feat: add support for expansion statements (P1306)

Introduces support for `template for ( init; for-range-decl : expansion-init )`
which is a compile-time way to expand (iterate over):
  - expansion expressions
  - anything destructurable via structured bindings
  - ranges with compile time size

It was sufficient to simply re-use the existing rule for
_for_range_loop_body for the part inside the parentheses, but add the
required sequence of `template` followed by `for`

grammar.js

@@ -887,6 +887,7 @@ module.exports = grammar(C, {
       $.co_return_statement,
       $.co_yield_statement,
       $.for_range_loop,
+      $.expansion_statement,
       $.try_statement,
       $.throw_statement,
     ),
@@ -896,6 +897,7 @@ module.exports = grammar(C, {
       $.co_return_statement,
       $.co_yield_statement,
       $.for_range_loop,
+      $.expansion_statement,
       $.try_statement,
       $.throw_statement,
     ),
@@ -1480,6 +1482,14 @@ module.exports = grammar(C, {
       seq('template', $._splice_specialization_specifier),
     )),
 
+    expansion_statement: $ => seq(
+      'template', 'for',
+      '(',
+      $._for_range_loop_body,
+      ')',
+      field('body', $.statement),
+    ),
+
     operator_name: $ => prec(1, seq(
       'operator',
       choice(

test/corpus/reflection.txt

@@ -933,3 +933,39 @@ struct C {
                 (false)))))
         (primitive_type)
         (field_identifier)))))
+
+================================================================================
+Expansion statements
+================================================================================
+
+template for (auto const& c : {Containers...}) {}
+template for (constexpr int I : std::views::iota(0zu, sizeof...(Ts))) {}
+
+--------------------------------------------------------------------------------
+
+(translation_unit
+  (expansion_statement
+    (placeholder_type_specifier
+      (auto))
+    (type_qualifier)
+    (reference_declarator
+      (identifier))
+    (initializer_list
+      (parameter_pack_expansion
+        (identifier)))
+    (compound_statement))
+  (expansion_statement
+    (type_qualifier)
+    (primitive_type)
+    (identifier)
+    (call_expression
+      (qualified_identifier
+        (namespace_identifier)
+        (qualified_identifier
+          (namespace_identifier)
+          (identifier)))
+      (argument_list
+        (number_literal)
+        (sizeof_expression
+          (identifier))))
+    (compound_statement)))