https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77660
Bug ID: 77660
Summary: Conversion operator unknown or ambiguous: gcc or clang
behave differently
Product: gcc
Version: 5.4.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: naupacte at sfr dot fr
Target Milestone: ---
In relation with bug #50306:
(using gcc5.4.1 or gcc6.1 from macports)
struct A;
struct B;
template <class T> struct Ptr
{
template <class U> operator const Ptr<U>& () const;
#ifdef GCC_BUT_CLANG_DECLARES_IT_IS_AMBIGUOUS
template <class U> operator Ptr<U> () const; // required by gcc
but skipped for clang
#endif
};
const Ptr<A>& foo1 (const Ptr<B>& b) { return b;}//gcc ok
Ptr<A> foo2 (const Ptr<B>& b) { return b;}// needs
GCC_BUT_CLANG_DECLARES_IT_IS_AMBIGUOUS to be set
Ptr<A> foo2a(const Ptr<B>& b) { return static_cast<Ptr<A>>(b);} //gcc
patch
Who is right? Gcc cannot convert from const Ptr<B>& to Ptr<A> via const
Ptr<A>&. I guess it has an influence on its inability to get the common type
for brace-lists or ?: pairs when only the reference conversion is given.
To avoid error: define GCC_BUT_CLANG_DECLARES_IT_IS_AMBIGUOUS only for gcc.
Otherwise, we get:
GCC
====
/opt/local/bin/gcc-mp-6 -x c++ -std=c++1y -c program.cpp
program.cpp: In function 'Ptr<A> foo2(const Ptr<B>&)':
program.cpp:22:47: error: could not convert 'b' from 'const Ptr<B>' to 'Ptr<A>'
Ptr<A> foo2 (const Ptr<B>& b) { return b;}// needs
GCC_BUT_CLANG_DECLARES_IT_IS_AMBIGUOUS to be set
CLANG
=====
clang-mp-3.8 -x c++ -std=c++1y -c program.cpp
-DGCC_BUT_CLANG_DECLARES_IT_IS_AMBIGUOUS
program.cpp:22:47: error: conversion from 'const Ptr<B>' to 'Ptr<A>' is
ambiguous
Ptr<A> foo2 (const Ptr<B>& b) { return b;}// needs
GCC_BUT_CLANG_DECLARES_IT_IS_AMBIGUOUS to be set
^
program.cpp:13:76: note: candidate function [with U = A]
template <class U> operator const Ptr<U>& () const;
^
program.cpp:15:76: note: candidate function [with U = A]
template <class U> operator Ptr<U> () const;
^
program.cpp:23:47: error: ambiguous conversion for static_cast from 'const
Ptr<B>' to 'Ptr<A>'
Ptr<A> foo2a(const Ptr<B>& b) { return static_cast<Ptr<A>>(b);} //gcc
patch
^~~~~~~~~~~~~~~~~~~~~~
program.cpp:11:27: note: candidate is the implicit copy constructor
template <class T> struct Ptr
^
program.cpp:11:27: note: candidate is the implicit move constructor
2 errors generated.
