Language: Default parameter values and optional arguments

[PurHur]

Problem

compileParam() in lib/Compiler.php asserts:

assert(null === $param->defaultBlock);

Most real PHP functions use defaults: function greet(string $name = 'Guest').

Goal

Support default values for function parameters in VM and JIT.

Tasks

• Compile defaultBlock expressions when parameter omitted at call site
• TYPE_ARG_RECV path: if missing arg, evaluate default block
• JIT: emit branch or init from constant defaults where possible
• PHPT: optional args, skipped middle params (after Language: Named arguments in function and method calls #168 named args)
• Interaction with strict_types (Language: declare(strict_types=1) enforcement in VM #156)

Acceptance criteria

function f($a = 1, $b = 2) { return $a + $b; }
echo f(), f(10), f(10, 20); // 3, 12, 30

Dependencies

• Language: JIT support for non-void function return types #55 non-void returns in JIT
• Language: Named arguments in function and method calls #168 named arguments (later)

Files

• lib/Compiler.php, lib/VM.php, lib/JIT.php