[cfe-dev] [ms-mode] Delayed parsing for a UnaryExprOrTypeTraitExpr?

[Will Wilson]

Hi All,

I've been attempting to find a clean way of dealing with an issue parsing
the core MS ATL headers in clang. The issue is a missing typename keyword
in a sizeof expression containing a dependent name in the header atlstr.h.
Annoyingly, this is the last error I'm encountering in the core MS headers.

A simplified example is:

template <class T>
size_t GetSize() {
return sizeof(T::SubType); // Should have typename keyword
}

struct TestStruct {
typedef int SubType;
};

void test_typename_missing_msext() {
GetSize<TestStruct>();
}


This passes in MSVC as it leaves almost all template evaluation
until instantiation by which time it can deduce if T::SubType refers to a
typename or a static member.

Clang does the right thing and relies on the typename keyword to decide
before institution whether the dependent expression refers to a type or a
non-type member. Of note, clang already supports (with warnings) parsing
dependent names missing the typename keyword - but only in locations where
only a type is permitted (as implemented by Francois in r129425). The
problem is dealing with a sizeof expression where the ambiguity cannot be
resolved until instantiation.

So, in short are there any safe approaches I could take with this (with a
view to patching clang)? From my perspective it looks like there needs to
be a third "delayed evaluation" state for UnaryExprOrTypeTraitExpr in order
to support MicrosoftMode correctly. Or even a new
DelayedUnaryExprOrTypeTraitExpr for MicrosoftMode. Does this sound
reasonable or is there a better approach?

- Will.
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