Improve error reporting (#357)

fmt/format.h

@@ -1121,6 +1121,9 @@ struct Not { enum { value = 0 }; };
 template <>
 struct Not<false> { enum { value = 1 }; };
 
+template <typename T>
+struct False { enum { value = 0 }; };
+
 template <typename T, T> struct LConvCheck {
   LConvCheck(int) {}
 };
@@ -1136,6 +1139,35 @@ inline StringRef thousands_sep(
 
 inline fmt::StringRef thousands_sep(...) { return ""; }
 
+#define FMT_CONCAT(a, b) a##b
+
+#if FMT_GCC_VERSION >= 407
+# define FMT_UNUSED __attribute__((unused))
+#else
+# define FMT_UNUSED
+#endif
+
+#ifndef FMT_USE_STATIC_ASSERT
+# define FMT_USE_STATIC_ASSERT 0
+#endif
+
+#if FMT_USE_STATIC_ASSERT || FMT_HAS_FEATURE(cxx_static_assert) || \
+  (FMT_GCC_VERSION >= 403 && FMT_HAS_GXX_CXX11) || _MSC_VER >= 1600
+# define FMT_STATIC_ASSERT(cond, message) static_assert(cond, message)
+#else
+# define FMT_CONCAT_(a, b) FMT_CONCAT(a, b)
+# define FMT_STATIC_ASSERT(cond, message) \
+  typedef int FMT_CONCAT_(Assert, __LINE__)[(cond) ? 1 : -1] FMT_UNUSED
+#endif
+
+template <typename Formatter, typename Char, typename T>
+void format_arg(Formatter &, const Char *, const T &) {
+  FMT_STATIC_ASSERT(False<T>::value,
+                    "Cannot format argument. To enable the use of ostream "
+                    "operator<< include fmt/ostream.h. Otherwise provide "
+                    "an overload of format_arg.");
+}
+
 // Makes an Arg object from any type.
 template <typename Formatter>
 class MakeValue : public Arg {
@@ -1179,7 +1211,7 @@ class MakeValue : public Arg {
   template <typename T>
   static void format_custom_arg(
       void *formatter, const void *arg, void *format_str_ptr) {
-    format(*static_cast<Formatter*>(formatter),
+    format_arg(*static_cast<Formatter*>(formatter),
                *static_cast<const Char**>(format_str_ptr),
                *static_cast<const T*>(arg));
   }
@@ -3323,7 +3355,6 @@ void arg(WStringRef, const internal::NamedArg<Char>&) FMT_DELETED_OR_UNDEFINED;
 #define FMT_ARG_N(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, N, ...) N
 #define FMT_RSEQ_N() 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0
 
-#define FMT_CONCAT(a, b) a##b
 #define FMT_FOR_EACH_(N, f, ...) \
   FMT_EXPAND(FMT_CONCAT(FMT_FOR_EACH, N)(f, __VA_ARGS__))
 #define FMT_FOR_EACH(f, ...) \

fmt/ostream.h

@@ -73,7 +73,7 @@ void write(std::ostream &os, Writer &w);
 
 // Formats a value.
 template <typename Char, typename ArgFormatter, typename T>
-void format(BasicFormatter<Char, ArgFormatter> &f,
+void format_arg(BasicFormatter<Char, ArgFormatter> &f,
                 const Char *&format_str, const T &value) {
   internal::MemoryBuffer<Char, internal::INLINE_BUFFER_SIZE> buffer;
 

fmt/posix.h

@@ -51,25 +51,6 @@
 # endif
 #endif
 
-#if FMT_GCC_VERSION >= 407
-# define FMT_UNUSED __attribute__((unused))
-#else
-# define FMT_UNUSED
-#endif
-
-#ifndef FMT_USE_STATIC_ASSERT
-# define FMT_USE_STATIC_ASSERT 0
-#endif
-
-#if FMT_USE_STATIC_ASSERT || FMT_HAS_FEATURE(cxx_static_assert) || \
-  (FMT_GCC_VERSION >= 403 && FMT_HAS_GXX_CXX11) || _MSC_VER >= 1600
-# define FMT_STATIC_ASSERT(cond, message) static_assert(cond, message)
-#else
-# define FMT_CONCAT_(a, b) FMT_CONCAT(a, b)
-# define FMT_STATIC_ASSERT(cond, message) \
-  typedef int FMT_CONCAT_(Assert, __LINE__)[(cond) ? 1 : -1] FMT_UNUSED
-#endif
-
 // Retries the expression while it evaluates to error_result and errno
 // equals to EINTR.
 #ifndef _WIN32

fmt/time.h

@@ -15,7 +15,7 @@
 
 namespace fmt {
 template <typename ArgFormatter>
-void format(BasicFormatter<char, ArgFormatter> &f,
+void format_arg(BasicFormatter<char, ArgFormatter> &f,
                 const char *&format_str, const std::tm &tm) {
   if (*format_str == ':')
     ++format_str;

test/format-test.cc

@@ -1366,7 +1366,7 @@ TEST(FormatterTest, FormatCStringRef) {
   EXPECT_EQ("test", format("{0}", CStringRef("test")));
 }
 
-void format(fmt::BasicFormatter<char> &f, const char *, const Date &d) {
+void format_arg(fmt::BasicFormatter<char> &f, const char *, const Date &d) {
   f.writer() << d.year() << '-' << d.month() << '-' << d.day();
 }
 
@@ -1379,7 +1379,7 @@ TEST(FormatterTest, FormatCustom) {
 class Answer {};
 
 template <typename Char>
-void format(fmt::BasicFormatter<Char> &f, const Char *, Answer) {
+void format_arg(fmt::BasicFormatter<Char> &f, const Char *, Answer) {
   f.writer() << "42";
 }
 

test/util-test.cc

@@ -64,7 +64,7 @@ namespace {
 struct Test {};
 
 template <typename Char>
-void format(fmt::BasicFormatter<Char> &f, const Char *, Test) {
+void format_arg(fmt::BasicFormatter<Char> &f, const Char *, Test) {
   f.writer() << "test";
 }
 
@@ -581,7 +581,7 @@ struct CustomFormatter {
   typedef char Char;
 };
 
-void format(CustomFormatter &, const char *&s, const Test &) {
+void format_arg(CustomFormatter &, const char *&s, const Test &) {
   s = "custom_format";
 }