c++23: support additional lambda expression syntax (P1102R2)

P1102R2 relaxed lambda syntax so that () (empty parameter list) is
optional in more cases.

Author: jdrouhard

grammar.js

@@ -83,6 +83,7 @@ module.exports = grammar(C, {
     [$.expression, $._lambda_capture],
     [$.expression, $.structured_binding_declarator, $._lambda_capture_identifier],
     [$.structured_binding_declarator, $._lambda_capture_identifier],
+    [$.lambda_declarator],
     [$.parameter_list, $.argument_list],
     [$.type_specifier, $.call_expression],
     [$._declaration_specifiers, $._constructor_specifiers],
@@ -1085,13 +1086,46 @@ module.exports = grammar(C, {
       field('requirements', $.requirement_seq),
     ),
 
+    lambda_declarator: $ => choice(
+      // main declarator form, includes parameter list
+      seq(
+        field('parameters', $.parameter_list),
+        repeat($.attribute_declaration),
+        optional($.type_qualifier),
+        optional($._function_exception_specification),
+        repeat($.attribute_declaration),
+        optional($.trailing_return_type),
+        optional($.requires_clause),
+      ),
+
+      // forms supporting omitted parameter list
+      repeat1($.attribute_declaration),
+      seq(
+        repeat($.attribute_declaration),
+        $.trailing_return_type,
+      ),
+      seq(
+        repeat($.attribute_declaration),
+        $._function_exception_specification,
+        repeat($.attribute_declaration),
+        optional($.trailing_return_type),
+      ),
+      seq(
+        repeat($.attribute_declaration),
+        $.type_qualifier,
+        optional($._function_exception_specification),
+        repeat($.attribute_declaration),
+        optional($.trailing_return_type),
+      ),
+    ),
+
     lambda_expression: $ => seq(
       field('captures', $.lambda_capture_specifier),
       optional(seq(
         field('template_parameters', $.template_parameter_list),
         optional(field('constraint', $.requires_clause)),
       )),
-      optional(field('declarator', $.abstract_function_declarator)),
+      optional(field('declarator', $.lambda_declarator)),
       field('body', $.compound_statement),
     ),
 

test/corpus/expressions.txt

@@ -302,6 +302,10 @@ auto f = [&arg, &...etc = etc] {};
 
 auto f = [=, *this, &args...] {};
 
+auto f = [] -> int {};
+
+auto f = [s = S{}] mutable {};
+
 --------------------------------------------------------------------------------
 
 (translation_unit
@@ -313,7 +317,7 @@ auto f = [=, *this, &args...] {};
       (lambda_expression
         (lambda_capture_specifier
           (lambda_default_capture))
-        (abstract_function_declarator
+        (lambda_declarator
           (parameter_list
             (parameter_declaration
               (primitive_type)
@@ -333,7 +337,7 @@ auto f = [=, *this, &args...] {};
         (lambda_capture_specifier
           (identifier)
           (identifier))
-        (abstract_function_declarator
+        (lambda_declarator
           (parameter_list
             (parameter_declaration
               (primitive_type)
@@ -361,7 +365,7 @@ auto f = [=, *this, &args...] {};
         (template_parameter_list
           (type_parameter_declaration
             (type_identifier)))
-        (abstract_function_declarator
+        (lambda_declarator
           (parameter_list))
         (compound_statement
           (return_statement
@@ -408,7 +412,7 @@ auto f = [=, *this, &args...] {};
                       (type_identifier)))))
               (argument_list
                 (identifier)))))
-        (abstract_function_declarator
+        (lambda_declarator
           (parameter_list
             (parameter_declaration
               (placeholder_type_specifier
@@ -468,6 +472,33 @@ auto f = [=, *this, &args...] {};
           (this)
           (parameter_pack_expansion
             (identifier)))
+        (compound_statement))))
+  (declaration
+    (placeholder_type_specifier
+      (auto))
+    (init_declarator
+      (identifier)
+      (lambda_expression
+        (lambda_capture_specifier)
+        (lambda_declarator
+          (trailing_return_type
+            (type_descriptor
+              (primitive_type))))
+        (compound_statement))))
+  (declaration
+    (placeholder_type_specifier
+      (auto))
+    (init_declarator
+      (identifier)
+      (lambda_expression
+        (lambda_capture_specifier
+          (lambda_capture_initializer
+            (identifier)
+            (compound_literal_expression
+              (type_identifier)
+              (initializer_list))))
+        (lambda_declarator
+          (type_qualifier))
         (compound_statement)))))
 
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