🐛 Recognize missing pattern in alternative as syntax error

Author: fruno-bulax

compiler-core/src/parse.rs

@@ -1609,11 +1609,18 @@ where
         match &patterns.first() {
             Some(lead) => {
                 let mut alternative_patterns = vec![];
-                loop {
-                    if self.maybe_one(&Token::Vbar).is_none() {
-                        break;
+                while let Some((vbar_start, vbar_end)) = self.maybe_one(&Token::Vbar) {
+                    let patterns = self.parse_patterns(PatternPosition::CaseClause)?;
+                    if patterns.is_empty() {
+                        return parse_error(
+                            ParseErrorType::ExpectedPattern,
+                            SrcSpan {
+                                start: vbar_start,
+                                end: vbar_end,
+                            },
+                        );
                     }
-                    alternative_patterns.push(self.parse_patterns(PatternPosition::CaseClause)?);
+                    alternative_patterns.push(patterns);
                 }
                 let guard = self.parse_case_clause_guard()?;
                 let (arr_s, arr_e) = self

compiler-core/src/parse/snapshots/gleam_core__parse__tests__case_alternative_clause_no_subject.snap

@@ -0,0 +1,20 @@
+---
+source: compiler-core/src/parse/tests.rs
+expression: "\nfn main() {\n    case 1 {\n      1 | -> 1\n      _ -> 1\n    }\n}\n"
+---
+----- SOURCE CODE
+
+fn main() {
+    case 1 {
+      1 | -> 1
+      _ -> 1
+    }
+}
+
+
+----- ERROR
+error: Syntax error
+  ┌─ /src/parse/error.gleam:4:9
+  │
+4 │       1 | -> 1
+  │         ^ I was expecting a pattern after this

compiler-core/src/parse/snapshots/gleam_core__parse__tests__case_clause_no_subject.snap

@@ -0,0 +1,24 @@
+---
+source: compiler-core/src/parse/tests.rs
+expression: "\nfn main() {\n    case 1 {\n      -> 1\n      _ -> 2\n    }\n}\n"
+---
+----- SOURCE CODE
+
+fn main() {
+    case 1 {
+      -> 1
+      _ -> 2
+    }
+}
+
+
+----- ERROR
+error: Syntax error
+  ┌─ /src/parse/error.gleam:4:7
+  │
+4 │       -> 1
+  │       ^^ I was not expecting this
+
+Found `->`, expected one of: 
+- `}`
+- a case clause

compiler-core/src/parse/tests.rs

@@ -1144,6 +1144,34 @@ fn main() {
     );
 }
 
+#[test]
+fn case_clause_no_subject() {
+    assert_module_error!(
+        "
+fn main() {
+    case 1 {
+      -> 1
+      _ -> 2
+    }
+}
+"
+    );
+}
+
+#[test]
+fn case_alternative_clause_no_subject() {
+    assert_module_error!(
+        "
+fn main() {
+    case 1 {
+      1 | -> 1
+      _ -> 1
+    }
+}
+"
+    );
+}
+
 #[test]
 fn use_invalid_assignments() {
     assert_module_error!(