[RFC] Implement block expressions

[iluuu1994]

@nikic

This implementation replaces the statement list ('{' inner_statement_list '}') by a block expression that CAN contain an expression. After implementing it this way I don't think it's the right approach.

Two things to note:

$x = {
    // Doesn't work because the expression belongs to the statement list instead of the terminating expression
    { 1 }
};

The reason for this is that the semicolon is optional with expression statements that only contain a single block expression to maintain backwards compatibility. Additionally the semicolon won't be enforced here:

// No error or warning
function foo() {
    {
        echo 'Foo';
        'Bar'
    } // Should require a semicolon IMO
}

Alternative approach

Instead of replacing the statement list completely we add the new block expression as '{' inner_statement_list expr '}'. This avoids the ambiguity with the existing statement list.

To support blocks in arrow functions without returning a value we accept statement lists in the arrow functions body and transform it to a block with a null return value ({ ...; null }). This would solve the two cases above and require much fewer changes in the grammar.

WDYT?

I'll create a PR for this alternative approach to demonstrate what I mean. The whole terminology here can be very confusing.

Side note: This breaks the optimizer in the same way the throw expression does (see 005.phpt).

[iluuu1994]

It's also worth noting that neither of those approaches fix the issue of having to differentiate between match expressions that allow blocks vs those that don't. Instead now we have to differentiate between blocks that require a return value vs those that don't. Pretty much the same thing.

match ($x) {
    // This is fine
    1 => {},
}

$x = match($y) {
    // This isn't
    1 => {},
};

[nikic]

I'd expect this to break precedence annotations.

[iluuu1994]

Closing in favor of #5407.