Class template argument deduction for template type which is itself an argument for a template

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Is class template argument deduction supported for template types with default arguments which are used within another template type declaration? The following code does not compile with both Clang/GCC trunk, it fails on the line defining y:

#include <optional>

template <class T = char>
struct C {};

int main() {
    C x;
    std::optional<C> y;
}

(https://godbolt.org/z/SgxY90)

Modifying the code to read std::optional<C<>> y fixes the issue, but I am a bit surprised this is needed. Is this a compiler issue, or a known language limitation?

option-type c++17 templates c++
1 Answers

The compiler has to verify that what is passed to the template matches the anticipated template parameter. Now consider I were to write this with your example, where you don't see foo:

foo<C> f;

Is it a instance of CTAD, or am I passing the template itself? Since one can already pass templates as arguments to other templates.

template< template<typename> class T > struct foo {};

Allowing for CTAD in that context would make the use of C context dependent. In contrast, a template name has no other use when declaring a variable. There is no ambiguity when writing...

C c;

... that can mean something other than CTAD, so it's allowed here. But context matters when the template name is used as a template argument. C++ has a lot of context dependent constructs already, so adding more is usually a bad idea.

user_8176430
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