From 99768695498147627ff1c70a5c6d97d1dc7c2543 Mon Sep 17 00:00:00 2001
From: Alexey Ochapov <alexez@alexez.com>
Date: Sun, 30 May 2021 03:46:12 +0300
Subject: [PATCH] fix custom types formatting at compile-time, add test

---
 include/fmt/core.h   |  2 +-
 include/fmt/format.h |  9 +++++----
 test/compile-test.cc | 37 ++++++++++++++++++++++++++-----------
 3 files changed, 32 insertions(+), 16 deletions(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index d71a2ab1..d2746615 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -1140,7 +1140,7 @@ template <typename Context> class value {
   FMT_INLINE value(const named_arg_info<char_type>* args, size_t size)
       : named_args{args, size} {}
 
-  template <typename T> FMT_INLINE value(const T& val) {
+  template <typename T> FMT_CONSTEXPR FMT_INLINE value(const T& val) {
     custom.value = &val;
     // Get the formatter type through the context to allow different contexts
     // have different extension points, e.g. `formatter<T>` for `format` and
diff --git a/include/fmt/format.h b/include/fmt/format.h
index b03449e1..2de69927 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -1935,10 +1935,11 @@ OutputIt write(OutputIt out, const T* value,
 }
 
 template <typename Char, typename OutputIt, typename T>
-auto write(OutputIt out, const T& value) -> typename std::enable_if<
-    mapped_type_constant<T, basic_format_context<OutputIt, Char>>::value ==
-        type::custom_type,
-    OutputIt>::type {
+FMT_CONSTEXPR auto write(OutputIt out, const T& value) ->
+    typename std::enable_if<
+        mapped_type_constant<T, basic_format_context<OutputIt, Char>>::value ==
+            type::custom_type,
+        OutputIt>::type {
   using context_type = basic_format_context<OutputIt, Char>;
   using formatter_type =
       conditional_t<has_formatter<T, context_type>::value,
diff --git a/test/compile-test.cc b/test/compile-test.cc
index 58fd057d..fb84ab28 100644
--- a/test/compile-test.cc
+++ b/test/compile-test.cc
@@ -53,17 +53,6 @@ TEST(iterator_test, truncating_back_inserter) {
   EXPECT_EQ(buffer, "42");
 }
 
-struct test_formattable {};
-
-FMT_BEGIN_NAMESPACE
-template <> struct formatter<test_formattable> : formatter<const char*> {
-  template <typename FormatContext>
-  auto format(test_formattable, FormatContext& ctx) -> decltype(ctx.out()) {
-    return formatter<const char*>::format("foo", ctx);
-  }
-};
-FMT_END_NAMESPACE
-
 TEST(compile_test, compile_fallback) {
   // FMT_COMPILE should fallback on runtime formatting when `if constexpr` is
   // not available.
@@ -71,6 +60,27 @@ TEST(compile_test, compile_fallback) {
 }
 
 #ifdef __cpp_if_constexpr
+struct test_formattable {};
+
+FMT_BEGIN_NAMESPACE
+template <> struct formatter<test_formattable> : formatter<const char*> {
+  char word_spec = 'f';
+  constexpr auto parse(format_parse_context& ctx) {
+    auto it = ctx.begin(), end = ctx.end();
+    if (it == end || *it == '}') return it;
+    if (it != end && (*it == 'f' || *it == 'b')) word_spec = *it++;
+    if (it != end && *it != '}') throw format_error("invalid format");
+    return it;
+  }
+  template <typename FormatContext>
+  constexpr auto format(test_formattable, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    return formatter<const char*>::format(word_spec == 'f' ? "foo" : "bar",
+                                          ctx);
+  }
+};
+FMT_END_NAMESPACE
+
 TEST(compile_test, format_default) {
   EXPECT_EQ("42", fmt::format(FMT_COMPILE("{}"), 42));
   EXPECT_EQ("42", fmt::format(FMT_COMPILE("{}"), 42u));
@@ -336,6 +346,11 @@ TEST(compile_time_formatting_test, combination) {
   EXPECT_EQ(" -42", test_format<5>(FMT_COMPILE("{:{}}"), -42, 4));
 }
 
+TEST(compile_time_formatting_test, custom_type) {
+  EXPECT_EQ("foo", test_format<4>(FMT_COMPILE("{}"), test_formattable()));
+  EXPECT_EQ("bar", test_format<4>(FMT_COMPILE("{:b}"), test_formattable()));
+}
+
 TEST(compile_time_formatting_test, multibyte_fill) {
   EXPECT_EQ("жж42", test_format<8>(FMT_COMPILE("{:ж>4}"), 42));
 }