[libc++][test] `flat_set` std test uses `EmplaceUnsafeContainer` which is non-standard

[AlexGuteniev]

EmplaceUnsafeContainer is used in tests for flat_set and flat_map to test the ability to have basic exception guarantee when underlying container throws on insert / emplace of a single element.

llvm-project/libcxx/test/std/containers/container.adaptors/flat_helpers.h

Line 122 in ffb9bbf

struct EmplaceUnsafeContainer : std::vector<T> {

Per [container.reqmts]/66, a sequence container should provide strong exception guarantee in this case.

So supporting underlying containers that don't provide that is nonstandard extension, and should be tested in libcxx test suite instead.

[huixie90]

I don't think agree this,

Unless otherwise specified (see [associative.reqmts.except], [unord.req.except], [deque.modifiers], [inplace.vector.modifiers], and [vector.modifiers]) all container types defined in this Clause meet the following additional requirements:

• If an exception is thrown by an insert() or emplace() function while inserting a single element, that function has no effects.

"Unless otherwise specified, container types defined in this Clause" only applies to the containers in the library. There is no requirements on the user defined Sequence Container about the exception guarantee on emplace

I tempted to close this issue as is.