EDG's __is_convertible_to and default_initializable mishandle various scenarios #266

Issue Summary

I'm the primary maintainer of microsoft/STL, and we've encountered a bug in EDG, which we're working around in the STL. Using VS Insiders / MSVC Build Tools Preview, the following code is correctly accepted by the MSVC Compiler 19.52.36815 and Clang 22.1.3, but incorrectly rejected by EDG 6.9. This is using MSVC's undocumented /BE option to invoke VS's copy of the EDG frontend used for IntelliSense.

When we reported this (as three separate bugs) to EDG before it was open-sourced, we were told that they were all duplicates of the same underlying bug, being tracked as EDGcpfe/23985. (For my own reference, this was also MSVC-internal VSO-1898937, VSO-1898941, and VSO-1898945.)

Reproducing Source Code

struct Base {
    int x;
};
struct PrivateDerived : private Base {
    void f();
};
struct ExplicitDefault {
    explicit ExplicitDefault() = default;
};
struct AggregatesExplicitDefault {
    ExplicitDefault meow;
};

template <class D, class B>
concept converts_to = __is_convertible_to(D*, B*);
void PrivateDerived::f() {
    // EDG's `__is_convertible_to` depends on whether the private members can be accessed in the current context.
    // Check these in a member to verify that access doesn't depend on context:
    static_assert(!converts_to<PrivateDerived, Base>);
}

void* operator new(decltype(sizeof(int)), void*) noexcept;
template <class Ty>
concept default_initializable = requires {
    Ty{};
    ::new (static_cast<void*>(nullptr)) Ty;
};
static_assert(!default_initializable<const Base>);
static_assert(!default_initializable<AggregatesExplicitDefault>);

Command-line Options

cl /EHsc /nologo /W4 /std:c++latest /c /BE /dE--version meow.cpp

Type of Issue

[Front End] This code SHOULD compile with these options.

Additional Details

Compiler output showing how MSVC and Clang accept while EDG rejects:

C:\Temp>cl
Microsoft (R) C/C++ Optimizing Compiler Version 19.52.36815 for x64 (PREVIEW)
Copyright (C) Microsoft Corporation.  All rights reserved.

usage: cl [ option... ] filename... [ /link linkoption... ]

C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /c meow.cpp
meow.cpp

C:\Temp>clang-cl -v
clang version 22.1.3 (https://github.com/llvm/llvm-project e9846648fd6183ee6d8cbdb4502213fcf902a211)
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Program Files\Microsoft Visual Studio\18\Insiders\VC\Tools\Llvm\x64\bin

C:\Temp>clang-cl /EHsc /nologo /W4 /std:c++latest /c meow.cpp

C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /c /BE /dE--version meow.cpp
meow.cpp
Edison Design Group C/C++ Front End, version 6.9 (Sep 15 2026 13:46:37)
Copyright 1988-2026 Edison Design Group, Inc.

"meow.cpp", line 19: error: static assertion failed
      static_assert(!converts_to<PrivateDerived, Base>);
      ^

"meow.cpp", line 28: error: static assertion failed
  static_assert(!default_initializable<const Base>);
  ^

"meow.cpp", line 29: error: static assertion failed
  static_assert(!default_initializable<AggregatesExplicitDefault>);
  ^

Reproducing (Standard) Configuration(s)

windows-msvc-release

Reproducing (Non-standard) Configuration

dump_configuration.txt

I acknowledge that:

• I have checked open issues and existing discussions before filing this report.
• The reproducing source code that I have provided does not depend on headers that I have not included with my submission.
• I have reduced the reproducing source code to the best of my ability.
• I have selected at least one standard configuration or uploaded a non-standard configuration dump.
• I understand code shared via this form is NOT confidential.
• I have permission to share the reproducing source code and grant permission for the project to use the source code (or derivatives of the source code) to create tests.
• I agree that any tests created using the reproducing source code (or deriatives of it) will be licensed under project license unless I included a LICENSE.txt file in my .zip or .tar.gz upload. If I included a LICENSE.txt file, any tests created using the reproducing source code will be governed by the terms of the license I provided instead.