Clang chokes on BOOST_HANA_STRING inside nested polymorphic lambdas

[BillyDonahue]

Clang chokes when BOOST_HANA_STRING is enclosed by 2 layers of polymorphic lambdas.

cannot initialize return object of type 'decltype(auto)' with an lvalue of type 'const char [4]'

Tested with Boost-1.75.0, Clang-11.0.1, MacOS & CompilerExplorer. GCC not affected. https://gcc.godbolt.org/z/6WsY7e

If either of the lambdas is not polymorphic, there's no problem.

#include <cstdio>
#include <boost/hana.hpp>

template <typename T>
void print(T&& t) {
    printf(t.c_str());
}

void ok() {
    [](auto) {
        [](int) {
            auto str = BOOST_HANA_STRING("xxx");
            printf("%s\n", str.c_str());
        }(2);
    }(1);
}

void err() {
    [](auto) {
        [](auto) {  // <== changed to auto and now build fails under clang.
            auto str = BOOST_HANA_STRING("xxx");
            printf("%s\n", str.c_str());
        }(2);
    }(1);
}

int main() {
    ok();
    err();
    return 0;
}

https://raw.githubusercontent.com/BillyDonahue/experimental/hana_repro/per_file_template/hana_string_repro.cpp

<source>:21:42: error: cannot initialize return object of type 'decltype(auto)' with an lvalue of type 'const char [4]'
            auto str = BOOST_HANA_STRING("xxx");
                                         ^~~~~
/opt/compiler-explorer/libs/boost_1_75_0/boost/hana/string.hpp:105:60: note: expanded from macro 'BOOST_HANA_STRING'
            static constexpr decltype(auto) get() { return s; }             \
                                                           ^
<source>:21:24: note: in instantiation of member function 'err()::(anonymous class)::operator()(int)::(anonymous class)::operator()(auto)::(anonymous class)::operator()()::tmp::get' requested here
            auto str = BOOST_HANA_STRING("xxx");
                       ^
/opt/compiler-explorer/libs/boost_1_75_0/boost/hana/string.hpp:104:16: note: expanded from macro 'BOOST_HANA_STRING'
        struct tmp {                                                        \
               ^
<source>:24:6: note: in instantiation of function template specialization 'err()::(anonymous class)::operator()<int>' requested here
    }(1);
     ^
1 error generated.

[ldionne]

This definitely looks like a compiler bug to me, can you please report against Clang?

Here's a reduced reproducer: https://godbolt.org/z/d4hMevrPo

int main() {
    // OK
    [](auto) {
        [](int) {
            struct tmp {                                                        
                static constexpr auto get() { return "xxx"; }         
            };
        }(2);
    }(1);

    // NOT OK
    [](auto) {
        [](auto) {
            struct tmp {                                                        
                static constexpr auto get() { return "xxx"; }         
            };
        }(2);
    }(1);
}