Constexpr formatted_size (fmtlib#3026)

* Constexpr formatted_size

* Add C++20 tests for gcc 9 and 10

* Adjust unit test to require __cpp_lib_bit_cast

Author: marksantaniello

include/fmt/compile.h

@@ -14,7 +14,7 @@ FMT_BEGIN_NAMESPACE
 namespace detail {
 
 template <typename Char, typename InputIt>
-inline counting_iterator copy_str(InputIt begin, InputIt end,
+FMT_CONSTEXPR inline counting_iterator copy_str(InputIt begin, InputIt end,
                                   counting_iterator it) {
   return it + (end - begin);
 }
@@ -568,7 +568,7 @@ format_to_n_result<OutputIt> format_to_n(OutputIt out, size_t n,
 
 template <typename S, typename... Args,
           FMT_ENABLE_IF(detail::is_compiled_string<S>::value)>
-size_t formatted_size(const S& format_str, const Args&... args) {
+FMT_CONSTEXPR20 size_t formatted_size(const S& format_str, const Args&... args) {
   return fmt::format_to(detail::counting_iterator(), format_str, args...)
       .count();
 }

include/fmt/format.h

@@ -1231,7 +1231,7 @@ FMT_CONSTEXPR20 auto format_decimal(Char* out, UInt value, int size)
 
 template <typename Char, typename UInt, typename Iterator,
           FMT_ENABLE_IF(!std::is_pointer<remove_cvref_t<Iterator>>::value)>
-inline auto format_decimal(Iterator out, UInt value, int size)
+FMT_CONSTEXPR inline auto format_decimal(Iterator out, UInt value, int size)
     -> format_decimal_result<Iterator> {
   // Buffer is large enough to hold all digits (digits10 + 1).
   Char buffer[digits10<UInt>() + 1];
@@ -2137,29 +2137,29 @@ class counting_iterator {
   FMT_UNCHECKED_ITERATOR(counting_iterator);
 
   struct value_type {
-    template <typename T> void operator=(const T&) {}
+    template <typename T> FMT_CONSTEXPR void operator=(const T&) {}
   };
 
-  counting_iterator() : count_(0) {}
+  FMT_CONSTEXPR counting_iterator() : count_(0) {}
 
-  size_t count() const { return count_; }
+  FMT_CONSTEXPR size_t count() const { return count_; }
 
-  counting_iterator& operator++() {
+  FMT_CONSTEXPR counting_iterator& operator++() {
     ++count_;
     return *this;
   }
-  counting_iterator operator++(int) {
+  FMT_CONSTEXPR counting_iterator operator++(int) {
     auto it = *this;
     ++*this;
     return it;
   }
 
-  friend counting_iterator operator+(counting_iterator it, difference_type n) {
+  FMT_CONSTEXPR friend counting_iterator operator+(counting_iterator it, difference_type n) {
     it.count_ += static_cast<size_t>(n);
     return it;
   }
 
-  value_type operator*() const { return {}; }
+  FMT_CONSTEXPR value_type operator*() const { return {}; }
 };
 
 template <typename Char, typename OutputIt>

test/compile-test.cc

@@ -227,10 +227,14 @@ TEST(compile_test, format_to_n) {
   EXPECT_STREQ("2a", buffer);
 }
 
-TEST(compile_test, formatted_size) {
-  EXPECT_EQ(2, fmt::formatted_size(FMT_COMPILE("{0}"), 42));
-  EXPECT_EQ(5, fmt::formatted_size(FMT_COMPILE("{0:<4.2f}"), 42.0));
+#ifdef __cpp_lib_bit_cast
+TEST(compile_test, constexpr_formatted_size) {
+  FMT_CONSTEXPR20 size_t s1 = fmt::formatted_size(FMT_COMPILE("{0}"), 42);
+  EXPECT_EQ(2, s1);
+  FMT_CONSTEXPR20 size_t s2 = fmt::formatted_size(FMT_COMPILE("{0:<4.2f}"), 42.0);
+  EXPECT_EQ(5, s2);
 }
+#endif
 
 TEST(compile_test, text_and_arg) {
   EXPECT_EQ(">>>42<<<", fmt::format(FMT_COMPILE(">>>{}<<<"), 42));