Support optional else-branch for if-then-elif-end

[wader]

Fixes #2481

[wader]

Not sure if seen as bug fix or feature, but can maybe wait until after current master has been released.

[wader]

This can probably be done in some cleverer way but this way feels straight forward

[itchyny]

Having optional then in different places looks confusing. You can move both in ElseBody;

Exp:
...
"if" Exp "then" Exp ElseBody {
  $$ = gen_cond($2, $4, $5);
} |
"if" Exp "then" error {
  FAIL(@$, "Possibly unterminated 'if' statement");
  $$ = $2;
} |
...

ElseBody:
"elif" Exp "then" Exp ElseBody {
  $$ = gen_cond($2, $4, $5);
} |
"else" Exp "end" {
  $$ = $2;
} |
"end" {
  $$ = gen_noop();
}

[wader]

@owenthereal CI for a PR seems to work

[owenthereal]

[wader]

I think we should maybe wait with any changes that are not absolutely needed for next release or what do ppl think? @itchyny @liquidaty

[liquidaty]

Personally I think it would be nice to see changes incorporated in stages and/or separate releases:

1. Pure QC/CI/CD. In theory, the artifacts should not change, and only the distribution mechanism (i.e. where to pick up binaries) changes
2. Bug fixes
3. Simple enhancements that can be made to only apply on an opt-in basis such as comment stripping-- basic PR here (yes selfish, I know)
4. New functions (e.g. utf8 upper/lower)
5. Other changes such as those that impact syntax (e.g. if-elif-option-else)

[wader]

Rebased on master. macOS build fail seems to be some homebrew temporary error

[itchyny]

Having optional then in different places looks confusing. You can move both in ElseBody;

Exp:
...
"if" Exp "then" Exp ElseBody {
  $$ = gen_cond($2, $4, $5);
} |
"if" Exp "then" error {
  FAIL(@$, "Possibly unterminated 'if' statement");
  $$ = $2;
} |
...

ElseBody:
"elif" Exp "then" Exp ElseBody {
  $$ = gen_cond($2, $4, $5);
} |
"else" Exp "end" {
  $$ = $2;
} |
"end" {
  $$ = gen_noop();
}

[wader]

@itchyny Thanks, yes that is better

[itchyny]

LGTM!