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C++17 has that requirement in the deduction guide.

1. An object is not a class.
2. An object is an instantiation of a class, an array, or built-in-type.
3. Subobjects are class member objects, array elements, or base classes of an object.
4. Derived objects (and most-derived objects) only make sense in the context of class inheritance.

Yes, as of now, the One Definition Rule in the C++ standard doesn't include enumerators.

However, the "the second a is a redeclaration of the first a" explanation doesn't work too.
From [dcl.enum#nt:enumerator-list] we can know that an enumerator-list is a list of enumerator-definition, so they're all definitions.

Maybe I'm wrong, but it seems that last part:

I think this is insufficient reduction as a result of the adoption of <=> in P1614. Before that paper, there were three ==s in the example:

Here's my current understanding, which could be incomplete or incorrect. A verification would be appreciated.

C++20 renamed strongly happens before to simply happens before, and introduced a new, more relaxed definition for strongly happens before, which imposes less ordering.

Simply happens before is used to reason about the presence of data races in your code. (Actually that would be the plain 'happens before', but the two are equivalent in absence of consume operations, the use of which is discouraged by the standard, since most (all?) major compilers treat them as acquires.)

The weaker strongly happens before is used to reason about the global order of seq-cst operations.

This change was introduced in proposal P0668R5: Revising the C++ memory model, which is based on the paper Repairing Sequential Consistency in C/C++11 by Lahav et al (which I didn't fully read).

The proposal explains why the change was made. Long story short, the way most compilers implement atomics on Power and ARM architectures turned out to be non-conformant in rare edge cases, and fixing the compilers had a performance cost, so they fixed the standard instead.

The change only affects you if you mix seq-cst operations with acquire-release operations on the same atomic variable (i.e. if an acquire operation reads a value from a seq-cst store, or a seq-cst operation reads a value from a release store).

If you don't mix operations in this manner, then you're not affected (i.e. can treat simply happens before and strongly happens before as equivalent).

The gist of the change is that the synchronization between a seq-cst operation and the corresponding acquire/release operation no longer affects the position of this specific seq-cst operation in the global seq-cst order, but the synchronization itself is still there.

This makes the seq-cst order for such seq-cst operations very moot, see below.

The proposal presents following example, and I'll try to explain my understanding of it:

All the algorithms require copy-constructible function objects, and views are basically lazy algorithms.

Historically, when these adaptors were added, views were required to be copyable, so we required the function objects to be copy_constructible (we couldn't require copyable without ruling out captureful lambdas). The change to make view only require movable came later.

It is probably possible to relax the restriction, but it will need a paper and isn't really high priority.

GCC and Clang report that S is trivially copyable in C++11 through C++23 standard modes. MSVC reports that S is not trivially copyable in C++14 through C++20 standard modes.

N3337 (~ C++11) and N4140 (~ C++14) say:

A trivially copyable class is a class that:

• has no non-trivial copy constructors,
• has no non-trivial move constructors,
• has no non-trivial copy assignment operators,
• has no non-trivial move assignment operators, and
• has a trivial destructor.

By this definition, S is trivially copyable.

N4659 (~ C++17) says:

A trivially copyable class is a class:

• where each copy constructor, move constructor, copy assignment operator, and move assignment operator is either deleted or trivial,
• that has at least one non-deleted copy constructor, move constructor, copy assignment operator, or move assignment operator, and
• that has a trivial, non-deleted destructor

By this definition, S is not trivially copyable.

N4860 (~ C++20) says:

A trivially copyable class is a class:

• that has at least one eligible copy constructor, move constructor, copy assignment operator, or move assignment operator,
• where each eligible copy constructor, move constructor, copy assignment operator, and move assignment operator is trivial, and
• that has a trivial, non-deleted destructor.

By this definition, S is not trivially copyable.

Thus, as published, S was trivally copyable in C++11 and C++14, but not in C++17 and C++20.

The change was adopted from DR 1734 in February 2016. Implementors generally treat DRs as though they apply to all prior language standards by convention. Thus, by the published standard for C++11 and C++14, S was trivially copyable, and by convention, newer compiler versions might choose to treat S as not trivially copyable in C++11 and C++14 modes. Thus, all compilers could be said to be correct for C++11 and C++14.

For C++17 and beyond, S is unambiguously not trivially copyable so GCC and Clang are incorrect. This is GCC bug #96288 and LLVM bug #39050

Interoperability with the associated C interface. From N2320 (Multi-threading Library for Standard C++):

The C level interface has been removed from this proposal with the following rationale:

• As long as we specify that the key types in this proposal are standard-layout types (which we have done), WG14 is still free to standardize a C interface which interoperates with this C++ interface.
• WG14 is in a better position to solve the cancellation interoperability problem than WG21 is. [...]
• WG14 asked WG21 to take the lead on this issue. We feel we can best take lead by specifying only a C++ interface which has the minimum hooks in it to support a future C interoperating interface (i.e. types are standard-layout types). We feel we should stop short of actually specifying that C interface in the C++ standard. WG14 can do a better job with the C interface and a future C++ standard can then import it by reference.