<ranges>: Temporarily disable join_view for non-forward_ranges, pending resolution of LWG-3698 (microsoft#2727)

Co-authored-by: Casey Carter <[email protected]>
Co-authored-by: timsong-cpp <[email protected]>

Author: 3 people

stl/inc/ranges

@@ -3068,6 +3068,19 @@ namespace ranges {
         requires view<_Vw> && input_range<range_reference_t<_Vw>>
     class join_view : public _Join_view_base<_Vw> {
         // clang-format on
+
+#ifndef _USE_JOIN_VIEW_INPUT_RANGE
+        static_assert(forward_range<_Vw>,
+            "Due to a design flaw, join_view can misbehave "
+            "with some input-only ranges (see https://wg21.link/lwg3698). "
+            "We believe that WG21 will be unable to fix this problem without breaking ABI. "
+            "To minimize breakage when a fix is implemented, "
+            "we are temporarily disabling potentially problematic cases. "
+            "You can define _USE_JOIN_VIEW_INPUT_RANGE to suppress this diagnostic, "
+            "but be aware that you will almost certainly need to recompile "
+            "when we release a fix.");
+#endif // _USE_JOIN_VIEW_INPUT_RANGE
+
     private:
         template <bool _Const>
         using _InnerRng = range_reference_t<_Maybe_const<_Const, _Vw>>;

tests/libcxx/usual_matrix.lst

@@ -4,7 +4,7 @@
 RUNALL_INCLUDE ..\universal_prefix.lst
 RUNALL_CROSSLIST
 # TRANSITION, LLVM-53957: _SILENCE_ALL_CXX23_DEPRECATION_WARNINGS belongs to llvm-project/libcxx/test/support/msvc_stdlib_force_include.h
-PM_CL="/EHsc /MTd /std:c++latest /permissive- /FImsvc_stdlib_force_include.h /wd4643 /D_STL_CALL_ABORT_INSTEAD_OF_INVALID_PARAMETER /D_SILENCE_ALL_CXX23_DEPRECATION_WARNINGS"
+PM_CL="/EHsc /MTd /std:c++latest /permissive- /FImsvc_stdlib_force_include.h /wd4643 /D_STL_CALL_ABORT_INSTEAD_OF_INVALID_PARAMETER /D_SILENCE_ALL_CXX23_DEPRECATION_WARNINGS /D_USE_JOIN_VIEW_INPUT_RANGE"
 RUNALL_CROSSLIST
 PM_CL="/analyze:autolog- /Zc:preprocessor"
 PM_COMPILER="clang-cl" PM_CL="-fno-ms-compatibility -fno-delayed-template-parsing"

tests/std/tests/P0896R4_views_join/test.cpp

@@ -1,6 +1,8 @@
 // Copyright (c) Microsoft Corporation.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+#define _USE_JOIN_VIEW_INPUT_RANGE
+
 #include <algorithm>
 #include <array>
 #include <cassert>