[clang] Inconsistent (stateful) compilation of class template default member initializer from parenthesized aggregate initialization

[ecatmur]

Since 16.0.0 and on trunk 0acb639:

template<int I> struct S { int x, y, z = I; };
constexpr S<7> s1(1, 2);
using _ = decltype(S<7>{});
constexpr S<7> s2(3, 4);
static_assert(s1.z == s2.z);

fails with:

<source>:6:15: error: static assertion failed due to requirement 's1.z == s2.z'
static_assert(s1.z == s2.z);
              ^~~~~~~~~~~~
<source>:6:20: note: expression evaluates to '0 == 7'
static_assert(s1.z == s2.z);
              ~~~~~^~~~~~~

Condensed to a single evaluation:

template<int I> struct S { int x, y, z = I; };
static_assert(S<7>(1, 2).z == S<S<7>{}.z>(3, 4).z);

<source>:8:15: error: static assertion failed due to requirement 'S<7>((1, 2, /*implicit*/(int)0)).z == S<7>((3, 4, )).z'
static_assert(S<7>(1, 2).z == S<S<7>{}.z>(3, 4).z);
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:8:28: note: expression evaluates to '0 == 7'
static_assert(S<7>(1, 2).z == S<S<7>{}.z>(3, 4).z);
              ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~

My guess would be that the default member initializer is lazy initialized in templates but this is not provoked by parenthesized aggregate initialization.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[shafik]

Confirmed: https://godbolt.org/z/s63oh9eMM

Modified a bit:

template<int I> struct S { int x, y, z = I; };
constexpr S<7> s1a(1, 2);
constexpr S<7> s1{1, 2};

constexpr S<7> s2a(3, 4);
constexpr S<7> s2{3, 4};


int main() {
    return s1.z + s2.z + s1a.z + s2a.z;
}

It does look related to parens init of aggregates maybe @alanzhao1 might have some ideas here.

[llvmbot]

@llvm/issue-subscribers-c-20

[alanzhao1]

Candidate patch: https://reviews.llvm.org/D149389

[alanzhao1]

@shafik any thoughts on backporting this to 16.0.xx?

[alanzhao1]

Closing for now since https://reviews.llvm.org/D149389 has been merged.

Feel free to reopen if you think this should be backported to the 16.0.X release branch.

[alanzhao1]

/branch alanzhao1/llvm-project/backport-62266

[alanzhao1]

Backporting per discussion in https://reviews.llvm.org/D150122

[llvmbot]

/pull-request llvm/llvm-project-release-prs#441