Is __declspec a core language extension?

Reid Kleckner rnk at
Wed May 20 15:17:45 PDT 2015

+1 for throwing it under -fms-extensions.

On Wed, May 20, 2015 at 2:54 PM, Aaron Ballman <aaron at>

> While refactoring some attribute parsing code, I noticed that we do
> not guard __declspec as being a Microsoft-specific extension that we
> support. This is consistent with the behavior we have for
> __attribute__. In both cases, the keyword is available everywhere, and
> specific attributes determine their target requirements. However, I
> wonder if that's the behavior we want.
> With __attribute__, it makes some degree of sense that we support it
> everywhere and own it as a core language extension. For instance, we
> allow users to add GNU-style attribute spellings that GCC does not
> support and no one takes issue.
> Do we want the same level of ownership over __declspec? If someone
> wants to add a new attribute with a __declspec spelling that is not
> supported by MSVC, will we allow it? I struggle to imagine us allowing
> that as it can cause future compatibility issues if Microsoft were to
> use the same attribute name with different semantics. For that reason,
> I think __declspec should be guarded by -fms-extensions, but I wanted
> to get community feedback before making such a change.
> Thanks!
> ~Aaron
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