incorrect compiler error "expression is not an integral constant expression" in enum declaration

[djnz00]

clang 18 incorrectly reports an error declaring an enum as the result of a compile-time evaluated integer expression.

reduced test case below. clang version 18.1.8 Target: x86_64-pc-linux-gnu

#include <iostream>
#include <memory>

struct B { int i; };

struct A {
  constexpr A(int i) { std::construct_at(__builtin_addressof(b), i); }
  constexpr ~A() { std::destroy_at(__builtin_addressof(b)); }
  union { B b; };
};

consteval int i() { return A{42}.b.i; } // wrapper to circumvent compiler bug

int main() {
  // enum { I = i() };		// no error with both clang and gcc
  enum { I = A{42}.b.i };	// error with clang, but not with gcc
  std::cout << I << '\n';
}

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Huw Rogers (djnz00)

clang 18 incorrectly reports an error declaring an enum as the result of a compile-time evaluated integer expression.

reduced test case below. clang version 18.1.8 Target: x86_64-pc-linux-gnu

#include <iostream>
#include <memory>

struct B { int i; };

struct A {
  constexpr A(int i) { std::construct_at(__builtin_addressof(b), i); }
  constexpr ~A() { std::destroy_at(__builtin_addressof(b)); }
  union { B b; };
};

consteval int i() { return A{42}.b.i; } // wrapper to circumvent compiler bug

int main() {
  // enum { I = i() };		// no error with both clang and gcc
  enum { I = A{42}.b.i };	// error with clang, but not with gcc
  std::cout << I << '\n';
}

[tbaederr]

The evaluation works just fine and returns the expected 42, but we reject it anyway because we record that is has side effects:

llvm-project/clang/lib/Sema/SemaExpr.cpp

Line 17184 in d140182

EvalResult.Val.isInt() && !EvalResult.HasSideEffects &&

[tbaederr]

... and the side effect is coming from the Cleanup of A being non-trivial, i.e. QualType::isDestructedType() returns true.