Added default as an expression in argument lists

[Bilge]

This PR adds default as an expression in argument lists; it is not valid in any context other than argument lists. At runtime, default will be substituted with the default value as declared at the call site of the function so called. For full details, see the default expression RFC.

TODO

• Decide which error/exception types to throw at both compile and runtime.

Limitations

The engine cannot retrieve the default value of a trampoline call.

Special thanks

• Thanks to @iluuu1994 for hand-holding me through the implementation.
• Thanks to @bwoebi for the Bison patch.
• Thanks to all the R11 folk who put up with me.

• Thanks to @gooh for the above.

[iluuu1994]

Not commenting on things listed in the todos.

[iluuu1994]

As mentioned off-list, this should check only for opcodes within the current call (between fcall and opline), not the entire function. It would be nice to also skip nested functions, but that will require listing all init/do opcodes.

[Bilge]

Thanks for the note. I am still trying to figure out how to do this.

[iluuu1994]

Store fcall in a local var, loop, and bump until reaching opline.

[derickr]

There is code for that in PHP already somewhere, and now also in Xdebug: xdebug/xdebug@551b203#diff-846d4897469701b1d805f3574922dc47b632fd189087a2589b2e01e848585a8bR378 — that should probably become a PHP API function.

[Bilge]

I'll be honest, I still have no idea what to do here, and no idea how the Xdebug code relates to this. Anyway, I'll deal with this if the feature gets accepted.

[iluuu1994]

for (zend_op *op = fcall + 1; op != opline; op++) {
	// Don't inline calls with explicit default arguments.
	if (op->opcode == ZEND_FETCH_DEFAULT_ARG) {
		return;
	}
}

[Bilge]

Thanks. Clearly I need to understand more about what oparrays, oplines and opcodes really are, and how they relate to each other. I tend to imagine that an oparray contains oplines which contain opcodes, but I still don't really know why oparray is even a thing, and when you would bundle oplines together.