From 03c7e28965684f3faa62c704d1ef6000230e94e5 Mon Sep 17 00:00:00 2001
From: Fatih BAKIR <mfatihbakir@gmail.com>
Date: Wed, 8 Oct 2025 12:23:57 -0700
Subject: [PATCH] write_demangled_name supports libc++ + clang-cl (#4560)

The current implementation assumes whenever we're on an FMT_MSC_VERSION
compiler, the standard library is MSVC's STL. However, with clang-cl we
have the possibility of using LLVM libc++ instead of MSVC STL. In that
scenario, the previous implementation produced the wrong demangled names
for RTTI types.

This patch detects the different combinations, and combines the existing
demangling implementations to produce the correct names and make all
tests pass on libc++ + clang-cl.
---
 include/fmt/std.h | 102 ++++++++++++++++++++++++++++------------------
 1 file changed, 62 insertions(+), 40 deletions(-)

diff --git a/include/fmt/std.h b/include/fmt/std.h
index e32d82f5..3928082f 100644
--- a/include/fmt/std.h
+++ b/include/fmt/std.h
@@ -139,50 +139,39 @@ template <typename Variant, typename Char> class is_variant_formattable {
 #endif  // FMT_CPP_LIB_VARIANT
 
 #if FMT_USE_RTTI
-
-template <typename OutputIt>
-auto write_demangled_name(OutputIt out, const std::type_info& ti) -> OutputIt {
-#  ifdef FMT_HAS_ABI_CXA_DEMANGLE
-  int status = 0;
-  size_t size = 0;
-  std::unique_ptr<char, void (*)(void*)> demangled_name_ptr(
-      abi::__cxa_demangle(ti.name(), nullptr, &size, &status), &std::free);
-
-  string_view demangled_name_view;
-  if (demangled_name_ptr) {
-    demangled_name_view = demangled_name_ptr.get();
-
-    // Normalization of stdlib inline namespace names.
-    // libc++ inline namespaces.
-    //  std::__1::*       -> std::*
-    //  std::__1::__fs::* -> std::*
-    // libstdc++ inline namespaces.
-    //  std::__cxx11::*             -> std::*
-    //  std::filesystem::__cxx11::* -> std::filesystem::*
-    if (demangled_name_view.starts_with("std::")) {
-      char* begin = demangled_name_ptr.get();
-      char* to = begin + 5;  // std::
-      for (char *from = to, *end = begin + demangled_name_view.size();
-           from < end;) {
-        // This is safe, because demangled_name is NUL-terminated.
-        if (from[0] == '_' && from[1] == '_') {
-          char* next = from + 1;
-          while (next < end && *next != ':') next++;
-          if (next[0] == ':' && next[1] == ':') {
-            from = next + 2;
-            continue;
-          }
+string_view normalize_libcxx_inline_namespaces(string_view demangled_name_view,
+                                               char* begin) {
+  // Normalization of stdlib inline namespace names.
+  // libc++ inline namespaces.
+  //  std::__1::*       -> std::*
+  //  std::__1::__fs::* -> std::*
+  // libstdc++ inline namespaces.
+  //  std::__cxx11::*             -> std::*
+  //  std::filesystem::__cxx11::* -> std::filesystem::*
+  if (demangled_name_view.starts_with("std::")) {
+    char* to = begin + 5;  // std::
+    for (const char *from = to, *end = begin + demangled_name_view.size();
+         from < end;) {
+      // This is safe, because demangled_name is NUL-terminated.
+      if (from[0] == '_' && from[1] == '_') {
+        const char* next = from + 1;
+        while (next < end && *next != ':') next++;
+        if (next[0] == ':' && next[1] == ':') {
+          from = next + 2;
+          continue;
         }
-        *to++ = *from++;
       }
-      demangled_name_view = {begin, detail::to_unsigned(to - begin)};
+      *to++ = *from++;
     }
-  } else {
-    demangled_name_view = string_view(ti.name());
+    demangled_name_view = {begin, detail::to_unsigned(to - begin)};
   }
-  return detail::write_bytes<char>(out, demangled_name_view);
-#  elif FMT_MSC_VERSION
-  const string_view demangled_name(ti.name());
+  return demangled_name_view;
+}
+
+template <class OutputIt>
+auto normalize_msvc_abi_name(string_view abi_name_view, OutputIt out)
+    -> OutputIt {
+  const string_view demangled_name(abi_name_view);
   for (size_t i = 0; i < demangled_name.size(); ++i) {
     auto sub = demangled_name;
     sub.remove_prefix(i);
@@ -201,6 +190,39 @@ auto write_demangled_name(OutputIt out, const std::type_info& ti) -> OutputIt {
     if (*sub.begin() != ' ') *out++ = *sub.begin();
   }
   return out;
+}
+
+template <typename OutputIt>
+auto write_demangled_name(OutputIt out, const std::type_info& ti) -> OutputIt {
+#  ifdef FMT_HAS_ABI_CXA_DEMANGLE
+  int status = 0;
+  size_t size = 0;
+  std::unique_ptr<char, void (*)(void*)> demangled_name_ptr(
+      abi::__cxa_demangle(ti.name(), nullptr, &size, &status), &std::free);
+
+  string_view demangled_name_view;
+  if (demangled_name_ptr) {
+    demangled_name_view = normalize_libcxx_inline_namespaces(
+        demangled_name_ptr.get(), demangled_name_ptr.get());
+  } else {
+    demangled_name_view = string_view(ti.name());
+  }
+  return detail::write_bytes<char>(out, demangled_name_view);
+#  elif FMT_MSC_VERSION && defined(_MSVC_STL_UPDATE)
+  return normalize_msvc_abi_name(ti.name(), out);
+#  elif FMT_MSC_VERSION && defined(_LIBCPP_VERSION)
+  const string_view demangled_name(ti.name());
+  std::string name_copy(demangled_name.size(), '\0');
+  // normalize_msvc_abi_name removes class, struct, union etc that MSVC has in
+  // front of types
+  name_copy.erase(normalize_msvc_abi_name(demangled_name, name_copy.begin()),
+                  name_copy.end());
+  // normalize_libcxx_inline_namespaces removes the inline __1, __2, etc
+  // namespaces libc++ uses for ABI versioning On MSVC ABI + libc++
+  // environments, we need to eliminate both of them.
+  const string_view normalized_name =
+      normalize_libcxx_inline_namespaces(name_copy, name_copy.data());
+  return detail::write_bytes<char>(out, normalized_name);
 #  else
   return detail::write_bytes<char>(out, string_view(ti.name()));
 #  endif