From db9ffa14055b38dc1a208e764c36bed511bea50b Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Sun, 29 Oct 2017 07:32:14 -0700
Subject: [PATCH] Make parse_format_string constexpr

---
 include/fmt/format.h | 18 +++++++++++-------
 test/format-test.cc  | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 7 deletions(-)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index 4f193308..07e96132 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -3519,7 +3519,7 @@ constexpr Iterator parse_format_specs(Iterator it, SpecHandler &handler) {
 }
 
 template <typename Iterator, typename Handler>
-void parse_format_string(Iterator it, Handler &handler) {
+constexpr void parse_format_string(Iterator it, Handler &handler) {
   using char_type = typename std::iterator_traits<Iterator>::value_type;
   auto start = it;
   while (*it) {
@@ -3530,20 +3530,24 @@ void parse_format_string(Iterator it, Handler &handler) {
       start = ++it;
       continue;
     }
-    if (ch == '}')
+    if (ch == '}') {
       handler.on_error("unmatched '}' in format string");
+      return;
+    }
     handler.on_text(start, it - 1);
 
     struct id_adapter {
-      explicit id_adapter(Handler &h): handler(h) {}
+      constexpr explicit id_adapter(Handler &h): handler(h) {}
 
-      void operator()() { handler.on_arg_id(); }
-      void operator()(unsigned id) { handler.on_arg_id(id); }
-      void operator()(basic_string_view<char_type> id) {
+      constexpr void operator()() { handler.on_arg_id(); }
+      constexpr void operator()(unsigned id) { handler.on_arg_id(id); }
+      constexpr void operator()(basic_string_view<char_type> id) {
         handler.on_arg_id(id);
       }
 
-      void on_error(const char *message) { handler.on_error(message); }
+      constexpr void on_error(const char *message) {
+        handler.on_error(message);
+      }
 
       Handler &handler;
     } adapter(handler);
diff --git a/test/format-test.cc b/test/format-test.cc
index 9cf8fb27..5d90b17f 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -1780,3 +1780,35 @@ TEST(FormatTest, ConstexprSpecsChecker) {
   static_assert(check_specs("d").type == 'd', "");
   static_assert(check_specs("{<").res == handler::ERROR, "");
 }
+
+struct test_format_string_handler {
+  constexpr void on_text(const char *, const char *) {}
+
+  constexpr void on_arg_id() {}
+
+  template <typename T>
+  constexpr void on_arg_id(T) {}
+
+  constexpr void on_replacement_field(const char *) {}
+
+  constexpr const char *on_format_specs(const char *s) { return s; }
+
+  constexpr void on_error(const char *) { error = true; }
+
+  bool error = false;
+};
+
+constexpr bool parse_string(const char *s) {
+  test_format_string_handler h;
+  fmt::internal::parse_format_string(s, h);
+  return !h.error;
+}
+
+TEST(FormatTest, ConstexprParseFormatString) {
+  static_assert(parse_string("foo"), "");
+  static_assert(!parse_string("}"), "");
+  static_assert(parse_string("{}"), "");
+  static_assert(parse_string("{42}"), "");
+  static_assert(parse_string("{foo}"), "");
+  static_assert(parse_string("{:}"), "");
+}