Add specific parser error for angle-bracket generics in function definitions

Author: AaronC81

compiler-core/src/parse.rs

@@ -2081,6 +2081,15 @@ where
                 n,
             ));
         }
+        if let Some((less_start, less_end)) = self.maybe_one(&Token::Less) {
+            return Err(ParseError {
+                error: ParseErrorType::FunctionDefinitionAngleGenerics,
+                location: SrcSpan {
+                    start: less_start,
+                    end: less_end,
+                },
+            });
+        }
         let _ = self
             .expect_one(&Token::LeftParen)
             .map_err(|e| self.add_anon_function_hint(e))?;

compiler-core/src/parse/error.rs

@@ -117,6 +117,7 @@ pub enum ParseErrorType {
     EmptyGuardBlock,
     // When the use tries to define a constant inside a function
     ConstantInsideFunction,
+    FunctionDefinitionAngleGenerics, // fn something<T>() { ... }
 }
 
 pub(crate) struct ParseErrorDetails {
@@ -651,6 +652,21 @@ functions are not necessary.",
                 label_text: "Constants are not allowed inside functions".into(),
                 extra_labels: vec![],
             },
+
+            ParseErrorType::FunctionDefinitionAngleGenerics => ParseErrorDetails {
+                text: "\
+Generic function type variables do not need to be predeclared like they
+would be in some other languages, instead they are written with lowercase
+names.
+
+    fn example(argument: generic) -> generic
+
+See: https://tour.gleam.run/functions/generic-functions/"
+                    .into(),
+                hint: None,
+                label_text: "I was expecting `(` here.".into(),
+                extra_labels: vec![],
+            },
         }
     }
 }

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

@@ -0,0 +1,21 @@
+---
+source: compiler-core/src/parse/tests.rs
+expression: "fn id<T>(x: T) { x }"
+---
+----- SOURCE CODE
+fn id<T>(x: T) { x }
+
+----- ERROR
+error: Syntax error
+  ┌─ /src/parse/error.gleam:1:6
+  │
+1 │ fn id<T>(x: T) { x }
+  │      ^ I was expecting `(` here.
+
+Generic function type variables do not need to be predeclared like they
+would be in some other languages, instead they are written with lowercase
+names.
+
+    fn example(argument: generic) -> generic
+
+See: https://tour.gleam.run/functions/generic-functions/

compiler-core/src/parse/tests.rs

@@ -1942,3 +1942,8 @@ pub fn main() {
 "
     );
 }
+
+#[test]
+fn function_definition_angle_generics_error() {
+    assert_module_error!("fn id<T>(x: T) { x }");
+}