Add constexpr if statements

Co-Authored-By: Thomas Johansen <thomasjo@gmail.com>

corpus/statements.txt

@@ -46,3 +46,28 @@ T main() {
         (identifier)
         (compound_statement
           (expression_statement (shift_expression (identifier) (identifier))))))))
+
+===========================================
+Constexpr if statements
+===========================================
+
+T f() {
+  if constexpr (std::is_pointer_v<T>)
+    return *t;
+  else
+    return t;
+}
+
+---
+
+(translation_unit
+  (function_definition
+    (type_identifier)
+    (function_declarator (identifier) (parameter_list))
+    (compound_statement
+      (if_statement
+        (template_function
+          (scoped_identifier (namespace_identifier) (identifier))
+          (template_argument_list (type_descriptor (type_identifier))))
+        (return_statement (pointer_expression (identifier)))
+        (return_statement (identifier))))))

grammar.js

@@ -388,6 +388,17 @@ module.exports = grammar(C, {
       $.for_range_loop
     ),
 
+    if_statement: $ => prec.right(seq(
+      'if',
+      optional('constexpr'),
+      '(', $._expression, ')',
+      $._statement,
+      optional(seq(
+        'else',
+        $._statement
+      ))
+    )),
+
     for_range_loop: $ => seq(
       'for',
       '(',

src/grammar.json

@@ -2880,6 +2880,18 @@
             "type": "STRING",
             "value": "if"
           },
+          {
+            "type": "CHOICE",
+            "members": [
+              {
+                "type": "STRING",
+                "value": "constexpr"
+              },
+              {
+                "type": "BLANK"
+              }
+            ]
+          },
           {
             "type": "STRING",
             "value": "("