Fixes #4984

compiler-core/src/parse.rs

@@ -2117,6 +2117,22 @@ where
         )?;
         let (_, rpar_e) =
             self.expect_one_following_series(&Token::RightParen, "a function parameter")?;
+
+        // Check for TypeScript-style return type annotation (:) instead of arrow (->)
+        if let Some((colon_start, colon_end)) = self.maybe_one(&Token::Colon) {
+            return Err(ParseError {
+                error: ParseErrorType::UnexpectedToken {
+                    token: Token::Colon,
+                    expected: vec!["`->`".into()],
+                    hint: Some("Maybe you meant to write return value type annotation?\nReturn type annotations are written using `->`, not `:`".into()),
+                },
+                location: SrcSpan {
+                    start: colon_start,
+                    end: colon_end,
+                },
+            });
+        };
+
         let return_annotation = self.parse_type_annotation(&Token::RArrow)?;
 
         let (body_start, body, end, end_position) = match self.maybe_one(&Token::LeftBrace) {

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

@@ -0,0 +1,20 @@
+---
+source: compiler-core/src/parse/tests.rs
+expression: "\npub fn main(): Nil {}\n        "
+---
+----- SOURCE CODE
+
+pub fn main(): Nil {}
+
+
+----- ERROR
+error: Syntax error
+  ┌─ /src/parse/error.gleam:2:14
+  │
+2 │ pub fn main(): Nil {}
+  │              ^ I was not expecting this
+
+Found `:`, expected one of: 
+- `->`
+Hint: Maybe you meant to write return value type annotation?
+Return type annotations are written using `->`, not `:`

compiler-core/src/parse/tests.rs

@@ -1992,3 +1992,12 @@ pub fn main() {
 "#
     );
 }
+
+#[test]
+fn wrong_function_return_type_declaration_using_colon_instead_of_right_arrow() {
+    assert_module_error!(
+        r#"
+pub fn main(): Nil {}
+        "#
+    );
+}