Do we still need to write the empty angle brackets when using transparent std function objects?

Question

With class template argument deduction we can write:

std::less Fn;

However, G++ 8.2 rejects this code:

#include <algorithm>
#include <vector>
#include <functional>

int main()
{
std::vector v= { 1, 3, 2, 7, 5, 4 };

std::sort(v.begin(),v.end(),std::greater());
}

emitting the following error:

error: cannot deduce template arguments for 'greater' from ()

Clang++ 7.0 and MSVC 15.8.0 compile it without warnings. Which compiler is right?

Answer 1

GCC is wrong. There is already a bug report.

[dcl.type.simple]/2 says:

A type-specifier of the form typename_opt nested-name-specifier_opt template-name is a placeholder for a deduced class type ([dcl.type.class.deduct]).

And [dcl.type.class.deduct]/2 says:

A placeholder for a deduced class type can also be used in the type-specifier-seq in the new-type-id or type-id of a new-expression, as the simple-type-specifier in an explicit type conversion (functional notation) ([expr.type.conv]), or as the type-specifier in the parameter-declaration of a template-parameter. A placeholder for a deduced class type shall not appear in any other context.

Such use is allowed.

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[temp.arg]/4 describes the syntax error that a template-id is required but there is no <>. However here std::greater is not resolved as a template-id so that paragraph does not apply.