty
+
+                    Bool.isEq s1 s1 && Bool.isEq s2 s2
+                #   ^^^^^^^^^          ^^^^^^^^^
+                "#
+            ),
+        @r###"
+        Set#Bool.isEq(17) : Set U8, Set U8 -[[Set.isEq(17)]]-> Bool
+        Set#Bool.isEq(17) : Set Str, Set Str -[[Set.isEq(17)]]-> Bool
+        "###
+        );
+    }
 }