report error for invalid alias argument patterns

compiler/can/src/def.rs

@@ -79,7 +79,10 @@ enum PendingDef<'a> {
         ann: &'a Located<ast::TypeAnnotation<'a>>,
     },
 
-    ShadowedAlias,
+    /// An invalid alias, that is ignored in the rest of the pipeline
+    /// e.g. a shadowed alias, or a definition like `MyAlias 1 : Int`
+    /// with an incorrect pattern
+    InvalidAlias,
 }
 
 #[derive(Clone, Debug, PartialEq)]
@@ -899,9 +902,8 @@ fn canonicalize_pending_def<'a>(
                 .union(&can_ann.introduced_variables);
         }
 
-        ShadowedAlias => {
-            // Since this alias was shadowed, it gets ignored and has no
-            // effect on the output.
+        InvalidAlias => {
+            // invalid aliases (shadowed, incorrect patterns) get ignored
         }
         TypedBody(loc_pattern, loc_can_pattern, loc_ann, loc_expr) => {
             let ann =
@@ -1365,7 +1367,13 @@ fn to_pending_def<'a>(
                                 });
                             }
                             _ => {
-                                panic!("TODO gracefully handle an invalid pattern appearing where a type alias rigid var should be.");
+                                // any other pattern in this position is a syntax error.
+                                env.problems.push(Problem::InvalidAliasRigid {
+                                    alias_name: symbol,
+                                    region: loc_var.region,
+                                });
+
+                                return PendingDef::InvalidAlias;
                             }
                         }
                     }
@@ -1386,7 +1394,7 @@ fn to_pending_def<'a>(
                         shadow: loc_shadowed_symbol,
                     });
 
-                    PendingDef::ShadowedAlias
+                    PendingDef::InvalidAlias
                 }
             }
         }

compiler/problem/src/can.rs

@@ -50,6 +50,10 @@ pub enum Problem {
         annotation_pattern: Region,
         def_pattern: Region,
     },
+    InvalidAliasRigid {
+        alias_name: Symbol,
+        region: Region,
+    },
 }
 
 #[derive(Clone, Debug, PartialEq)]

compiler/reporting/src/error/canonicalize.rs

@@ -247,6 +247,21 @@ pub fn can_problem<'b>(
             alloc.region(Region::span_across(annotation_pattern, def_pattern)),
             alloc.reflow("Is it a typo? If not, put either a newline or comment between them."),
         ]),
+        Problem::InvalidAliasRigid { alias_name, region } => alloc.stack(vec![
+            alloc.concat(vec![
+                alloc.reflow("This pattern in the definition of "),
+                alloc.symbol_unqualified(alias_name),
+                alloc.reflow(" is not what I expect:"),
+            ]),
+            alloc.region(region),
+            alloc.concat(vec![
+                alloc.reflow("Only type variables like "),
+                alloc.type_variable("a".into()),
+                alloc.reflow(" or "),
+                alloc.type_variable("value".into()),
+                alloc.reflow(" can occur in this position."),
+            ]),
+        ]),
         Problem::RuntimeError(runtime_error) => pretty_runtime_error(alloc, runtime_error),
     };
 

compiler/reporting/tests/test_reporting.rs

@@ -2714,6 +2714,41 @@ mod test_reporting {
         )
     }
 
+    #[test]
+    fn invalid_alias_rigid_var_pattern() {
+        report_problem_as(
+            indoc!(
+                r#"
+                MyAlias 1 : Int
+
+                4
+                "#
+            ),
+            indoc!(
+                r#"
+                -- SYNTAX PROBLEM --------------------------------------------------------------
+
+                This pattern in the definition of `MyAlias` is not what I expect:
+
+                1 ┆  MyAlias 1 : Int
+                  ┆          ^
+
+                Only type variables like `a` or `value` can occur in this position.
+
+                -- SYNTAX PROBLEM --------------------------------------------------------------
+
+                `MyAlias` is not used anywhere in your code.
+
+                1 ┆  MyAlias 1 : Int
+                  ┆  ^^^^^^^^^^^^^^^
+
+                If you didn't intend on using `MyAlias` then remove it so future readers
+                of your code don't wonder why it is there.
+                "#
+            ),
+        )
+    }
+
     #[test]
     fn invalid_num() {
         report_problem_as(