Add a CORRADE_MSVC_COMPATIBILITY option.

This will cover issues with MSVC2019+ that happen only when /permissive-
is not set -- most of the workarounds for MSVC 2019 were already
happening only when the standards conforming mode was not enabled, and I
expect this only get better with MSVC 2022.

The flag is impossible to detect via a preprocessor which means we can't
just disable / change certain parts of the API if the flag is missing,
thus it has to be done the old explicit way by setting it in CMake and
then remembering it inside configure.h. That way however avoids nasty
corner cases, especially when the API changes would change the ABI, so
that's nice.

Furthermore, to ensure /permissive- is always present when
CORRADE_MSVC_COMPATIBILITY is off, configure.h has a static_assert()
that fails when the flag is not present. This is the only (and not fully
robust) way to check the presence of the flag, but could be good enough.

CMakeLists.txt

@@ -60,6 +60,16 @@ project(Corrade CXX)
 set_property(GLOBAL PROPERTY USE_FOLDERS ON)
 
 include(CMakeDependentOption)
+
+# We want to present the option to the user, but we also want to implicitly
+# enable it during the first run, unless manually overriden. Thus we first
+# remember if it was defined, then define the option() and then below check the
+# remembered state.
+if(NOT DEFINED MSVC_COMPATIBILITY)
+    set(_CORRADE_MSVC_COMPATIBILITY_NOT_DEFINED ON)
+endif()
+option(MSVC_COMPATIBILITY "Enable compatibility mode for MSVC 2019+ without /permissive- set (might disable some features)" OFF)
+
 option(MSVC2019_COMPATIBILITY "Enable compatibility mode for MSVC 2019 (might disable some features)" OFF)
 option(MSVC2017_COMPATIBILITY "Enable compatibility mode for MSVC 2017 (might disable some features)" OFF)
 option(MSVC2015_COMPATIBILITY "Enable compatibility mode for MSVC 2015 (might disable some features)" OFF)
@@ -212,27 +222,42 @@ elseif(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
             set(MSVC2017_COMPATIBILITY ON)
             message(WARNING "MSVC 2017 detected, automatically enabling MSVC2017_COMPATIBILITY. Note that some features may not be available with this compiler.")
         endif()
-    elseif(CMAKE_CXX_COMPILER_VERSION VERSION_LESS "19.30")
-        if(NOT MSVC2019_COMPATIBILITY)
-            set(MSVC2019_COMPATIBILITY ON)
-            message(WARNING "MSVC 2019 detected, automatically enabling MSVC2019_COMPATIBILITY. Note that some features may not be available with this compiler.")
+    else()
+        if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS "19.30")
+            if(NOT MSVC2019_COMPATIBILITY)
+                set(MSVC2019_COMPATIBILITY ON)
+                message(WARNING "MSVC 2019 detected, automatically enabling MSVC2019_COMPATIBILITY. Note that some features may not be available with this compiler.")
+            endif()
+        endif()
+
+        # MSVC_COMPATIBILITY is enforced by MSVC2017_COMPATIBILITY and
+        # MSVC2015_COMPATIBILITY below, but can be disabled for 2019+.
+        if(_CORRADE_MSVC_COMPATIBILITY_NOT_DEFINED)
+            set(MSVC_COMPATIBILITY ON)
+            message(WARNING "MSVC 2019+ detected, automatically enabling MSVC_COMPATIBILITY which may cause some features to not be available. You can disable this option if you pass /permissive- to the compiler to enable a standards-conforming mode.")
         endif()
     endif()
 endif()
 
+if(MSVC_COMPATIBILITY)
+    set(CORRADE_MSVC_COMPATIBILITY 1)
+endif()
+
 if(MSVC2019_COMPATIBILITY)
     set(CORRADE_MSVC2019_COMPATIBILITY 1)
 endif()
 
 if(MSVC2017_COMPATIBILITY)
     set(CORRADE_MSVC2017_COMPATIBILITY 1)
     set(CORRADE_MSVC2019_COMPATIBILITY 1)
+    set(CORRADE_MSVC_COMPATIBILITY 1)
 endif()
 
 if(MSVC2015_COMPATIBILITY)
     set(CORRADE_MSVC2015_COMPATIBILITY 1)
     set(CORRADE_MSVC2017_COMPATIBILITY 1)
     set(CORRADE_MSVC2019_COMPATIBILITY 1)
+    set(CORRADE_MSVC_COMPATIBILITY 1)
 endif()
 
 if(TESTSUITE_TARGET_XCTEST)

doc/building-corrade.dox

@@ -572,17 +572,23 @@ for both debug and release configurations. This distinction is handled
 automatically when using the library in depending projects, see @ref corrade-cmake
 for more information.
 
-By default the library expects standards-conforming compiler. However, for
-portability, compatibility mode for some compilers is provided. The build
-system tries to autodect it, but in some rare cases you may need to enable
-the compatibility mode explicitly:
-
--   `MSVC2019_COMPATIBILITY` --- Enable for compiling with compatibility mode
-    for Microsoft Visual C++ 2019.
--   `MSVC2017_COMPATIBILITY` --- Enable for compiling with compatibility mode
-    for Microsoft Visual C++ 2017.
--   `MSVC2015_COMPATIBILITY` --- Enable for compiling with compatibility mode
-    for Microsoft Visual C++ 2015.
+By default the library expects a standards-conforming compiler. However, for
+portability, compatibility mode for some compilers is provided:
+
+-   `MSVC_COMPATIBILITY` --- Compiling with compatibility mode for Microsoft
+    Visual C++ 2019+ *without* the `/permissive-` flag set. Enabled implicitly
+    if MSVC (but not clang-cl) is detected. You can set it to `OFF` but then
+    you're required to specify `/permissive-` for all code using Corrade.
+-   `MSVC2019_COMPATIBILITY` --- Compiling with compatibility mode for
+    Microsoft Visual C++ 2019. Enabled implicitly if MSVC 2019 is detected,
+    implies `MSVC2022_COMPATIBILITY`.
+-   `MSVC2017_COMPATIBILITY` --- Compiling with compatibility mode for
+    Microsoft Visual C++ 2017. Enabled implicitly if MSVC 2017 is detected,
+    implies `MSVC2019_COMPATIBILITY` and `MSVC_COMPATIBILITY`.
+-   `MSVC2015_COMPATIBILITY` --- Compiling with compatibility mode for
+    Microsoft Visual C++ 2015. Enabled implicitly if MSVC 2015 is detected,
+    implies `MSVC2017_COMPATIBILITY`, `MSVC2019_COMPATIBILITY` and
+    `MSVC_COMPATIBILITY`.
 
 Corrade will detect the compiler used and refuses to build (or be used) if
 some required compatibility mode is not enabled. On the other hand, if any of

doc/corrade-changelog.dox

@@ -49,6 +49,8 @@ namespace Corrade {
     x86, ARM and WebAssembly
 -   New @ref CORRADE_TARGET_32BIT preprocessor variable for cross-platform
     detection of 32-bit builds
+-   New @ref CORRADE_MSVC_COMPATIBILITY flag that controls workarounds for MSVC
+    2019+ with the `/permissive-` flag not enabled.
 
 @subsubsection corrade-changelog-latest-new-containers Containers library
 

doc/corrade-cmake.dox

@@ -193,6 +193,8 @@ Features of found Corrade library are exposed in these variables, they are
 also available as preprocessor variables if you include
 @ref Corrade/Corrade.h "Corrade/Corrade.h":
 
+-   `CORRADE_MSVC_COMPATIBILITY` --- Defined if compiled with compatibility
+    mode for MSVC 2019+ without the `/permissive-` flag set
 -   `CORRADE_MSVC2019_COMPATIBILITY` --- Defined if compiled with compatibility
     mode for MSVC 2019
 -   `CORRADE_MSVC2017_COMPATIBILITY` --- Defined if compiled with compatibility

modules/FindCorrade.cmake

@@ -62,6 +62,8 @@
 #
 # Features of found Corrade library are exposed in these variables:
 #
+#  CORRADE_MSVC_COMPATIBILITY   - Defined if compiled with compatibility
+#   mode for MSVC 2019+ without the /permissive- flag set
 #  CORRADE_MSVC2019_COMPATIBILITY - Defined if compiled with compatibility
 #   mode for MSVC 2019
 #  CORRADE_MSVC2017_COMPATIBILITY - Defined if compiled with compatibility
@@ -313,6 +315,7 @@ set(_corradeFlags
     MSVC2015_COMPATIBILITY
     MSVC2017_COMPATIBILITY
     MSVC2019_COMPATIBILITY
+    MSVC_COMPATIBILITY
     BUILD_DEPRECATED
     BUILD_STATIC
     BUILD_STATIC_UNIQUE_GLOBALS

package/ci/appveyor.yml

@@ -86,7 +86,7 @@ environment:
     # probably due to a lot of "warning C4530: C++ exception handler used, but
     # unwind semantics are not enabled. Specify /EHsc" messages printed before.
     # Not sure why is that NOT a problem without /permissive-.
-    COMPILER_EXTRA: -DCMAKE_CXX_FLAGS="/permissive- /EHsc"
+    COMPILER_EXTRA: -DCMAKE_CXX_FLAGS="/permissive- /EHsc" -DMSVC_COMPATIBILITY=OFF
     PLATFORM: x64
     APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
     APPVEYOR_JOB_NAME: windows-conforming-msvc2019

src/Corrade/Corrade.h

@@ -35,6 +35,17 @@
 namespace Corrade {
 
 #ifdef DOXYGEN_GENERATING_OUTPUT
+/**
+@brief MSVC compatibility
+@m_since_latest
+
+Defined if compatibility mode for Microsoft Visual C++ 2019+ *without* the
+`/permissive-` flag set is enabled.
+@see @ref building-corrade, @ref corrade-cmake
+*/
+#define CORRADE_MSVC_COMPATIBILITY
+#undef CORRADE_MSVC_COMPATIBILITY
+
 /**
 @brief MSVC 2019 compatibility
 

src/Corrade/Test/TargetTest.cpp

@@ -239,7 +239,15 @@ void TargetTest::msvcPermissiveFlag() {
     CORRADE_INFO((sizeof(1 ? "" : "") == 1 ?
         "Standards-conforming C++ parser, compiled with the /permissive- flag" :
         "Non-conforming C++ parser, compiled without /permissive-"));
+
+    #ifdef CORRADE_MSVC_COMPATIBILITY
+    if(sizeof(1 ? "" : "") == 1)
+        CORRADE_WARN("CORRADE_MSVC_COMPATIBILITY set, but the parser is standards-conforming.");
     CORRADE_VERIFY(true);
+    #else
+    CORRADE_FAIL_IF(sizeof(1 ? "" : "") != 1,
+        "CORRADE_MSVC_COMPATIBILITY not set and the parser is not standards-conforming.");
+    #endif
 }
 #endif
 

src/Corrade/configure.h.cmake

@@ -26,6 +26,7 @@
     DEALINGS IN THE SOFTWARE.
 */
 
+#cmakedefine CORRADE_MSVC_COMPATIBILITY
 #cmakedefine CORRADE_MSVC2019_COMPATIBILITY
 #cmakedefine CORRADE_MSVC2017_COMPATIBILITY
 #cmakedefine CORRADE_MSVC2015_COMPATIBILITY
@@ -283,4 +284,34 @@
 #define CORRADE_LONG_DOUBLE_SAME_AS_DOUBLE
 #endif
 
+/* Kill switch for when absence of the /permissive- flag is detected and
+   CORRADE_MSVC_COMPATIBILITY is not defined. According to
+    https://developercommunity.visualstudio.com/t/pre-define-a-macro-when-compiling-under-permissive/1253982
+   there's STILL no macro that advertising presence of the flag (independently
+   verified with `cl /EP /Zc:preprocessor /PD empty.cpp 2>nul` on MSVC 2022).
+   I suppose it's because it would break PCHs?
+
+   However, because we need to remove various class members and do other nasty
+   stuff to work without /permissive-, we need a way to control it from the
+   preprocessor, which the CORRADE_MSVC_COMPATIBILITY macro is for. It's
+   enabled implicitly but the user can explicitly disable it during the build
+   of Corrade, which is then recorded into the generated configure.h file. The
+   /permissive- flag is then expected to be passed.
+
+   To avoid nasty issues with ABI mismatch, the following assert checks that
+   the flag is passed if CORRADE_MSVC_COMPATIBILITY is not enabled. The hope is
+   that CORRADE_MSVC_COMPATIBILITY gets eventually dropped altogether, with
+   this assert being checked always. But as /permissive- caused ICEs in some
+   MSVC 2017 versions, it's not feasible to enforce it yet.
+*/
+#if defined(CORRADE_TARGET_MSVC) && !defined(CORRADE_TARGET_CLANG_CL) && !defined(CORRADE_MSVC_COMPATIBILITY)
+/* Without /permissive- (or with /permissive- /Zc:ternary-, or with just
+   /Zc:ternary), it'd be sizeof(const char*) instead. Yes, it's not
+   bulletproof, but of all cases shown here it's the cheapest check:
+   https://docs.microsoft.com/en-us/cpp/build/reference/permissive-standards-conformance?view=msvc-170#ambiguous-conditional-operator-arguments */
+static_assert(sizeof(1 ? "" : "") == 1,
+    "Corrade was built without CORRADE_MSVC_COMPATIBILITY, but /permissive- " "doesn't seem to be enabled. Either rebuild Corrade with the option "
+    "enabled or ensure /permissive- is set for all files that include Corrade " "headers.");
+#endif
+
 #endif // kate: hl c++