[Bug c++/91292] Mangler incorrectly handles negative numbers in expressions

["richard-gccbugzilla at metafoo dot co.uk" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91292

--- Comment #7 from Richard Smith <richard-gccbugzilla at metafoo dot co.uk> ---
(In reply to Patrick Palka from comment #3)
> Hmm, but according to
> http://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangling.literal the
> mangling of a negative integer literal is prefixed with "n",

There is no such thing as a negative integer literal.

The ABI document says "negative integer *values* are preceded with "n""; this
case is reached when mangling fully-resolved template arguments via the

<template-arg> ::= <expr-primary>

production, not when mangling an instantiation-dependent expression. For
example, given

template<int> struct X {};
template<int N> void f(X<N * -1>, X<-1>) {}
template void f<1>(X<-1>, X<-1>);

the proper mangling is

_Z1fILi1EEv1XIXmlT_ngLi1EEES0_ILin1EE

using ngLi1E for the instantiation-dependent expression -1 and Lin1E for the
non-instantiation-dependent value -1.


(In reply to Patrick Palka from comment #4)
> And if -(1) is to be mangled the same as -1, then shouldn't
> 
>   template<typename T>
>   typename std::enable_if<(int)sizeof(T) >= -(1), int>::type size1(T *t);
> 
>   template<typename T>
>   typename std::enable_if<(int)sizeof(T) >= -1, int>::type size1(T *t);
> 
> be considered two declarations of the same function?  But IIUC that would
> contradict [temp.over.link]p5, which says
> 
>   Two expressions involving template parameters are considered equivalent if
> two function definitions containing the expressions would satisfy the
> one-definition rule
> 
> but IIUC the one-definition rule fails here because -1 is not the same
> (token-wise) as -(1).

These declarations are functionally-equivalent but not equivalent, so a program
is not permitted to contain both. That language rule exists in order to allow
implementations to do things like ignore parentheses in mangling, as the
Itanium C++ ABI does.

Note that parentheses are never mangled (except for a weird corner case
involving pointers to members), so if your argument were correct it would apply
very broadly. For example, that argument would imply that -1 and (-1) would
need different manglings.