EDG's __is_aggregate builtin should be updated to match LWG-3823's preconditions

[StephanTLavavej]

Issue Summary

I'm the primary maintainer of microsoft/STL, and we've encountered an issue 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 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 to EDG before it was open-sourced, we were told this was being tracked as EDGcpfe/25827. (For my own reference, this was also MSVC-internal VSO-1690654.)

The issue is that after LWG-3823 "Unnecessary precondition for is_aggregate" was accepted, N5054 [tab:meta.unary.prop] says:

Template	Condition	Preconditions
template<class T> struct is_aggregate;	T is an aggregate type ([dcl.init.aggr])	T shall be an array type, a complete type, or cv void.

This allows arrays of incomplete types (and arrays are always aggregates).

We're working around this in MSVC's STL by activating EDG-specific product code to detect arrays as a special case. We would prefer to use the same code for all compilers, directly using the __is_aggregate builtin. (This follows our long-standing philosophy in the <type_traits> header of Don't Help The Compiler, i.e. the compiler should be responsible for answering type traits queries without extra layers of Library code. This results in the highest throughput and least potential for bugs.)

Reproducing Source Code

template <class T>
constexpr bool is_aggregate_v = __is_aggregate(T);

static_assert(!is_aggregate_v<int>);
static_assert(is_aggregate_v<int[5]>);
static_assert(is_aggregate_v<int[]>);

struct Incomplete;

static_assert(is_aggregate_v<Incomplete[5]>);
static_assert(is_aggregate_v<Incomplete[]>);

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 2: error: type of array element must be complete
  constexpr bool is_aggregate_v = __is_aggregate(T);
                                  ^
          detected during instantiation of "const bool is_aggregate_v [with
                    T=Incomplete [5]]" at line 10

"meow.cpp", line 2: error: type of array element must be complete
  constexpr bool is_aggregate_v = __is_aggregate(T);
                                  ^
          detected during instantiation of "const bool is_aggregate_v [with
                    T=Incomplete []]" at line 11

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.