Fix crash on record update if constructor definition is invalid

Fixes #5296

Author: seafoamteal

CHANGELOG.md

@@ -325,3 +325,8 @@
   detected when rebuilding the root project, causing the compiler to report
   errors about missing modules.
   ([daniellionel01](https://github.com/daniellionel01))
+
+- Fixed a bug where the compiler would crash when type-checking record
+  updates if the constructor definition has a positional field defined after
+  labelled fields.
+  ([Hari Mohan](https://gituhub.com/seafoamteal))

compiler-core/src/type_/expression.rs

@@ -3251,10 +3251,19 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
                     }
                     .expect("Variant has already checked to be valid");
 
+                // Positional fields must always be defined before any labelled fields.
+                // As such, we expect that field indices 0..(positional_fields.length)
+                // all correspond to positional fields. If this is not the case, then the
+                // user currently has a mistake in the definition of their custom type.
+                //
+                // For example, when a user starts adding a new labelled field to a custom
+                // type constructor, the field label will be parsed as a custom type name
+                // until they type the colon i.e. as a new positional field.
                 let type_ = positional_fields
                     .get(index as usize)
-                    .expect("Field exists")
+                    .ok_or_else(|| Error::UnlabelledAfterlabelled { location })?
                     .clone();
+
                 let mut type_vars = im::HashMap::new();
                 let accessor_type = self.instantiate(accessor_type, &mut type_vars);
                 let type_ = self.instantiate(type_, &mut type_vars);

compiler-core/src/type_/tests/errors.rs

@@ -3424,3 +3424,20 @@ pub fn main() {
 "
     );
 }
+
+// https://github.com/gleam-lang/gleam/issues/5296
+#[test]
+fn no_crash_on_record_update_when_constructor_definition_is_invalid() {
+    assert_module_error!(
+        "
+pub type Wibble {
+  Wibble(a: Int, b: Int, Bool)
+}
+
+pub fn main() {
+  let one = Wibble(False, a: 5, b: 6)
+  let two = Wibble(..one, b: 1)
+}
+        "
+    );
+}

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__no_crash_on_record_update_when_constructor_definition_is_invalid.snap

@@ -0,0 +1,32 @@
+---
+source: compiler-core/src/type_/tests/errors.rs
+expression: "\npub type Wibble {\n  Wibble(a: Int, b: Int, Bool)\n}\n\npub fn main() {\n  let one = Wibble(False, a: 5, b: 6)\n  let two = Wibble(..one, b: 1)\n}\n        "
+---
+----- SOURCE CODE
+
+pub type Wibble {
+  Wibble(a: Int, b: Int, Bool)
+}
+
+pub fn main() {
+  let one = Wibble(False, a: 5, b: 6)
+  let two = Wibble(..one, b: 1)
+}
+        
+
+----- ERROR
+error: Unlabelled argument after labelled argument
+  ┌─ /src/one/two.gleam:3:26
+  │
+3 │   Wibble(a: Int, b: Int, Bool)
+  │                          ^^^^
+
+All unlabelled arguments must come before any labelled arguments.
+
+error: Unlabelled argument after labelled argument
+  ┌─ /src/one/two.gleam:8:13
+  │
+8 │   let two = Wibble(..one, b: 1)
+  │             ^^^^^^^^^^^^^^^^^^^
+
+All unlabelled arguments must come before any labelled arguments.