From 367b2e2ec75163bf79a6886adcf02547a7fc31dc Mon Sep 17 00:00:00 2001
From: Ihor Dutchak <ihor.youw@gmail.com>
Date: Tue, 11 Oct 2022 20:00:59 +0300
Subject: [PATCH] Explicitly export buffer<char> for MSVC

Some MSVC-specific behavior:

Class ostream by inheriting detail::buffer<char> implicitly exports it when fmt is built as a shared library.
Unless os.h is included, the compiler assumes detail::buffer<char> is not externally exported and instantiets a local copy of it,
which cases ODR violation.
---
 include/fmt/core.h | 8 ++++++++
 src/format.cc      | 7 +++++++
 2 files changed, 15 insertions(+)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 5f44a974..4feb385f 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -960,6 +960,14 @@ template <typename T> class buffer {
   }
 };
 
+#if defined(FMT_MSC_VERSION)
+#  ifdef FMT_EXPORT
+extern template class buffer<char>;
+#  else
+template class FMT_API buffer<char>;
+#  endif
+#endif
+
 struct buffer_traits {
   explicit buffer_traits(size_t) {}
   auto count() const -> size_t { return 0; }
diff --git a/src/format.cc b/src/format.cc
index 19dff444..03fc3321 100644
--- a/src/format.cc
+++ b/src/format.cc
@@ -22,6 +22,13 @@ template FMT_API auto locale_ref::get<std::locale>() const -> std::locale;
 
 // Explicit instantiations for char.
 
+#if defined(FMT_MSC_VERSION)
+// ostream does it implicitly, so we need make it explicitly
+// otherwise we get ODR violation when {fmt} is built as a shared library
+// and fmt/os.h is not included by the user
+template class FMT_API buffer<char>;
+#endif
+
 template FMT_API auto thousands_sep_impl(locale_ref)
     -> thousands_sep_result<char>;
 template FMT_API auto decimal_point_impl(locale_ref) -> char;