Issue 13538 & 13539 - Improve variable declaration grammar for old C style syntax

[9rnsr]

Issue 13538 - Divide old C style syntax from D style grammar rule
Issue 13539 - Disallow optional template parameters with C-style syntax

[quickfur]

Excellent idea!

[9rnsr]

@Hackerpilot How do you think about this separation?

[Hackerpilot]

This still allows int *a, *b; for D-style declarations, unless I misread it.

[9rnsr]

Right. This PR still accepts any C-style syntax, but after the change we can easily say "All AltXXX rules are now deprecated to recommend proper D style syntax." It's far better than current confusing grmmar rules.

[Hackerpilot]

VarDeclarator still allows BasicType2, so removing the Alt rules will still allow int *a, *b;.

[9rnsr]

VarDeclarator still allows BasicType2, so removing the Alt rules will still allow int *a, *b;.

The BasicType2 in VarDeclarator rule only consumes the first ** before a. The remain part *b is consumed by DeclaratorIdentifierList, but surprisingly, current grammar does not accept the token * before b.

To recognize all C-style declarations in grammar rule, I added AltDeclaratorIdentifier in the last commit.

[9rnsr]

@Hackerpilot Do you have a time re-review this PR?

@WalterBright You're going to make all C-style declarations errors. This PR will help it from grammar side.

[Hackerpilot]

It looks good to me.

[9rnsr]

Thanks!