Typecheck let bindings using annotated type

abs0luty · abs0luty · commit c30ec7cb6972 · 2025-10-28T00:00:26.000+04:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -29,6 +29,17 @@
 
   ([Surya Rose](https://github.com/GearsDatapacks))
 
+- Let bindings now continue to assume their annotated type after mismatches,
+  preventing misleading follow-up errors:
+  ```gleam
+  pub fn main() {
+    let x: String = 5 // type error: expected String, got Int
+    let y: Int = x    // valid
+    let z: String = x // type error: expected String, got Int
+  }
+  ```
+  ([Adi Salimgereyev](https://github.com/abs0luty))
+
 ### Build tool
 
 - The help text displayed by `gleam dev --help`, `gleam test --help`, and
diff --git a/compiler-core/src/type_/expression.rs b/compiler-core/src/type_/expression.rs
@@ -1976,44 +1976,48 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
         let type_ = value.type_();
         let kind = self.infer_assignment_kind(kind.clone());
 
-        // Ensure the pattern matches the type of the value
-        let mut pattern_typer = pattern::PatternTyper::new(
-            self.environment,
-            &self.implementations,
-            &self.current_function_definition,
-            &self.hydrator,
-            self.problems,
-            PatternPosition::LetAssignment,
-        );
+        let mut annotation_type_override = None;
 
-        let pattern = pattern_typer.infer_single_pattern(pattern, &value);
-
-        let minimum_required_version = pattern_typer.minimum_required_version;
-        if minimum_required_version > self.minimum_required_version {
-            self.minimum_required_version = minimum_required_version;
-        }
-
-        let pattern_typechecked_successfully = !pattern_typer.error_encountered;
-
-        // Check that any type annotation is accurate.
         if let Some(annotation) = &annotation {
             match self
                 .type_from_ast(annotation)
                 .map(|type_| self.instantiate(type_, &mut hashmap![]))
             {
                 Ok(annotated_type) => {
+                    let annotation_type_for_pattern = annotated_type.clone();
                     if let Err(error) = unify(annotated_type, type_.clone())
                         .map_err(|e| convert_unify_error(e, value.type_defining_location()))
                     {
                         self.problems.error(error);
                     }
+                    annotation_type_override = Some(annotation_type_for_pattern);
                 }
                 Err(error) => {
                     self.problems.error(error);
                 }
             }
         }
 
+        // Ensure the pattern matches the type of the value
+        let mut pattern_typer = pattern::PatternTyper::new(
+            self.environment,
+            &self.implementations,
+            &self.current_function_definition,
+            &self.hydrator,
+            self.problems,
+            PatternPosition::LetAssignment,
+        );
+
+        let pattern =
+            pattern_typer.infer_single_pattern(pattern, &value, annotation_type_override.clone());
+
+        let minimum_required_version = pattern_typer.minimum_required_version;
+        if minimum_required_version > self.minimum_required_version {
+            self.minimum_required_version = minimum_required_version;
+        }
+
+        let pattern_typechecked_successfully = !pattern_typer.error_encountered;
+
         // The exhaustiveness checker expects patterns to be valid and to type check;
         // if they are invalid, it will crash. Therefore, if any errors were found
         // when type checking the pattern, we don't perform the exhaustiveness check.
@@ -2029,7 +2033,7 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
         }
 
         let (output, not_exhaustive_error) =
-            self.check_let_exhaustiveness(location, value.type_(), &pattern);
+            self.check_let_exhaustiveness(location, type_.clone(), &pattern);
 
         match (&kind, not_exhaustive_error) {
             // The pattern is exhaustive in a let assignment, there's no problem here.
diff --git a/compiler-core/src/type_/pattern.rs b/compiler-core/src/type_/pattern.rs
@@ -317,7 +317,7 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
         for (pattern, subject) in multi_pattern.into_iter().zip(subjects) {
             let subject_variable = Self::subject_variable(subject);
 
-            let pattern = self.unify(pattern, subject.type_(), subject_variable);
+            let pattern = self.unify(pattern, subject.type_(), subject_variable, None);
             typed_multi.push(pattern);
         }
 
@@ -330,10 +330,16 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
         &mut self,
         pattern: UntypedPattern,
         subject: &TypedExpr,
+        subject_type_override: Option<Arc<Type>>,
     ) -> TypedPattern {
         let subject_variable = Self::subject_variable(subject);
-
-        let typed_pattern = self.unify(pattern, subject.type_(), subject_variable);
+        let subject_type = subject.type_();
+        let typed_pattern = self.unify(
+            pattern,
+            subject_type,
+            subject_variable,
+            subject_type_override,
+        );
         self.register_variables();
         typed_pattern
     }
@@ -453,7 +459,7 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
             .into_iter()
             .map(|option| {
                 analyse::infer_bit_array_option(option, |value, type_| {
-                    Ok(self.unify(value, type_, None))
+                    Ok(self.unify(value, type_, None, None))
                 })
             })
             .try_collect()
@@ -529,7 +535,7 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
             _ => segment_type,
         };
 
-        let typed_value = self.unify(*segment.value, type_.clone(), None);
+        let typed_value = self.unify(*segment.value, type_.clone(), None, None);
 
         match &typed_value {
             // We can't directly match on the contents of a `Box`, so we must
@@ -586,6 +592,18 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
         // in the inner scope, we can infer that the `some_wibble` variable is the `Wibble` variant
         //
         subject_variable: Option<EcoString>,
+        // An optional type annotation provided for the pattern.
+        //
+        // Example:
+        // ```gleam
+        // let assert [first, ..rest]: List(String) = numbers
+        // ```
+        //
+        // We pass `List(String)` annotation down so sub-patterns know
+        // that `first` must be a `String`. We still check `numbers`
+        // against its real inferred type and emit a mismatch if it is, say,
+        // `List(Int)`.
+        annotation_type: Option<Arc<Type>>,
     ) -> TypedPattern {
         match pattern {
             Pattern::Discard { name, location, .. } => {
@@ -595,7 +613,7 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
                     .discarded_names
                     .insert(name.clone(), location);
                 Pattern::Discard {
-                    type_,
+                    type_: annotation_type.unwrap_or_else(|| type_.clone()),
                     name,
                     location,
                 }
@@ -609,10 +627,11 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
                 origin,
                 ..
             } => {
-                self.insert_variable(&name, type_.clone(), location, origin.clone());
+                let variable_type = annotation_type.unwrap_or_else(|| type_.clone());
+                self.insert_variable(&name, variable_type.clone(), location, origin.clone());
 
                 Pattern::Variable {
-                    type_,
+                    type_: variable_type,
                     name,
                     location,
                     origin,
@@ -691,17 +710,24 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
                 pattern,
                 location,
             } => {
-                let pattern = self.unify(*pattern, type_, subject_variable);
+                let pattern_annotation = annotation_type.clone();
+                let pattern = self.unify(
+                    *pattern,
+                    type_.clone(),
+                    subject_variable,
+                    pattern_annotation,
+                );
 
                 if pattern.is_discard() {
                     self.problems.warning(Warning::UnusedDiscardPattern {
                         location,
                         name: name.clone(),
                     });
                 }
+                let assigned_type = annotation_type.unwrap_or_else(|| pattern.type_().clone());
                 self.insert_variable(
                     &name,
-                    pattern.type_().clone(),
+                    assigned_type.clone(),
                     location,
                     VariableOrigin {
                         syntax: VariableSyntax::AssignmentPattern,
@@ -766,28 +792,53 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
                 self.environment,
             ) {
                 Some(arguments) => {
-                    let type_ = arguments
+                    let element_type = arguments
                         .first()
                         .expect("Failed to get type argument of List")
                         .clone();
+                    let list_type = list(element_type.clone());
+                    let annotation_element_type = annotation_type.as_ref().and_then(|annotation| {
+                        annotation
+                            .get_app_arguments(
+                                Publicity::Public,
+                                PRELUDE_PACKAGE_NAME,
+                                PRELUDE_MODULE_NAME,
+                                "List",
+                                1,
+                                self.environment,
+                            )
+                            .and_then(|arguments| arguments.into_iter().next())
+                    });
                     let elements = elements
                         .into_iter()
-                        .map(|element| self.unify(element, type_.clone(), None))
+                        .map(|element| {
+                            self.unify(
+                                element,
+                                element_type.clone(),
+                                None,
+                                annotation_element_type.clone(),
+                            )
+                        })
                         .collect();
-                    let type_ = list(type_);
 
                     let tail = tail.map(|tail| {
+                        let tail_annotation = annotation_type.clone();
                         Box::new(TailPattern {
                             location: tail.location,
-                            pattern: self.unify(tail.pattern, type_.clone(), None),
+                            pattern: self.unify(
+                                tail.pattern,
+                                list_type.clone(),
+                                None,
+                                tail_annotation,
+                            ),
                         })
                     });
 
                     Pattern::List {
                         location,
                         elements,
                         tail,
-                        type_,
+                        type_: list_type,
                     }
                 }
 
@@ -819,10 +870,22 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
                         return Pattern::Invalid { location, type_ };
                     }
 
+                    let annotation_elements = annotation_type.as_ref().and_then(|annotation| {
+                        match collapse_links(annotation.clone()).deref() {
+                            Type::Tuple { elements } => Some(elements.clone()),
+                            _ => None,
+                        }
+                    });
                     let elements = elements
                         .into_iter()
                         .zip(type_elements)
-                        .map(|(pattern, type_)| self.unify(pattern, type_.clone(), None))
+                        .enumerate()
+                        .map(|(index, (pattern, type_))| {
+                            let annotation = annotation_elements
+                                .as_ref()
+                                .and_then(|elements| elements.get(index).cloned());
+                            self.unify(pattern, type_.clone(), None, annotation)
+                        })
                         .collect();
                     Pattern::Tuple { elements, location }
                 }
@@ -832,10 +895,22 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
                         .map(|_| self.environment.new_unbound_var())
                         .collect();
                     self.unify_types(tuple(elements_types.clone()), type_, location);
+                    let annotation_elements = annotation_type.as_ref().and_then(|annotation| {
+                        match collapse_links(annotation.clone()).deref() {
+                            Type::Tuple { elements } => Some(elements.clone()),
+                            _ => None,
+                        }
+                    });
                     let elements = elements
                         .into_iter()
                         .zip(elements_types)
-                        .map(|(pattern, type_)| self.unify(pattern, type_, None))
+                        .enumerate()
+                        .map(|(index, (pattern, type_))| {
+                            let annotation = annotation_elements
+                                .as_ref()
+                                .and_then(|elements| elements.get(index).cloned());
+                            self.unify(pattern, type_, None, annotation)
+                        })
                         .collect();
                     Pattern::Tuple { elements, location }
                 }
@@ -1213,7 +1288,8 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
                     .cloned()
                     .unwrap_or_else(|| self.environment.new_unbound_var());
 
-                let value = self.unify(value, type_, None);
+                let annotation = Some(type_.clone());
+                let value = self.unify(value, type_, None, annotation);
                 CallArg {
                     value,
                     location,
diff --git a/compiler-core/src/type_/tests/errors.rs b/compiler-core/src/type_/tests/errors.rs
@@ -743,6 +743,19 @@ fn module_could_not_unify6() {
     assert_module_error!("fn main() { let x: String = 5 x }");
 }
 
+#[test]
+fn module_could_not_unify_let_binding_annotation_follow_up() {
+    assert_module_error!(
+        "
+pub fn main() {
+  let x: String = 5
+  let y: Int = x
+  let z: String = x
+}
+"
+    );
+}
+
 #[test]
 fn module_could_not_unify7() {
     assert_module_error!("fn main() { let assert 5 = \"\" }");
diff --git a/compiler-core/src/type_/tests/functions.rs b/compiler-core/src/type_/tests/functions.rs
@@ -261,33 +261,6 @@ pub fn main() {
     );
 }
 
-#[test]
-fn multiple_bad_statement_assignment_with_annotation_fault_tolerance() {
-    assert_module_error!(
-        r#"
-pub fn main() {
-  let a: Int = "not an int"
-  let b: String = 1
-  let c = a + 2
-}
-"#
-    );
-}
-
-#[test]
-fn multiple_bad_statement_assignment_with_annotation_fault_tolerance2() {
-    assert_module_error!(
-        r#"
-pub fn main() {
-  // Since the value is invalid the type is the annotation
-  let a: Int = Junk
-  let b: String = 1
-  let c = a + 2
-}
-"#
-    );
-}
-
 #[test]
 fn multiple_bad_statement_assignment_with_pattern_fault_tolerance2() {
     assert_module_error!(
diff --git a/compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__module_could_not_unify9.snap b/compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__module_could_not_unify9.snap
@@ -1,6 +1,8 @@
 ---
 source: compiler-core/src/type_/tests/errors.rs
+assertion_line: 771
 expression: "fn main() { let assert [1, 2, ..x]: List(String) = [1,2,3] x }"
+snapshot_kind: text
 ---
 ----- SOURCE CODE
 fn main() { let assert [1, 2, ..x]: List(String) = [1,2,3] x }
diff --git a/compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__module_could_not_unify_let_binding_annotation_follow_up.snap b/compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__module_could_not_unify_let_binding_annotation_follow_up.snap
diff --git a/compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__functions__multiple_bad_statement_assignment_with_annotation_fault_tolerance.snap b/compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__functions__multiple_bad_statement_assignment_with_annotation_fault_tolerance.snap
diff --git a/compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__use___typed_pattern_wrong_type.snap b/compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__use___typed_pattern_wrong_type.snap
