Use std::allocator_traits (fmtlib#3804)

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>

include/fmt/format.h

@@ -43,9 +43,13 @@
 #include <cstring>           // std::memcpy
 #include <initializer_list>  // std::initializer_list
 #include <limits>            // std::numeric_limits
-#include <stdexcept>         // std::runtime_error
-#include <string>            // std::string
-#include <system_error>      // std::system_error
+#if defined(__GLIBCXX__) && !defined(_GLIBCXX_USE_DUAL_ABI)
+// Workaround for pre gcc 5 libstdc++.
+#  include <memory>  // std::allocator_traits
+#endif
+#include <stdexcept>     // std::runtime_error
+#include <string>        // std::string
+#include <system_error>  // std::system_error
 
 #include "base.h"
 
@@ -509,14 +513,6 @@ FMT_INLINE void assume(bool condition) {
 #endif
 }
 
-template <typename Allocator, typename Enable = void> struct allocator_size {
-  using type = size_t;
-};
-template <typename Allocator>
-struct allocator_size<Allocator, void_t<typename Allocator::size_type>> {
-  using type = typename Allocator::size_type;
-};
-
 template <typename Char, typename InputIt>
 auto copy_str(InputIt begin, InputIt end, appender out) -> appender {
   get_container(out).append(begin, end);
@@ -915,9 +911,8 @@ class basic_memory_buffer : public detail::buffer<T> {
   static FMT_CONSTEXPR20 void grow(detail::buffer<T>& buf, size_t size) {
     detail::abort_fuzzing_if(size > 5000);
     auto& self = static_cast<basic_memory_buffer&>(buf);
-    constexpr size_t max_size =
-        detail::max_value<typename detail::allocator_size<Allocator>::type>() /
-        sizeof(T);
+    const size_t max_size =
+        std::allocator_traits<Allocator>::max_size(self.alloc_);
     size_t old_capacity = buf.capacity();
     size_t new_capacity = old_capacity + old_capacity / 2;
     if (size > new_capacity)

test/format-test.cc

@@ -413,30 +413,37 @@ TEST(memory_buffer_test, exception_in_deallocate) {
   EXPECT_CALL(alloc, deallocate(&mem2[0], 2 * size));
 }
 
-class smol_allocator : public std::allocator<char> {
+template <typename Allocator, size_t MaxSize>
+class max_size_allocator : public Allocator {
  public:
-  using size_type = unsigned char;
-
-  auto allocate(size_t n) -> value_type* {
-    if (n > fmt::detail::max_value<size_type>())
+  using typename Allocator::value_type;
+  size_t max_size() const noexcept { return MaxSize; }
+  value_type* allocate(size_t n) {
+    if (n > max_size()) {
       throw std::length_error("size > max_size");
-    return std::allocator<char>::allocate(n);
+    }
+    return std::allocator_traits<Allocator>::allocate(
+        *static_cast<Allocator*>(this), n);
   }
   void deallocate(value_type* p, size_t n) {
-    std::allocator<char>::deallocate(p, n);
+    std::allocator_traits<Allocator>::deallocate(*static_cast<Allocator*>(this),
+                                                 p, n);
   }
 };
 
 TEST(memory_buffer_test, max_size_allocator) {
-  basic_memory_buffer<char, 10, smol_allocator> buffer;
-  buffer.resize(200);
-  // new_capacity = 200 + 200/2 = 300 > 256
-  buffer.resize(255);  // Shouldn't throw.
+  // 160 = 128 + 32
+  using test_allocator = max_size_allocator<std::allocator<char>, 160>;
+  basic_memory_buffer<char, 10, test_allocator> buffer;
+  buffer.resize(128);
+  // new_capacity = 128 + 128/2 = 192 > 160
+  buffer.resize(160);  // Shouldn't throw.
 }
 
 TEST(memory_buffer_test, max_size_allocator_overflow) {
-  basic_memory_buffer<char, 10, smol_allocator> buffer;
-  EXPECT_THROW(buffer.resize(256), std::exception);
+  using test_allocator = max_size_allocator<std::allocator<char>, 160>;
+  basic_memory_buffer<char, 10, test_allocator> buffer;
+  EXPECT_THROW(buffer.resize(161), std::exception);
 }
 
 TEST(format_test, exception_from_lib) {
@@ -2152,7 +2159,7 @@ TEST(format_int_test, format_int) {
   EXPECT_EQ(fmt::format_int(42ul).str(), "42");
   EXPECT_EQ(fmt::format_int(-42l).str(), "-42");
   EXPECT_EQ(fmt::format_int(42ull).str(), "42");
-  EXPECT_EQ(fmt::format_int(-42ll).str(), "-42");\
+  EXPECT_EQ(fmt::format_int(-42ll).str(), "-42");
   EXPECT_EQ(fmt::format_int(max_value<int64_t>()).str(),
             std::to_string(max_value<int64_t>()));
 }