Reference (section label): [expr.cond], [expr.const]
Issue descriptionIn some situations, the lvalue-to-rvalue conversion mandated by [expr.cond] paragraph 7 can apply to class types. Such lvalue-to-rvalue conversions may not be constant expressions (see [expr.const] paragraph 5, bullet 9) even though they result in a call to the copy constructor ([conv.lval] paragraph 3, bullet 2), and calling the copy constructor directly would be a constant expression.
struct S {};
S a;
constexpr S b = a; // OK, call to implicitly-declared copy constructor
S c = false ? S{} : a; // OK, call to copy constructor through lvalue-to-rvalue conversion of 'a'
constexpr S d = false ? S{} : a; // error: lvalue-to-rvalue conversion of 'a' is not a constant expression
None of MSVC, GCC, or Clang implement this behavior and permit constexpr S d = false ? S{} : a; instead.
The use of lvalue-to-rvalue conversion in [expr.cond] may be eliminated for class types.
Alternatively, update [expr.const] to permit this case.
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