GCC accepts `constexpr struct {} s;` but Clang rejects it. Who is correct?

Question

The following code compiles fine with GCC:

constexpr struct {} s; 

But Clang rejects it with the following error:

error: default initialization of an object of const type 'const struct (anonymous struct at …)' without a user-provided default constructor

I've tested all versions of GCC and Clang that I was able to find at https://gcc.godbolt.org/. Each version of GCC accepts the code and each version of Clang rejects it.

I wonder which compiler is correct in this case?
What does the standard say about this?

Answer 1

Clang adheres to the following passage in [dcl.init]/7:

If a program calls for the default initialization of an object of a const-qualified type T, T shall be a class type with a user-provided default constructor.

This wording is defective and hence ignored by GCC (and, as of v3.9, also by Clang).
_{The above quote differs from core issue 253's due to N2762 moving/adjusting paragraph 9 as cited.}