Wrong codegen for overload set with aggregate designated initializers and anonymous union

[ecatmur]

struct C {
  union {
    int a;
    const char* p;
  };
};
struct D {
  int a = 1;
  const char* p = "bad";
};
int f(C);
int f(D);
int i = f({.a = 2, .p = "ok"});

Clang should either reject as ambiguous, or remove f(C) from the overload set under https://wg21.link/dcl.init.aggr#4.1.sentence-2 and call f(D) with a D{.a = 2, .p = "ok"} (the standard is not particularly clear on this; this came up discussing CWG 2169). It should not call f(D) with a D{} (i.e. D{.a = 1, .p = "bad"}), but that's what it does.

Also, it comes up with a comedy warning:

warning: ISO C++ requires field designators to be specified in declaration order; field 'p' will be initialized after field '' [-Wreorder-init-list]
<source>:13:20: warning: initializer overrides prior initialization of this subobject [-Winitializer-overrides]
int i = f({.a = 2, .p = "ok"});
                   ^~~~~~~~~
<source>:13:17: note: previous initialization is here
int i = f({.a = 2, .p = "ok"});
                ^

If you remove the declaration f(C);, it DTRT.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[ecatmur]

It has been pointed out to me that overriding union member initializers are valid in C:

<source>:10:33: warning: initializer overrides prior initialization of this subobject [-Winitializer-overrides]
    return f((struct C){.a = 2, .p = "ok"});
                                ^~~~~~~~~
<source>:10:30: note: previous initialization is here
    return f((struct C){.a = 2, .p = "ok"});
                             ^

So presumably clang is performing enough aggregate initialization with C in mind to modify the initializer list (somehow), and removing f(C) from the overload set later.

Note that gcc leaves f(C) in the overload set, making the original code ambiguous.