What does the ms-extensions flag do exactly with gcc?

Question

GCC has a flag -fms-extensions.

What does this flag do exactly? Why is it sometimes on by default, and why does it exist?

Answer 1

According to the gcc 9.1.0 source code (grepped for flag_ms_extensions) the effects are:

• (C) Allow Microsoft's version of anonymous unions and struct. This includes support for C11 anonymous unions and structs as well as Microsoft-specific flavours, including omitting the braced member list entirely, and placing members in the parent namespace even if the struct/union had an identifier.
• (C++) Allow a class member to have the same name as its type (e.g. using foo = int; struct A { foo foo; }). With ms-extensions disabled, the behaviour is to accept this code in C (where it is legal); or an extern "C" block unless -pedantic flag was given. The error message for this is declaration of NAME changes meaning of NAME.
• (C++) Allow implicit int; any situation that would have produced the diagnostic ISO C++ forbids declaration of NAME with no type is now allowed, with int assumed as the type. Examples: const *p; or const f();.
• (C++) Allow implicit conversion from a qualified-id naming a non-static member function, to a pointer-to-member. In ISO C++ the & operator is required to perform that conversion.
• (C++) Allow &f to form a pointer-to-member, if f (an unqualified-id) names a non-overloaded member function in that context. ISO C++ requires explicit qualification with the class name.

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The flag is turned on by default if the Target ABI is a Microsoft ABI. It can be disabled by manually specifying -fno-ms-extensions.

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The rationale behind this is a tougher question. The documentation has to say:

Accept some non-standard constructs used in Microsoft header files.

Disable Wpedantic warnings about constructs used in MFC.

So I assume the rationale is to allow g++ to build MFC applications which depend on non-standard code in MSVC vendor-supplied headers.

I am not sure how relevant that still is in 2019 and I think a good case could be made for gcc to default to having this flag turned off. (Users can always specify it if they want to build an old MFC app).

For example MSVC 19.xx (the latest version to date) no longer allows the last three bullet points in its default mode. (It does still allow foo foo; even with /Za flag).