Erroneous if constexpr instantiation in lambda template

[llvmbot]

Bugzilla Link	45506
Version	trunk
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@hubert-reinterpretcast,@zygoloid

Extended Description

clang++ erroneously instantiates the if constexpr statement commented "pfft" below; g++ and VS2019 accept it on local consensus is that it is a clang++ bug:

#include <cstdint>
#include <type_traits>
#include <variant>
#include <iostream>

template <typename T>
T foo(const std::variant<int64_t, double, std::string>& v)
{
    T result;
    std::visit([&result](auto&& a)
    {
        typedef std::decay_t<decltype(a)> vt;
        if constexpr (std::is_same_v<vt, double>)
            result = static_cast<T>(a);
        else if constexpr (std::is_same_v<vt, int64_t>)
            result = static_cast<T>(a);
        else if constexpr (std::is_same_v<vt, std::string> && std::is_class_v<T>)
            result = T::from_string(a); // pfft
        else
            throw std::logic_error("wibble");
    }, v);
    return result;
}

int main()
{
    std::cout << foo<double>(int64_t{});
}

[hubert-reinterpretcast]

Reduced:

template <typename T> int foo() {
  int ans;
  auto &&x = [&ans](auto a) {
    if constexpr (sizeof(double) != sizeof a)
      ans = sizeof(T::type);
  };
  x(0.);
  return ans;
}

int main() {
  foo<double>();
}

ICC produces an error similar to Clang's.
The question is whether or not instantiation of foo<double> needs to defer substitution into the subexpression of the constexpr-if or accept formation of impossible types.

[zygoloid]

This code is only valid after the resolution of http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0588r1.html, which Clang does not yet implement. Prior to that change, instantiation of a generic lambda in general requires full substitution into the body of the lambda in order to determine the set of captures.

[zygoloid]

Note, however, that P0588R1 was accepted into C++ as a defect report resolution, so Clang will apply it retroactively to all language modes when it is implemented.