no matching function for call to 'format_base2e' if `#include<iterator>` after `#include <fmt>`

[yujincheng08]

Reproduction:

https://godbolt.org/z/GqrsPY5jb

and

https://godbolt.org/z/xf8PPv5qY

[sunmy2019]

minimized to https://godbolt.org/z/qo9EW7rf5

// #include <iterator>
#include "fmt/format.h"
#include <iterator>

int main() {
  using Container =
      fmt::v12::detail::iterator_buffer<char*, char,
                                        fmt::v12::detail::fixed_buffer_traits>;
  fmt::detail::adl::invoke_back_inserter<Container>();
}

---

If format.h is included BEFORE <iterator>, the following code cannot find std::back_inserter by normal lookup (forward lookup) or argument dependent lookup (ADL).

If format.h is included AFTER <iterator>, the following code can find std::back_inserter by normal lookup (forward lookup).

fmt/include/fmt/base.h

Lines 482 to 488 in 790b938

	namespace adl {
	using namespace std;
	template <typename Container>
	auto invoke_back_inserter()
	-> decltype(back_inserter(std::declval<Container&>()));
	} // namespace adl

---

To fix this, we can include <iterator> in format.h.

[sunmy2019]

Confirmed to both gcc and clang. I provide 2 possible fixes. Apply either one is able to solve the issue. Apply both is even better.

---

Possible Fix 1:

Include <iterator> in format.h for symbol std::back_inserter. This also helps to avoid possible ODR violation.
I also provide one test case which fails on current main.

diff --git a/include/fmt/base.h b/include/fmt/base.h
index 72756344..02c4766b 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -17,6 +17,7 @@
 #  include <stdio.h>   // FILE
 #  include <string.h>  // memcmp
 
+#  include <iterator>     // std::back_inserter
 #  include <type_traits>  // std::enable_if
 #endif
 
@@ -480,7 +481,7 @@ FMT_CONSTEXPR auto compare(const Char* s1, const Char* s2, size_t n) -> int {
 }
 
 namespace adl {
-using namespace std;
+using std::back_inserter;
 
 template <typename Container>
 auto invoke_back_inserter()
diff --git a/test/format-test.cc b/test/format-test.cc
index b7496085..fe1f70c2 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -1723,6 +1723,19 @@ TEST(format_test, write_uintptr_fallback) {
   EXPECT_EQ(s, "0xface");
 }
 
+TEST(format_test, write_ptr_custom) {
+  // Test that formatting a pointer into a custom buffer works.
+  char data[10]{};
+  using traits = fmt::detail::fixed_buffer_traits;
+  auto buf =
+      fmt::detail::iterator_buffer<char*, char, traits>(data, sizeof(data));
+  fmt::detail::write_ptr<char>(std::back_inserter(buf),
+                               fmt::detail::bit_cast<fmt::detail::uintptr_t>(
+                                   reinterpret_cast<void*>(0xface)),
+                               nullptr);
+  EXPECT_STREQ(data, "0xface");
+}
+
 enum class color { red, green, blue };
 
 namespace test_ns {

---

Possible Fix 2:
I investigated why it is only triggered in compiled string. We can also rewrite in this way. The problem just gone.

diff --git a/include/fmt/compile.h b/include/fmt/compile.h
index eeba72db..b2183596 100644
--- a/include/fmt/compile.h
+++ b/include/fmt/compile.h
@@ -522,7 +522,7 @@ auto format_to_n(OutputIt out, size_t n, const S& fmt, T&&... args)
     -> format_to_n_result<OutputIt> {
   using traits = detail::fixed_buffer_traits;
   auto buf = detail::iterator_buffer<OutputIt, char, traits>(out, n);
-  fmt::format_to(std::back_inserter(buf), fmt, std::forward<T>(args)...);
+  fmt::format_to(appender(buf), fmt, std::forward<T>(args)...);
   return {buf.out(), buf.count()};
 }

[sunmy2019]

(Edited)
I provide 2 possible fixes. Apply either one is able to solve the issue. Apply both is even better.

---

Possible Fix 1: (Fixing the root cause)

Include <iterator> in format.h for symbol std::back_inserter. This fix also helps to avoid possible ODR violation.
I also provide one test case which fails on current main.

diff --git a/include/fmt/base.h b/include/fmt/base.h
index 72756344..02c4766b 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -17,6 +17,7 @@
 #  include <stdio.h>   // FILE
 #  include <string.h>  // memcmp
 
+#  include <iterator>     // std::back_inserter
 #  include <type_traits>  // std::enable_if
 #endif
 
@@ -480,7 +481,7 @@ FMT_CONSTEXPR auto compare(const Char* s1, const Char* s2, size_t n) -> int {
 }
 
 namespace adl {
-using namespace std;
+using std::back_inserter;
 
 template <typename Container>
 auto invoke_back_inserter()
diff --git a/test/format-test.cc b/test/format-test.cc
index b7496085..fe1f70c2 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -1723,6 +1723,19 @@ TEST(format_test, write_uintptr_fallback) {
   EXPECT_EQ(s, "0xface");
 }
 
+TEST(format_test, write_ptr_custom) {
+  // Test that formatting a pointer into a custom buffer works.
+  char data[10]{};
+  using traits = fmt::detail::fixed_buffer_traits;
+  auto buf =
+      fmt::detail::iterator_buffer<char*, char, traits>(data, sizeof(data));
+  fmt::detail::write_ptr<char>(std::back_inserter(buf),
+                               fmt::detail::bit_cast<fmt::detail::uintptr_t>(
+                                   reinterpret_cast<void*>(0xface)),
+                               nullptr);
+  EXPECT_STREQ(data, "0xface");
+}
+
 enum class color { red, green, blue };
 
 namespace test_ns {

---

Possible Fix 2: (bypass the problem)
I investigated why it is only triggered in compiled string.
We can also mimic the function in format.h. The problem just gone away.

diff --git a/include/fmt/compile.h b/include/fmt/compile.h
index eeba72db..b2183596 100644
--- a/include/fmt/compile.h
+++ b/include/fmt/compile.h
@@ -522,7 +522,7 @@ auto format_to_n(OutputIt out, size_t n, const S& fmt, T&&... args)
     -> format_to_n_result<OutputIt> {
   using traits = detail::fixed_buffer_traits;
   auto buf = detail::iterator_buffer<OutputIt, char, traits>(out, n);
-  fmt::format_to(std::back_inserter(buf), fmt, std::forward<T>(args)...);
+  fmt::format_to(appender(buf), fmt, std::forward<T>(args)...);
   return {buf.out(), buf.count()};
 }

[yujincheng08]

In my case, only the first fix works; the second one gives me link error like:

  ld.lld: error: undefined symbol: std::__ndk1::basic_streambuf<char, std::__ndk1::char_traits<char>>::sgetc()

FYI, I explicitly exclude STL's stream to reduce binary size.

[vitaut]

Applied the fix in 2d839bb, thanks @sunmy2019 for investigating! The <iterator> include is intentionally avoided because of compile times.

The linker error doesn't look related. @yujincheng08 could you provide a godbolt repro for it?

[yujincheng08]

@vitaut i compiled my own libcxx, and explictly remove stream, so it's hard to reproduce from godblot. But I think appender will include some codes from stream, which will make the binary size expolde.

[vitaut]

Does b3f642b fix the locale issue?

[vitaut]

Also note that you can disable the use of locales by compiling with -DFMT_USE_LOCALE=0: https://fmt.dev/dev/api/#macros.

[yujincheng08]

i will try when i am free, thanks!

[yujincheng08]

Confirmed the fix works!

[vitaut]

Thanks for the confirmation, merged.