Union has no active member after assignment from a value-initialized temporary in constant evaluation

[jonatanlinden]

Issue Summary

In constant evaluation, assigning a value-initialized temporary to an object of class type, x = R(), loses the active member of every union subobject. Reading the first union member afterwards is diagnosed as "invalid access to inactive member ... (no field is active)". Zero-initialization should make the first named member of each union active (n4868 [dcl.init.general]/6.3).

Reproducing Source Code

struct R {
  union { int a; long b; };
};

constexpr int f() {
  R x;
  x = R();        // assignment from a value-initialized temporary
  return x.a;     // a is the active member after zero-initialization
}
static_assert(f() == 0);

Command-line Options

cpfe --c++20 --no_code_gen

Type of Issue

[Front End] This code SHOULD compile with these options.

Additional Details

This breaks libc++'s constexpr std::string move constructor, which does __str.__r_.first() = __rep(); (__rep is a struct holding an anonymous union).

Expected: no diagnostics.
Observed:

"<source>", line 10: error: expression must have a constant value
  static_assert(f() == 0);
                ^
"<source>", line 8: note: invalid access to inactive member "R::a" of union (no field is active)
    return x.a;     // a is the active member after zero-initialization
            ^

x = R{} and R x = R(); are accepted; only the assignment from R() fails. That temporary has a dik_zero initializer and is initialized with init_subobject_to_zero.

Reproducing (Standard) Configuration(s)

linux-gcc-debug

Reproducing (Non-standard) Configuration

No response

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