"Block body" needs updates in light of expression bodied members

[BillWagner]

The standard has historically used "block body" to describe a member that isn't abstract, to contrast with a body that is a single semi-colon.

These terms should be updated because a "block body" doesn't include members whose definition is an expression bodied member.

Sweep the spec for this term, and replace with "abstract" for ; and "body" for either a block body or expression body.

See #681 (comment)

Also see #681 (comment)

[jnm2]

Related: #1430

[jnm2]

I would not use "abstract" for ; because extern is also bodyless, but is not abstract. The defining feature of ; is that no implementation is being provided ("no body").

[jnm2]

Also, get; is often not abstract, nor extern, but the lack of body implies a compiler-generated implementation.

[KalleOlaviNiemitalo]

Also partial methods.

[jskeet]

(Self-assigned for the sake of triage/investigation, not actually fixing the issue...)

[jskeet]

Proposal: punt to the pre-C#-9 milestone. I suspect this change needs to be done really carefully and even with care, we'll make mistakes (or more likely omissions) that should be fixed during the course of a whole version. While leaving it until C# 9 will leave C# 8 slightly inconsistent, I think it's probably still in a better state than it might be if we try to do this relatively quickly.