[cxx-abi-dev] mangling for fold-expressions

[Richard Smith]

On 10 November 2014 09:11, Jason Merrill <jason at redhat.com> wrote:

> On 11/08/2014 12:16 PM, Richard Smith wrote:
>
>> <expression> ::=
>>    fl <binary operator-name> <expression>   # ( ... op pack )
>>    fr <binary operator-name> <expression>   # ( pack op ... )
>>    fx <binary operator-name> <expression> <expression>   # ( expr op ...
>> op
>> expr )
>>
>
> Do we really need mangling for the third case?  I was thinking it would
> reduce to one of the others plus a normal binary operation.


The extra element is always the innermost element, so it's not equivalent
to a normal binary operator combined with one of the other forms. For
instance:

  (1 / ... / divisors)

expands to

  ((1 / d1) / ...) / dn

There is no way to get the same result with (... / divisors).


And as an extreme case,

template<int> struct X {};
template<int ...N> using Foldr = X<(N + ...)>;
template<int ...A> struct Partial {
  template<int ...B> void foldr(Foldr<A..., B..., A..., B...>);
};
template void Partial<1,2>::foldr<3,4>(X<20>);

would mangle as

_ZN7PartialIJLi1ELi2EEE5foldrIJLi3ELi4EEEEv1XIXplLi1EplLi2EfxplT_plLi1EplLi2EfrplT_EE

Note that Foldr<A..., B..., A..., B...> becomes X<(A + ... + (B + ... + (A
+ ... + (B + ...))))> (the unary form turns into the binary form after a
partial substitution).
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