clang-19 returns true for __is_literal_type on union with non trivial member starting with C++-23

[AMS21]

Given the following code

struct non_trivial {
    ~non_trivial();
};

union non_trivial_union {
    int i;
    non_trivial n;
};

static_assert(!__is_literal_type(non_trivial), "");
static_assert(!__is_literal_type(non_trivial_union), "");

clang-18.1.0 compiles fine while clang-19 fails the second static_assert.

godbolt

Note that this does not happen in C++-20 mode godbolt

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (AMS21)

Given the following code

struct non_trivial {
    ~non_trivial();
};

union non_trivial_union {
    int i;
    non_trivial n;
};

static_assert(!__is_literal_type(non_trivial), "");
static_assert(!__is_literal_type(non_trivial_union), "");

clang-18.1.0 compiles fine while clang-19 fails the second static_assert.

godbolt

Note that this does not happen in C++-20 mode godbolt

[Sirraide]

This is a bit strange. Afaik a union is literal if it contains at least one literal nsdm (provided there is no other reason for it to be non-literal). It is a bit strange that the result is different in C++23 and C++20 though.

CC @zygoloid @shafik

[AMS21]

Just took another look at my test code and there seems to be a lot of disagreement if this type is literal or not.

gcc before 10.0 says it's always: not literal
gcc since 10.0 says it's literal starting with C++-20
MSVC 2019/2022 say it's literal starting with C++-17

With that and the fact that is_literal_type was removed in C++-20 feel free to close as won't fix.

[frederick-vs-ja]

Seems related to #77753. The implicitly declared destructor of non_trivial_union is non-trivial and deleted. IIUC non_trivial_union becomes a literal type since C++20.

[Sirraide]

With that and the fact that is_literal_type was removed in C++-20 feel free to close as won't fix.

That would explain the discrepancy, yeah. I’d leave the issue open in case I either misread the standard and the type is in fact not supposed to be literal (because we currently think it is, except in C++23 mode, apparently) or in case we do decide to do something about this.

[Fznamznon]

I think it works as expected in C++23 mode. Union with at least one literal member is literal. The fact why it is not considered literal by clang in C++20 mode is the destructor. If a class doesn't have a constexpr destructor then it is not literal. Before C++23 clang takes into account destructors for all members of a class to decide that its destructor is constexpr (i.e. for destructor of a class to be constexpr, destructors of all of its members should be constexpr). I also double checked that union is a class https://eel.is/c++draft/class.union#general-1 , so this all worked for unions in pre C++23 too. non_trivial_union has constexpr destructor only after C++23 since its members don't matter anymore, hence the outcome.