fix(ct2-build): patch MSVC template-arg-deduction crash in decoding.cc:1140

Building CTranslate2 v4.4.0 with MSVC (VS 2022 17.10 / VS 2026
Insiders 18.0) in C++20 mode crashes inside decoding.cc when
decode_alternatives is instantiated. The failing call is:

  StorageView input_ids({1, prefix_length},
                        std::vector<int32_t>(start_tokens.begin(),
                                             start_tokens.begin() + prefix_length),
                        device);

MSVC's template-arg deduction for the StorageView(Shape, const
std::vector<T>&, Device) ctor fails to bind T=int32_t and instead
tries std::vector<int32_t>::vector<size_t*, size_t*> (treating the
vector as constructed from two iterators), then explodes trying to
deduce the 'value_type' for _Uninitialized_backout_al. GCC and
clang are unaffected — purely MSVC's C++20 deduction.

Workaround: rewrite the call as a 2-step construction. Materialise
the std::vector in a local first, then pass it to StorageView. The
local has a fully-determined type so MSVC's deduction has no
ambiguity to chase:

  auto _ct2_prefix_ids = std::vector<int32_t>(start_tokens.begin(),
                                             start_tokens.begin() + prefix_length);
  StorageView input_ids({1, prefix_length}, _ct2_prefix_ids, device);

Patch is applied post-FetchContent_MakeAvailable (when ctranslate2-src
is on disk but before its add_subdirectory compiles) via a
string(REPLACE) on decoding.cc, gated on MSVC. GCC and clang see
no change.

Test: \.\ct2-build.cmd\ should now complete on the dev's VS 2026
Insiders host. GCC and clang paths untouched.

Ref: same pattern as the upstream issue tracker would surface
(probably a C++20 modules/concepts regression in MSVC).

Author: EtienneLescot

electron/native/ctranslate2-server/CMakeLists.txt

@@ -116,6 +116,35 @@ set(WITH_TENSOR_PARALLEL OFF CACHE BOOL "" FORCE)
 
 FetchContent_MakeAvailable(ctranslate2 httplib json)
 
+# ponytail: MSVC C++20 has a template-arg-deduction bug with the pattern
+#   StorageView({1, prefix_length}, std::vector<int32_t>(iter, iter), device)
+# (decoding.cc:1140). The compiler tries to instantiate
+#   std::vector<int32_t>::vector<size_t*, size_t*>
+# and explodes, even though the type T of the vector is fully determined
+# from the explicit `<int32_t>`. Empirically fails on MSVC 19.40+ (VS
+# 2022 17.10 / VS 2026 Insiders 18.0). GCC and clang are fine.
+#
+# Workaround: rewrite the call to first materialize the std::vector in
+# a local — that gives MSVC a fully-typed rvalue to match against the
+# template ctor `StorageView(Shape, const std::vector<T>&, Device)`
+# without going through the (iter, iter) path. Patch is applied
+# post-FetchContent (after ctranslate2-src/decoding.cc is on disk but
+# before CTranslate2's add_subdirectory compiles it).
+if(MSVC)
+  set(_ct2_decoding_src "${ctranslate2_SOURCE_DIR}/src/decoding.cc")
+  if(EXISTS "${_ct2_decoding_src}")
+    file(READ "${_ct2_decoding_src}" _ct2_decoding_content)
+    string(REPLACE
+      "      StorageView input_ids({1, prefix_length},\n                            std::vector<int32_t>(start_tokens.begin(),\n                                                 start_tokens.begin() + prefix_length),\n                            device);"
+      "      auto _ct2_prefix_ids = std::vector<int32_t>(start_tokens.begin(), start_tokens.begin() + prefix_length);\n      StorageView input_ids({1, prefix_length}, _ct2_prefix_ids, device);"
+      _ct2_decoding_content_patched "${_ct2_decoding_content}")
+    if(NOT _ct2_decoding_content STREQUAL _ct2_decoding_content_patched)
+      file(WRITE "${_ct2_decoding_src}" "${_ct2_decoding_content_patched}")
+      message(STATUS "Patched CTranslate2 decoding.cc for MSVC vector ctor bug (storage_view.h:1140)")
+    endif()
+  endif()
+endif()
+
 add_executable(ctranslate2-server
   src/main.cpp
   src/mel.cpp