`<variant>`: Constructor inheritance causes ICE when class in named module inherits from class imported from `module std`

[JMazurkiewicz]

Basic Info

• I have read the rules for the Standard Library Modules Bug Bash.
• I have pulled and rebuilt the bug-bash branch in the last 24 hours.
• My current commit, as printed by git rev-parse HEAD, is: 6c96fe702ab23df846cca2110bc8dc08e2aa6d3a
• This bug is: ICE-on-valid

Command-Line Test Case

My test case that works with classic includes but fails with named modules:

PS D:\stl-playground> type .\mod.ixx

module;
#ifdef USE_CLASSIC_INCLUDES
#include <variant>
#endif
export module part;

#ifndef USE_CLASSIC_INCLUDES
import std;
#endif

using Base = std::variant<double, int>;

export struct DoubleOrInt : public Base {
    using Base::Base;
};

Other source file:

PS D:\stl-playground> type .\main.cpp

import part;

int main() {
    [[maybe_unused]] DoubleOrInt it_is_ice_time;
}

PS D:\stl-playground> $CXXFLAGS = '/EHsc', '/nologo', '/W4', '/std:c++latest', '/MTd', '/Od'
PS D:\stl-playground> $STL = 'D:\stl'      
PS D:\stl-playground> # Correct compilation
PS D:\stl-playground> cl $CXXFLAGS /c mod.ixx /reference $STL\std.ifc /DUSE_CLASSIC_INCLUDES    
mod.ixx
PS D:\stl-playground> cl $CXXFLAGS main.cpp mod.obj /reference $STL\std.ifc $STL\std.obj    
main.cpp
PS D:\stl-playground> .\main.exe
PS D:\stl-playground> # Bad compilation
PS D:\stl-playground> cl $CXXFLAGS /c mod.ixx /reference $STL\std.ifc  
mod.ixx
PS D:\stl-playground> cl $CXXFLAGS main.cpp mod.obj /reference $STL\std.ifc $STL\std.obj    
main.cpp
main.cpp(4): fatal error C1001: Internal compiler error.
(compiler file 'msc1.cpp', line 1576)
 To work around this problem, try simplifying or changing the program near the locations listed above.
If possible please provide a repro here: https://developercommunity.visualstudio.com
Please choose the Technical Support command on the Visual C++
 Help menu, or open the Technical Support help file for more information
INTERNAL COMPILER ERROR in 'C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.33.31627\bin\HostX64\x64\cl.exe'
    Please choose the Technical Support command on the Visual C++
    Help menu, or open the Technical Support help file for more information

Additional Context

• I've already reported similar issue about header units: https://developercommunity.visualstudio.com/t/C20-Exporting-Importing-module-par/10078204?space=62&sort=newest&q=module. The main difference is that header units fail only when they are used in module partition (not in primary module).
• I've also tested this with some other std:: classes:
  • When Base = std::optional<T>, std::unique_ptr<T>, std::shared_ptr<T> the ICE is present,
  • When Base = std::vector<T>, std::deque<T>, std::unordered_map<T, U> the code compiles.

[StephanTLavavej]

Thanks for the bug report! 😺 I'll send this to the compiler team later today.

[StephanTLavavej]

Confirmed, still repros with 17.3 Preview 5. Reported as internal VSO-1582916 "Standard Library Modules: ICE when class in named module inherits from variant with inheriting constructors".

[StephanTLavavej]

A compiler fix for this ICE is undergoing review!

[StephanTLavavej]

Compiler fix will ship in 17.4 Preview 3.