fmt::ptr: Support function pointers (#2131)

Passing a function pointer to fmt::ptr results in:

 In file included from /home/mac/git/fmt/test/gmock/gmock.h:238,
                  from /home/mac/git/fmt/test/format-test.cc:31:
 .../fmt/test/format-test.cc: In member function ‘virtual void FormatterTest_FormatPointer_Test::TestBody()’:
 .../fmt/test/format-test.cc:1486:56: error: no matching function for call to ‘ptr(void (&)(int, double, std::__cxx11::string))’
              format("{}", fmt::ptr(function_pointer_test)));

with GCC and Clang. Let's add an overload to support that usage.

Unfortunately, MSVC would
consider the overload to be ambiguous for unknown reasons:

 D:\a\fmt\fmt\test\format-test.cc(1485,1): error C2668: 'fmt::v7::ptr': ambiguous call to overloaded function [D:\a\fmt\build\test\format-test.vcxproj]
 D:\a\fmt\fmt\include\fmt/format.h(3742,60): message : could be 'const void *fmt::v7::ptr<void,int,double,std::string>(T (__cdecl *)(int,double,std::string))' [D:\a\fmt\build\test\format-test.vcxproj]
           with
           [
               T=void
           ]
 D:\a\fmt\fmt\include\fmt/format.h(3735,42): message : or       'const void *fmt::v7::ptr<void(int,double,std::string)>(T (__cdecl *))' [D:\a\fmt\build\test\format-test.vcxproj]
           with
           [
               T=void (int,double,std::string)
           ]
 D:\a\fmt\fmt\test\format-test.cc(1486,1): message : while trying to match the argument list '(overloaded-function)' [D:\a\fmt\build\test\format-test.vcxproj]

but luckily this means that the overload is unnecessary in that case
anyway, so we can just make it conditional.

doc/api.rst

@@ -314,6 +314,7 @@ Utilities
 .. doxygenfunction:: fmt::ptr(const T *p)
 .. doxygenfunction:: fmt::ptr(const std::unique_ptr<T> &p)
 .. doxygenfunction:: fmt::ptr(const std::shared_ptr<T> &p)
+.. doxygenfunction:: fmt::ptr(T (*fn)(Args...))
 
 .. doxygenfunction:: fmt::to_string(const T &value)
 

include/fmt/format.h

@@ -3739,6 +3739,13 @@ template <typename T> inline const void* ptr(const std::unique_ptr<T>& p) {
 template <typename T> inline const void* ptr(const std::shared_ptr<T>& p) {
   return p.get();
 }
+#if !FMT_MSC_VER
+// MSVC lets function pointers decay to void pointers, so this
+// overload is unnecessary.
+template <typename T, typename... Args> inline const void* ptr(T (*fn)(Args...)) {
+  return detail::bit_cast<const void *>(fn);
+}
+#endif
 
 class bytes {
  private:

test/format-test.cc

@@ -1467,6 +1467,9 @@ TEST(FormatterTest, FormatUCharString) {
   EXPECT_EQ("test", format("{0:s}", ptr));
 }
 
+void function_pointer_test(int, double, std::string) {
+}
+
 TEST(FormatterTest, FormatPointer) {
   check_unknown_types(reinterpret_cast<void*>(0x1234), "p", "pointer");
   EXPECT_EQ("0x0", format("{0}", static_cast<void*>(nullptr)));
@@ -1479,6 +1482,8 @@ TEST(FormatterTest, FormatPointer) {
   EXPECT_EQ(format("{}", fmt::ptr(up.get())), format("{}", fmt::ptr(up)));
   std::shared_ptr<int> sp(new int(1));
   EXPECT_EQ(format("{}", fmt::ptr(sp.get())), format("{}", fmt::ptr(sp)));
+  EXPECT_EQ(format("{}", fmt::detail::bit_cast<const void *>(&function_pointer_test)),
+            format("{}", fmt::ptr(function_pointer_test)));
   EXPECT_EQ("0x0", format("{}", nullptr));
 }