[clang]Should operands in ternary operator be allowed to have explicit copy constructor? #123700
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Description
I've got a very confusing output with the following test:
struct QString
{
QString()=default;
explicit QString( const QString & );
};
QString def(){
return QString{};
};
void foo( bool b, QString s )
{
auto res = b ? def() : s;
}It turns out that GCC accepts this with -std=c++17 or above; EDG and ICC always accept, and clang and MSVC always reject even with -std=c++17 or above: https://godbolt.org/z/zhza8Krfa
The diagnostic of clang and MSVC points out that if s is taken, the conversion from s to res will implicitly use the copy ctor of QString, which is not allowed:
<source>:13:29: error: no matching constructor for initialization of 'QString'
13 | auto res = b ? def() : s;
| ^
<source>:4:14: note: explicit constructor is not a candidate
4 | explicit QString( const QString & );
| ^
<source>:3:5: note: candidate constructor not viable: requires 0 arguments, but 1 was provided
3 | QString()=default;
| ^
1 error generated.Although the diagnostic sounds reasonable here, I still wonder if the standard has been made some changes since C++17 according to the behavior of GCC.