Fix type inference for functions, constructors without explicit annotations

CHANGELOG.md

@@ -34,6 +34,37 @@
   than stopping at the syntax error.
   ([mxtthias](https://github.com/mxtthias))
 
+- Type inference now preserves generic type parameters when constructors or functions are used without explicit annotations, eliminating false errors in mutually recursive code:
+  ```gleam
+  type Test(a) {
+    Test(a)
+  }
+
+  fn it(value: Test(a)) {
+    it2(value)
+  }
+
+  fn it2(value: Test(a)) -> Test(a) {
+    it(value)
+  }
+  ```
+  Previously this could fail with an incorrect "Type mismatch" error:
+  ```
+  Type mismatch
+
+  The type of this returned value doesn't match the return type
+  annotation of this function.
+
+  Expected type:
+
+    Test(a)
+
+  Found type:
+
+    Test(a)
+  ```
+  ([Adi Salimgereyev](https://github.com/abs0luty))
+
 ### Build tool
 
 - The help text displayed by `gleam dev --help`, `gleam test --help`, and

compiler-core/src/analyse.rs

@@ -673,8 +673,18 @@ impl<'a, A> ModuleAnalyzer<'a, A> {
         }
 
         // Assert that the inferred type matches the type of any recursive call
-        if let Err(error) = unify(preregistered_type.clone(), type_) {
-            self.problems.error(convert_unify_error(error, location));
+        if let Err(error) = unify(preregistered_type.clone(), type_.clone()) {
+            let mut instantiated_ids = im::HashMap::new();
+            let flexible_hydrator = Hydrator::new();
+            let instantiated_annotation = environment.instantiate(
+                preregistered_type.clone(),
+                &mut instantiated_ids,
+                &flexible_hydrator,
+            );
+
+            if unify(instantiated_annotation, type_.clone()).is_err() {
+                self.problems.error(convert_unify_error(error, location));
+            }
         }
 
         // Ensure that the current target has an implementation for the function.
@@ -715,10 +725,13 @@ impl<'a, A> ModuleAnalyzer<'a, A> {
             purity,
         };
 
+        // Store the inferred type (not the preregistered type) in the environment.
+        // This ensures concrete type information flows through recursive calls - e.g., if we infer
+        // `fn() -> Test(Int)`, callers see that instead of the generic `fn() -> Test(a)`.
         environment.insert_variable(
             name.clone(),
             variant,
-            preregistered_type.clone(),
+            type_.clone(),
             publicity,
             deprecation.clone(),
         );
@@ -731,6 +744,8 @@ impl<'a, A> ModuleAnalyzer<'a, A> {
             ReferenceKind::Definition,
         );
 
+        // Use the inferred return type for the typed AST node.
+        // This matches the type stored in the environment above.
         let function = Function {
             documentation: doc,
             location,
@@ -741,7 +756,7 @@ impl<'a, A> ModuleAnalyzer<'a, A> {
             body_start,
             end_position: end_location,
             return_annotation,
-            return_type: preregistered_type
+            return_type: type_
                 .return_type()
                 .expect("Could not find return type for fn"),
             body,

compiler-core/src/language_server/code_action.rs

@@ -1371,13 +1371,187 @@ fn collect_type_variables(printer: &mut Printer<'_>, function: &ast::TypedFuncti
 }
 
 impl<'ast, 'a, 'b> ast::visit::Visit<'ast> for TypeVariableCollector<'a, 'b> {
+    fn visit_typed_function(&mut self, fun: &'ast ast::TypedFunction) {
+        for argument in fun.arguments.iter() {
+            if let Some(annotation) = &argument.annotation {
+                register_type_variables_from_annotation(
+                    self.printer,
+                    annotation,
+                    argument.type_.as_ref(),
+                );
+            }
+        }
+
+        if let Some(annotation) = &fun.return_annotation {
+            register_type_variables_from_annotation(
+                self.printer,
+                annotation,
+                fun.return_type.as_ref(),
+            );
+        }
+
+        ast::visit::visit_typed_function(self, fun);
+    }
+
+    fn visit_typed_expr_fn(
+        &mut self,
+        location: &'ast SrcSpan,
+        type_: &'ast Arc<Type>,
+        kind: &'ast FunctionLiteralKind,
+        arguments: &'ast [TypedArg],
+        body: &'ast Vec1<TypedStatement>,
+        return_annotation: &'ast Option<ast::TypeAst>,
+    ) {
+        if let Type::Fn {
+            arguments: argument_types,
+            return_: return_type,
+            ..
+        } = type_.as_ref()
+        {
+            for (argument, argument_type) in arguments.iter().zip(argument_types) {
+                if let Some(annotation) = &argument.annotation {
+                    register_type_variables_from_annotation(
+                        self.printer,
+                        annotation,
+                        argument_type.as_ref(),
+                    );
+                }
+            }
+
+            if let Some(annotation) = return_annotation {
+                register_type_variables_from_annotation(
+                    self.printer,
+                    annotation,
+                    return_type.as_ref(),
+                );
+            }
+        }
+
+        ast::visit::visit_typed_expr_fn(
+            self,
+            location,
+            type_,
+            kind,
+            arguments,
+            body,
+            return_annotation,
+        );
+    }
+
     fn visit_type_ast_var(&mut self, _location: &'ast SrcSpan, name: &'ast EcoString) {
         // Register this type variable so that we don't duplicate names when
         // adding annotations.
         self.printer.register_type_variable(name.clone());
     }
 }
 
+fn register_type_variables_from_annotation(
+    printer: &mut Printer<'_>,
+    annotation: &ast::TypeAst,
+    type_: &Type,
+) {
+    // fn wibble(a, b, c) {
+    //   fn(a: b, b: c) -> d { ... }
+    //                     ^
+    // Without this tracking the printer could rename `d` to a fresh `h`.
+    match (annotation, type_) {
+        (ast::TypeAst::Var(ast::TypeAstVar { name, .. }), Type::Var { type_ }) => {
+            match &*type_.borrow() {
+                TypeVar::Generic { id } | TypeVar::Unbound { id } => {
+                    let id = *id;
+                    printer.register_type_variable(name.clone());
+                    printer.register_type_variable_with_id(id, name.clone());
+                }
+                TypeVar::Link { type_ } => {
+                    register_type_variables_from_annotation(printer, annotation, type_.as_ref());
+                }
+            }
+        }
+
+        (
+            ast::TypeAst::Fn(ast::TypeAstFn {
+                arguments: annotation_arguments,
+                return_: annotation_return,
+                ..
+            }),
+            Type::Fn {
+                arguments: type_arguments,
+                return_: type_return,
+                ..
+            },
+        ) => {
+            for (argument_annotation, argument_type) in
+                annotation_arguments.iter().zip(type_arguments)
+            {
+                // Maintain the names from each `fn(arg: name, ...)` position.
+                register_type_variables_from_annotation(
+                    printer,
+                    argument_annotation,
+                    argument_type.as_ref(),
+                );
+            }
+
+            // And likewise propagate the annotated return variable.
+            register_type_variables_from_annotation(
+                printer,
+                annotation_return.as_ref(),
+                type_return.as_ref(),
+            );
+        }
+
+        (
+            ast::TypeAst::Constructor(ast::TypeAstConstructor {
+                arguments: annotation_arguments,
+                ..
+            }),
+            Type::Named {
+                arguments: type_arguments,
+                ..
+            },
+        ) => {
+            for (argument_annotation, argument_type) in
+                annotation_arguments.iter().zip(type_arguments)
+            {
+                // Track aliases introduced inside named type arguments.
+                register_type_variables_from_annotation(
+                    printer,
+                    argument_annotation,
+                    argument_type.as_ref(),
+                );
+            }
+        }
+
+        (
+            ast::TypeAst::Tuple(ast::TypeAstTuple {
+                elements: annotation_elements,
+                ..
+            }),
+            Type::Tuple {
+                elements: type_elements,
+                ..
+            },
+        ) => {
+            for (element_annotation, element_type) in annotation_elements.iter().zip(type_elements)
+            {
+                // Tuples can hide extra annotations; ensure each slot retains its label.
+                register_type_variables_from_annotation(
+                    printer,
+                    element_annotation,
+                    element_type.as_ref(),
+                );
+            }
+        }
+
+        (_, Type::Var { type_ }) => {
+            if let TypeVar::Link { type_ } = &*type_.borrow() {
+                register_type_variables_from_annotation(printer, annotation, type_.as_ref());
+            }
+        }
+
+        _ => {}
+    }
+}
+
 pub struct QualifiedConstructor<'a> {
     import: &'a Import<EcoString>,
     used_name: EcoString,

compiler-core/src/type_/expression.rs

@@ -4707,31 +4707,39 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
             if let Ok(body) = Vec1::try_from_vec(body) {
                 let mut body = body_typer.infer_statements(body);
 
-                // Check that any return type is accurate.
-                if let Some(return_type) = return_type
-                    && let Err(error) = unify(return_type, body.last().type_())
-                {
-                    let error = error
-                        .return_annotation_mismatch()
-                        .into_error(body.last().type_defining_location());
-                    body_typer.problems.error(error);
-
-                    // If the return type doesn't match with the annotation we
-                    // add a new expression to the end of the function to match
-                    // the annotated type and allow type inference to keep
-                    // going.
-                    body.push(Statement::Expression(TypedExpr::Invalid {
-                        // This is deliberately an empty span since this
-                        // placeholder expression is implicitly inserted by the
-                        // compiler and doesn't actually appear in the source
-                        // code.
-                        location: SrcSpan {
-                            start: body.last().location().end,
-                            end: body.last().location().end,
-                        },
-                        type_: body_typer.new_unbound_var(),
-                        extra_information: None,
-                    }))
+                // Check that any return type is compatible with the annotation.
+                if let Some(return_type) = return_type {
+                    let mut instantiated_ids = hashmap![];
+                    let flexible_hydrator = Hydrator::new();
+                    let instantiated_annotation = body_typer.environment.instantiate(
+                        return_type.clone(),
+                        &mut instantiated_ids,
+                        &flexible_hydrator,
+                    );
+
+                    if let Err(error) = unify(instantiated_annotation, body.last().type_()) {
+                        let error = error
+                            .return_annotation_mismatch()
+                            .into_error(body.last().type_defining_location());
+                        body_typer.problems.error(error);
+
+                        // If the return type doesn't match with the annotation we
+                        // add a new expression to the end of the function to match
+                        // the annotated type and allow type inference to keep
+                        // going.
+                        body.push(Statement::Expression(TypedExpr::Invalid {
+                            // This is deliberately an empty span since this
+                            // placeholder expression is implicitly inserted by the
+                            // compiler and doesn't actually appear in the source
+                            // code.
+                            location: SrcSpan {
+                                start: body.last().location().end,
+                                end: body.last().location().end,
+                            },
+                            type_: body_typer.new_unbound_var(),
+                            extra_information: None,
+                        }))
+                    }
                 };
 
                 Ok((arguments, body.to_vec()))

compiler-core/src/type_/printer.rs

@@ -454,6 +454,12 @@ impl<'a> Printer<'a> {
         _ = self.printed_type_variable_names.insert(name);
     }
 
+    /// Record that the given type-variable id is already using the supplied name.
+    pub fn register_type_variable_with_id(&mut self, id: u64, name: EcoString) {
+        _ = self.printed_type_variable_names.insert(name.clone());
+        _ = self.printed_type_variables.insert(id, name);
+    }
+
     pub fn print_type(&mut self, type_: &Type) -> EcoString {
         let mut buffer = EcoString::new();
         self.print(type_, &mut buffer, PrintMode::Normal);