return-type deduction failure for lambda inside decltype.

[jehelset]

This fails to compile on clang-trunk.

template<int I>
using X = decltype(
    []{
        if constexpr(I == 0)
            return int{};
        else
            return char{};
    }()
);

template<int I>
inline constexpr auto Y = X<I>{};

static_assert(Y<1> == char{});

Need to inject a template-param () for it to work.

Works on gcc. Godbolt: https://godbolt.org/z/3Ydv8b866.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (jehelset)

This fails to compile on clang-trunk. ```cpp template<int I> using X = decltype( []{ if constexpr(I == 0) return int{}; else return char{}; }() );

template<int I>
inline constexpr auto Y = X<I>{};

static_assert(Y<1> == char{});

Need to inject a template-param (<int J = I>) for it to work.

Works on gcc. Godbolt: https://godbolt.org/z/3Ydv8b866.
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