[dcl.fct] Overbroad grammar for parameter-declaration CWG2537

[opensdh]

It should have a defining-type-specifier-seq instead of a decl-specifier-seq, since no other decl-specifier (a storage-class-specifier, a function-specifier, friend, typedef, constexpr, consteval, constinit, or inline) can be used there (unless we expect to get constexpr function parameters any day now!).

[cpplearner]

In the past the auto and register storage-class-specifiers were permitted. Nowadays neither is a storage-class-specifier.

[jensmaurer]

@zygoloid , any opinion here?
@tkoeppe, changing the grammar is not prima facie editorial, I believe.

[tkoeppe]

I'd welcome at least a bit of CWG consensus. Is this something you could look at during a telecon? I wouldn't want to make such a change without any oversight, but if CWG says OK then we don't need to create an issue.

[jensmaurer]

Editorial meeting 2021-04-16: This is not editorial.

[tkoeppe]

@tkoeppe to email CWG.

[jensmaurer]

See cplusplus/CWG#7

[jensmaurer]

Created CWG2537