> On Feb 4, 2015, at 11:17 PM, Richard Smith <[email protected]> wrote:
>
> On Wed, Feb 4, 2015 at 11:08 PM, John McCall <[email protected]
> <mailto:[email protected]>> wrote:
>> On Feb 4, 2015, at 9:36 PM, Richard Smith <[email protected]
>> <mailto:[email protected]>> wrote:
>> On Wed, Feb 4, 2015 at 6:38 PM, John McCall <[email protected]
>> <mailto:[email protected]>> wrote:
>> On Jun 5, 2014, at 4:17 PM, Richard Smith <[email protected]
>> <mailto:[email protected]>> wrote:
>> > John: any chance we could get the ABI document updated with these?
>> > (http://sourcerytools.com/pipermail/cxx-abi-dev/2012-January/000024.html
>> > <http://sourcerytools.com/pipermail/cxx-abi-dev/2012-January/000024.html>)
>>
>> After much delay, added. We don’t seem to get this right, though, at least
>> not when the destination type isn’t dependent:
>>
>> template <class T, class U> T fst(T, U);
>> struct A {
>> int x[3];
>> };
>> template <class T> decltype(fst(A{1,2},T())) foo(T t) {}
>>
>> int main() {
>> foo(1);
>> }
>>
>> We produce:
>> _Z3fooIiEDTcl3fstcv1AililLi1ELi2EEEcvT__EEES1_
>> It should be:
>> _Z3fooIiEDTcl3fsttl1ALi1ELi2EcvT__EEES1_
>>
>> There are quite a few bugs conspiring to give that result :( Our AST is also
>> poorly-suited to this mangling, because the braces are not considered to be
>> part of the functional cast itself; they're part of its subexpression.
>>
>> If you parenthesize the argument to A:
>> template <class T> decltype(fst(A({1,2}),T())) foo(T t) {}
>> We produce:
>> _Z3fooIiEDTcl3fstcv1AcvS0_ililLi1ELi2EEEcvT__EEES1_
>> It should be:
>> _Z3fooIiEDTcl3fstcv1AliLi1ELi2EcvT__EEES1_
>>
>> Somewhat related, we also get this wrong:
>>
>> struct X { X(int); };
>> int f(X);
>> template<typename T> void f(decltype(f(0), T())) { f(0); }
>> void g() { f<int>(0); }
>>
>> ... because we explicitly mangle the implicit conversion from int to X. I see
>>
>> _Z1fIiEvDTcmcl1fLi0EEcvT__EE from EDG
>> _Z1fIiEvDTcmclL_Z1f1XELi0EEcvT__EE from GCC
>> _Z1fIiEvDTcmclL_Z1f1XEcvS0_cvS0_Li0EEcvT__EE from Clang
>
> Ugh, that’s awful.
>
>> I think GCC and Clang are right to use the resolved name L_Z1f1XE rather
>> than the unresolved name 1f here, and GCC's mangling is right overall. Do
>> you agree?
>
>
>> As an aside: if we have a fully-resolved call in an instantiation-dependent
>> expression, should we really be putting any used default arguments into the
>> mangling?
>
> I feel like both of these points need to be asked on the cxx-abi-dev. I
> definitely don’t think we should be mangling default arguments, but I’m not
> sure that resolving ‘f’ here is really consistent with the general dictate to
> follow the syntactic tree.
>
>> All of the above fixed in r228274. I'm not really very happy with our AST
>> representation here; we've overloaded CXXConstructExpr to mean too many
>> different syntactic things that it's hard to reconstruct the right mangling.
>
> The rule used to be that a “bare" CXXConstructExpr — neither a specific
> subclass nor the implementation of a cast — was always implicit, and that
> there were subclasses which provided additional syntactic information. I
> think it would make sense to have a dedicated subclass for the truly implicit
> case as well. The implicit case is always a constructor conversion or
> copy-construction, right?
>
> Right. The oddball cases are CXXConstructExpr-used-for-list-initialization:
>
> f({1}, {2})
>
> (which clearly isn't a CXXFunctionalCastExpr but probably shouldn't be just a
> CXXConstructExpr either)
Yeah, this feels like an obvious candidate for a CXXConstructExpr subclass. Do
we just forget the brace locations completely in this case? Or is there some
sort of “I was written with braces instead of parens” flag on CXXConstructExpr?
> and CXXConstructExpr-used-for-direct-intiialization:
>
> T var(1, 2); // #1
> T var{1, 2}; // #2
Ah, right, this is a weird case.
> According to the rules we use in -ast-print, the parens belong to the
> initialization of the variable, not to the CXXConstructExpr, so that we can
> support
>
> int var(1);
>
> with no additional AST nodes beyond the IntegerLiteral expression, but the
> braces in #2 usually belong to the CXXConstructExpr. Except when T has a
> constructor that takes std::initializer_list<int>, when they don't, because
> the braces belong to the construction of the underlying array of int. *sigh*
Yuck.
John._______________________________________________
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