fix Issue 16627 - non-static structs with postblit/dtor fields now nested

[MartinNowak]

• Since [REG2.068.0] Issue 15726 - forward reference error for circular classes, RefCounted #5500 buildPostblit/Dtor/OpAssign are run before
  the struct size is finalized, thereby now making structs nested
  that previously were not.
• Not sure if the old behavior was an intended feature. If not
  we might want to deprecate it.

[dlang-bot]

Fix	Bugzilla	Description
✓	16627	[Reg 2.072] non-static structs with postblit/dtor fields are now nested

[WalterBright]

This is rather hackish to use the storage class to transmit a flag. I can feel the technical debt :-) Perhaps use a FUNCFLAG bit instead?

[MartinNowak]

I can feel the technical debt :-)

In the mid-term, I'd be in favor to deprecating these non-static, but unnested, structs, requiring people to always explicitly use static struct. Can you clarify the original intention behind this? Was it just a simple PODs don't need a context optimization?

[MartinNowak]

Found that I can reuse the generated flag in FuncDeclaration.
Still clarifying the specs for the current behavior would be appreciated, it wasn't inferable from the code.

Nested structs have member functions

The spec page mentions this (see Nested Structs mentions), could use a bit more explanation though.

[WalterBright]

Can you clarify the original intention behind this?

I don't recall it. But being in the spec, it is clearly intentional, and seems like a good idea :-)