Re: [C++ Patch] PR 65579 ("gcc requires definition of a static constexpr member...")

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi again,

On 24/10/2017 20:58, Jason Merrill wrote:
> This seems like an odd place to add the complete_type call.  What
> happens if we change the COMPLETE_TYPE_P (type) in
> cp_apply_type_quals_to_decl to COMPLETE_TYPE_P (complete_type (type))?
Finally I'm back with some information.

Simply doing the above doesn't fully work. The first symptom is the 
failure of g++.dg/init/mutable1.C which is precisely the testcase that 
you added together with the "|| !COMPLETE_TYPE_P (type)" itself: 
clearly, the additional condition isn't able anymore to do its work, 
because, first, when the type isn't complete, TYPE_HAS_MUTABLE_P (type) 
is false and then, when in fact it would be found true, we check 
!COMPLETE_TYPE_P (complete_type (type)) which is false, because 
completing succeeded.

Thus it seems we need at least something like:

   TREE_TYPE (decl) = type = complete_type (type);

   if (TYPE_HAS_MUTABLE_P (type) || !COMPLETE_TYPE_P (type))
     type_quals &= ~TYPE_QUAL_CONST;

But then, toward the end of the testsuite, we notice a more serious 
issue, which is unrelated to the above: g++.old-deja/g++.pt/poi1.C

// { dg-do assemble  }
// Origin: Gerald Pfeifer <pfeifer@dbai.tuwien.ac.at>

template <class T>
class TLITERAL : public T
    {
    int x;
    };

class GATOM;

typedef TLITERAL<GATOM> x;
extern TLITERAL<GATOM> y;

also fails:

poi1.C: In instantiation of ‘class TLITERAL<GATOM>’:
poi1.C:13:24:   required from here
poi1.C:5:7: error: invalid use of incomplete type ‘class GATOM’
 class TLITERAL : public T
poi1.C:10:7: note: forward declaration of ‘class GATOM’
 class GATOM;

that is, trying to complete GATOM at the 'extern TLITERAL<GATOM> y;" 
line obviously fails. Note, in case isn't obvious, that this happens 
exactly for the cp_apply_type_quals_to_decl call at the end of 
grokdeclarator which I tried to change in my first try: the failure of 
poi1.C seems rather useful to figure out what we want to do for this bug.

Well, as expected, explicitly checking VAR_P && 
DECL_DECLARED_CONSTEXPR_P works again - it seems to me that after all it 
could make sense given the comment precisely talking about the 
additional complexities related to constexpr. Anyway, I'm attaching the 
corresponding complete patch.

Thanks!
Paolo.

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