[BUG] Allow empty if blocks

[Sola85]

Environment

• qBraid-QIR version: 0.4.0
• Python version: 3.11.5
• Operating system: Linux c5ce507e9ef3 6.6.87.2-microsoft-standard-WSL2

What happened?

Is there a reason why

OPENQASM 3.0;
include "stdgates.inc";
bit[1] c;
qubit[1] q;

if (c[0]) {
  z q[0];
} else {
  
}

gets accepted but

OPENQASM 3.0;
include "stdgates.inc";
bit[1] c;
qubit[1] q;

if (c[0]) {

} else {
  z q[0];
}

doesn't? The latter raises a pyqasm.exceptions.ValidationError: Missing if block.

Suggestions (Optional)

I think this should be accepted (as it would be in e.g. C)

[ryanhill1]

Hi @Sola85, thanks for raising this issue.

Seems like this could be more of a pyqasm issue, rather than a qBraid-QIR issue. If so, we'll transfer it there.

@TheGupta2012 Can you confirm?

[TheGupta2012]

Yep, seems like a pyqasm issue, will transfer and fix!

[TheGupta2012]

I guess C does allow the empty if blocks but can you elaborate upon the use-case for empty if blocks in QASM?

[Sola85]

We found this while minimizing a test-case for our later compiler stages. Not sure if there are any usecases other than testing/debugging partially complete programs.

But the QASM3 grammar does allow it:

scope: LBRACE statementOrScope* RBRACE; // <- includes no statement
statementOrScope: statement | scope;
ifStatement: IF LPAREN expression RPAREN if_body=statementOrScope (ELSE else_body=statementOrScope)?;

But I agree that this is probably a low-priority issue.

[TheGupta2012]

Okay, if it is a part of the grammar we will support it as well!