Disable fallback_formatter for arrays

include/fmt/ostream.h

@@ -70,8 +70,7 @@ void_t<> operator<<(std::basic_ostream<char, Traits>&, signed char);
 template <typename Traits>
 void_t<> operator<<(std::basic_ostream<char, Traits>&, unsigned char);
 
-// Checks if T has a user-defined operator<< (e.g. not a member of
-// std::ostream).
+// Checks if T has a user-defined operator<< e.g. not a member of std::ostream.
 template <typename T, typename Char> class is_streamable {
  private:
   template <typename U>
@@ -90,6 +89,11 @@ template <typename T, typename Char> class is_streamable {
   static const bool value = result::value;
 };
 
+// Formatting of arrays is intentionally disabled to prevent conflicts with
+// standard (non-ostream) formatters.
+template <typename T, size_t N, typename Char>
+struct is_streamable<T[N], Char> : std::false_type {};
+
 // Write the content of buf to os.
 template <typename Char>
 void write_buffer(std::basic_ostream<Char>& os, buffer<Char>& buf) {

test/header-only-test.cc

 // Header-only configuration test
 
 #include "fmt/core.h"
+#include "fmt/ostream.h"
+#include "gtest/gtest.h"
 
 #ifndef FMT_HEADER_ONLY
 #  error "Not in the header-only mode."
 #endif
+
+TEST(header_only_test, format) { EXPECT_EQ(fmt::format("foo"), "foo"); }