C++20: Deduction guides ignored for class type placeholder template argument

[devreal]

I experimented with placeholders for deduced class types in templates using std::array (passing it as a template argument without explicitly specifying std::array during instantiation, instead relying on deduction guides). I found that GCC uses deduction guides to deduce the argument type while Clang seems to ignore them.

Here is the smallest example I could come up with based on std::array:

// trimmed-down std::array
template<class T, int I>
struct A {
    T x[I];
};

// deduction guide (like std::array)
template< class T, class... U >
A( T, U... ) -> A<T, 1 + sizeof...(U)>;

template<A a> void foo() { }

void bar() {
    foo<{1}>(); // works with GCC, fails with Clang
}

See also: https://godbolt.org/z/7vWa3zKoj

Apologies if this was reported before, I tried to search the open issues but couldn't find anything related to this. Also, I am not sure whether this is legal in C++20, I was just surprised to see it work in GCC but not in Clang. Please feel free to close this issue if this is not legal.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Joseph Schuchart (devreal)

I experimented with placeholders for deduced class types in templates using `std::array` (passing it as a template argument without explicitly specifying `std::array` during instantiation, instead relying on deduction guides). I found that GCC uses deduction guides to deduce the argument type while Clang seems to ignore them.

Here is the smallest example I could come up with based on std::array:

// trimmed-down std::array
template<class T, int I>
struct A {
    T x[I];
};

// deduction guide (like std::array)
template< class T, class... U >
A( T, U... ) -> A<T, 1 + sizeof...(U)>;

template<A a> void foo() { }

void bar() {
    foo<{1}>(); // works with GCC, fails with Clang
}

See also: https://godbolt.org/z/7vWa3zKoj

Apologies if this was reported before, I tried to search the open issues but couldn't find anything related to this. Also, I am not sure whether this is legal in C++20, I was just surprised to see it work in GCC but not in Clang. Please feel free to close this issue if this is not legal.

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Joseph Schuchart (devreal)

I experimented with placeholders for deduced class types in templates using `std::array` (passing it as a template argument without explicitly specifying `std::array` during instantiation, instead relying on deduction guides). I found that GCC uses deduction guides to deduce the argument type while Clang seems to ignore them.

Here is the smallest example I could come up with based on std::array:

// trimmed-down std::array
template<class T, int I>
struct A {
    T x[I];
};

// deduction guide (like std::array)
template< class T, class... U >
A( T, U... ) -> A<T, 1 + sizeof...(U)>;

template<A a> void foo() { }

void bar() {
    foo<{1}>(); // works with GCC, fails with Clang
}

See also: https://godbolt.org/z/7vWa3zKoj

Apologies if this was reported before, I tried to search the open issues but couldn't find anything related to this. Also, I am not sure whether this is legal in C++20, I was just surprised to see it work in GCC but not in Clang. Please feel free to close this issue if this is not legal.

[shafik]

This seems like it should MSVC accepts it as well but edg does not: https://godbolt.org/z/vccfzr5sW

Clang does accept this case: https://godbolt.org/z/vKTxEEr7z

So I wonder if this comes down to: #58434

[cor3ntin]

Should be fixed by #73103