[SwayamInSync]

Plan item E2 (located messages in user terms) and exit criterion 2: "no message names a Clang AST class". The converter named every construct it had no words for by Clang's class (this statement is unsupported (OMPSimdDirective)), and its test said so: "A construct without a name of its own is named by Clang's class".

On the base

$ cpp-verify --lower-only probe.cpp -- -std=c++20 -fopenmp
probe.cpp:7:1: error: simd_sum: this statement is unsupported (OMPSimdDirective)
probe.cpp:13:67: error: atom: this expression is unsupported (AtomicExpr)
probe.cpp:7:33: error: coro: this statement is unsupported (CoroutineBodyStmt)
probe.cpp:4:42: error: elvis: this expression is unsupported (BinaryConditionalOperator)
probe.cpp:6:49: error: rewritten: this expression is unsupported (CXXRewrittenBinaryOperator)
probe.cpp:4:45: error: pseudo: indirect function calls are unsupported

With this PR

error: simd_sum: OpenMP directives are unsupported
error: atom: atomic operations are unsupported: the memory model of threads is not modeled
error: coro: coroutines are unsupported
error: elvis: this expression `x ?: 1` is unsupported
error: pseudo: a destructor call of a scalar is unsupported

and rewritten verifies.

Change

• Names. unsupportedConstruct names, as C++ does, OpenMP directives (any OMPExecutableDirective), coroutines (co_return, co_await, co_yield, a coroutine body), __try, atomic operations (__atomic_*, AtomicExpr), typeid, dynamic_cast, va_arg, __uuidof, vector and matrix operations, and a pseudo-destructor call (y.Int::~Int(), which was called an indirect call).
• Quotes. A construct C++ gives no name of its own is quoted from the source instead of named by its Clang class: the first line of its text, at most 40 characters (quotedSource, through the expansion range, so a macro shows its use). No message of the converter names a Clang class now; getStmtClassName has no other use in clang/lib/Verify.
• Rewritten comparisons. One of the quoted constructs is ordinary C++20, so it is supported instead of named: a comparison rewritten in terms of another operator (a != b with only operator==, which is !(a == b); 5 == r with only R::operator==(int), reversed to r == 5) is converted as its semantic form, the call Clang already built, and checked against that operator's contract. Nothing else about member operators changes: the object must be a reference, an addressable local, or a dereference, as for a == b.

Tests

• unsupported_constructs.cpp: the OpenMP check now expects its name; new cases for an atomic load, a coroutine, a pseudo-destructor call, and x ?: 1 quoted.
• suite/rewritten_comparisons.cpp (new): a != b and reversed 5 == r and 5 != r verify; the same with the meaning of == claimed for != fails with a counterexample; rewritten comparisons in a condition and an operand verify.
• Both fail on main (the base's messages above). Mutants, each caught: the rewritten form not lowered (rewritten_comparisons.cpp), the class name back in the fallback (unsupported_constructs.cpp).
• The structs page's 19 examples, the new one included, pass check-doc-examples.py.

Docs

• Structs, "Member operators": rewritten comparisons, with a checked example;
• limitations (supported functions);
• release notes ("Errors name the construct, once", and "C++20 rewritten comparisons");
• ARCHITECTURE (the converter's messages).