Alias template CTAD is erroneously allowed in C++17 mode

[PJBoy]

Alias template CTAD is a C++20 feature ( https://en.cppreference.com/w/cpp/compiler_support/20#cpp_deduction_guides_201907L ) recently introduced in clang v19. Seems it's being allowed in C++17 mode with no diagnostic ( https://godbolt.org/z/x7zdxnrqx ). Though a (almost correct) diagnostic is issued with the -Wpre-c++17-compat flag ( https://godbolt.org/z/acKsG3dfj ).

Testcase is replicated here

template<typename T, typename TT>
struct S
{
    T t;
    TT tt;
};

template<typename T, typename TT>
S(T, TT) -> S<T, TT>;

template<typename T>
using S_int = S<T, int>;

int main()
{
    (void) S_int{1, 2};
}

[llvmbot]

@llvm/issue-subscribers-c-17

Author: Patrick Johnston (PJBoy)

Alias template CTAD is a C++20 feature ( https://en.cppreference.com/w/cpp/compiler_support/20#cpp_deduction_guides_201907L ) recently introduced in clang v19. Seems it's being allowed in C++17 mode with no diagnostic ( https://godbolt.org/z/x7zdxnrqx ). Though a (almost correct) diagnostic is issued with the `-Wpre-c++17-compat` flag ( https://godbolt.org/z/acKsG3dfj ).

Testcase is replicated here

template<typename T, typename TT>
struct S
{
    T t;
    TT tt;
};

template<typename T, typename TT>
S(T, TT) -> S<T, TT>;

template<typename T>
using S_int = S<T, int>;

int main()
{
    (void) S_int{1, 2};
}

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Patrick Johnston (PJBoy)

Alias template CTAD is a C++20 feature ( https://en.cppreference.com/w/cpp/compiler_support/20#cpp_deduction_guides_201907L ) recently introduced in clang v19. Seems it's being allowed in C++17 mode with no diagnostic ( https://godbolt.org/z/x7zdxnrqx ). Though a (almost correct) diagnostic is issued with the `-Wpre-c++17-compat` flag ( https://godbolt.org/z/acKsG3dfj ).

Testcase is replicated here

template<typename T, typename TT>
struct S
{
    T t;
    TT tt;
};

template<typename T, typename TT>
S(T, TT) -> S<T, TT>;

template<typename T>
using S_int = S<T, int>;

int main()
{
    (void) S_int{1, 2};
}

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor has already been assigned to this issue. If you believe that no one is actually working on it despite an assignment, ping the person. After one week without a response, the assignee may be changed.
2. In the comments of this issue, request for it to be assigned to you, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Patrick Johnston (PJBoy)

Alias template CTAD is a C++20 feature ( https://en.cppreference.com/w/cpp/compiler_support/20#cpp_deduction_guides_201907L ) recently introduced in clang v19. Seems it's being allowed in C++17 mode with no diagnostic ( https://godbolt.org/z/x7zdxnrqx ). Though a (almost correct) diagnostic is issued with the `-Wpre-c++17-compat` flag ( https://godbolt.org/z/acKsG3dfj ).

Testcase is replicated here

template<typename T, typename TT>
struct S
{
    T t;
    TT tt;
};

template<typename T, typename TT>
S(T, TT) -> S<T, TT>;

template<typename T>
using S_int = S<T, int>;

int main()
{
    (void) S_int{1, 2};
}

[GeorgeKA]

Hi @cor3ntin & @PJBoy . I'm interested in working on this. Can it be assigned to me?

[vinay-deshmukh]

I'd like to pick this one up, if ok with you @GeorgeKA

Haven't worked on the clang-frontend yet, so would appreciate any pointers on where to start looking at this issue. @cor3ntin