<variant>: Failed to use an empty class as template parameter

[DuKeM-CSGO]

Describe the bug

To compile the following file:

1. Comment line 1 module; and line 4 export module Module; so that don't treat this file as an interface module unit and compile successfully.
2. Uncomment line 7 int a; to add a member to class Dummy and compile successfully.

Or it can't be compiled due to internal compiler error.

Command-line test case

repro.ixx

module;
#include <variant>

export module Module;

struct Dummy{
	//int a;
};

using WrappedDummy = std::variant<Dummy>;

template<const WrappedDummy&>
struct Container {};

constexpr auto nttp = WrappedDummy{};

constexpr auto var = Container<nttp>{};

C:\Temp>cl /std:c++latest .\repro.ixx
Microsoft (R) C/C++ Optimizing Compiler Version 19.42.34436 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

/std:c++latest is provided as a preview of language features from the latest C++
working draft, and we're eager to hear about bugs and suggestions for improvements.
However, note that these features are provided as-is without support, and subject
to changes or removal as the working draft evolves. See
https://go.microsoft.com/fwlink/?linkid=2045807 for details.

.\repro.ixx(18): fatal error C1001: Internal compiler error.
(compiler file 'msc1.cpp', line 1599)
 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 'H:\Program Files\Microsoft Visual Studio\VC\Tools\MSVC\14.42.34433\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

Expected behavior

compile successfully

STL version

Microsoft Visual Studio Community 2022
Version 17.12.4

[StephanTLavavej]

Thanks for the bug report! ICEs are always compiler bugs by definition, so let's ping @cdacamar.

[StephanTLavavej]

Cameron believes that this is a duplicate of DevCom-10736790 (internal VSO-2238628) "Reference NTTPs broken in modules (ICE)", which is a bug in constexpr.