treat wildcards as rigids

compiler/can/src/module.rs

@@ -155,8 +155,12 @@ where
         }
     }
 
-    for (var, lowercase) in output.introduced_variables.name_by_var.clone() {
-        rigid_variables.insert(var, lowercase);
+    for (var, lowercase) in output.introduced_variables.name_by_var {
+        rigid_variables.insert(var, lowercase.clone());
+    }
+
+    for var in output.introduced_variables.wildcards {
+        rigid_variables.insert(var, "*".into());
     }
 
     let mut references = MutSet::default();

compiler/reporting/tests/test_reporting.rs

@@ -71,6 +71,10 @@ mod test_reporting {
             subs.rigid_var(var, name);
         }
 
+        for var in output.introduced_variables.wildcards {
+            subs.rigid_var(var, "*".into());
+        }
+
         let mut unify_problems = Vec::new();
         let (_content, mut subs) = infer_expr(subs, &mut unify_problems, &constraint, var);
 
@@ -6387,4 +6391,44 @@ mod test_reporting {
             ),
         )
     }
+
+    #[test]
+    fn num_too_general_wildcard() {
+        report_problem_as(
+            indoc!(
+                r#"
+                mult : Num *, F64 -> F64
+                mult = \a, b -> a * b
+
+                {}
+                "#
+            ),
+            indoc!(
+                r#"
+                ── TYPE MISMATCH ───────────────────────────────────────────────────────────────
+
+            "#
+            ),
+        )
+    }
+
+    #[test]
+    fn num_too_general_named() {
+        report_problem_as(
+            indoc!(
+                r#"
+                mult : Num a, F64 -> F64
+                mult = \a, b -> a * b
+
+                {}
+                "#
+            ),
+            indoc!(
+                r#"
+                ── TYPE MISMATCH ───────────────────────────────────────────────────────────────
+
+            "#
+            ),
+        )
+    }
 }