PHP: rfc:calls_in_constant_expressions

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====== PHP RFC: Allow function calls in constant expressions ======
  * Version: 0.5
  * Date: 2020-02-16
  * Author: Tyson Andre <tandre@php.net>
  * Status: Withdrawn
  * First Published at: https://wiki.php.net/rfc/calls_in_constant_expressions

===== Introduction =====

Currently, constant expressions in declarations allow a limited set of expression types such as literals, constants, and operations.
This RFC proposes allowing calls of a whitelist of global functions in some constant expressions. Calls would be allowed in declarations of class and global constants, defaults of static properties, static variables, and parameter defaults.
This will allow calling functions such as ''\count()'', ''\array_merge()'', ''\array_keys()'', and ''\in_array()'',
An Error will be thrown if the call would be invalid to use within a constant (due to references or non-constant return values).

===== Proposal =====

This RFC proposes allowing global function calls by name (in any namespace).
Method calls in constant expressions continue to be fatal errors (e.g. ''"MyClass::methodName"()'' or ''MyClass::methodName()'').

Function calls will be allowed in the following types of constant expressions:

  * Defaults of static properties, but **not** instance properties, due to changes required to the PHP internals expanding the scope of this RFC too much. PHP currently evaluates all of the default values of instance properties of a class once, and permanently stores them. \\ This preserves PHP's current evaluation order (the first time a static property is used, all property default values are evaluated)
  * Parameter defaults (expressions with function calls will always be evaluated each time the parameter is absent)
  * Global constants (evaluated immediately) and class constants (evaluated and cached the first time they are successfully fetched)
  * Defaults of static variables (evaluated and cached the first time the expression succeeds)


Only functions in the following whitelist of functions will be callable in constant expressions.
Attempting to use other functions in constant expressions will be a fatal compile error.
This whitelist includes functions that are actually deterministic and without side effects, and don't depend on ini
settings or locale. The functions must be unambiguously resolved.
This has the same implementation as the first option, with additional compile-time restrictions.

For example, allow ''\count()'', ''\array_merge()'', and ''\in_array()'',
but don't allow functions such as

  * ''\strtolower()'' (different in Turkish locale),
  * ''\sprintf()'' (Depends on locale and  ''ini_get('precision')'', but so does ''("" . EXPR)''),
  * ''\json_encode()'' (The ''json'' extension can be disabled),
or calls which aren't unambiguously resolved with ''\'' or ''use function''.

The function ''get_defined_functions_allowed_in_constant_expressions()'' will be added to provide this list of functions the current php version to end users.
''get_defined_functions_allowed_in_constant_expressions()'' will be unaffected by the ''disable_functions'' ini directive, the same way ''get_defined_functions()'' is.

==== Behaviors in any constant expression ====

Many of these edge cases aren't possible for functions in the whitelist, but will be checked for in the implementation.

If a function call's result contains anything that is invalid for a constant (e.g. objects or reference cycles),
an ''Error'' will be thrown and the result will be freed. (same as what ''define()'' would accept).

If a function call attempts to modify a parameter by reference, an ''Error'' will be thrown.

''func_get_args()'', ''func_get_arg()'', ''func_num_args()'', ''extract()'', ''compact()'', and ''get_defined_vars()''
will all throw an ''Error'' if they are called in a function expression (through the same mechanism that already forbids them in dynamic calls).

Argument unpacking is allowed. (e.g. ''count(...[CONST_ARRAY])'')

Function calls must be by name:

<code php>
// the following are not allowed by this RFC:
// const X1 = ('coun' . 't')([2]);
// const X2 = (MyClass::CALLBACK_NAME)([2]);
</code>


==== Evaluation Order ====

Global constants will continue to be evaluated immediately.

Class constants, static property defaults, and static variable defaults will be continue to be evaluated and permanently cached 
the first time they get used.

Parameter defaults of function/method/closure declarations (containing function calls) will get evaluated every time a function gets called without the parameter (unless a Throwable was thrown).
Opcache is free to cache the result if the parameter values are known and the function is definitely deterministic.

The behavior of constant expressions that don't contain function calls won't be modified by this RFC.

==== Handling functions not in the whitelist ====

A fatal CompileError will be emitted when including a file if any constant expression uses a function name that is outside of the whitelist.

<code php>
<?php
namespace NS;
use function count;
use function count as c;
class MyClass {
	const VALUES = [1, 0];
	const C1 = count(self::VALUES);  // this would not throw
	const C2 = c(self::VALUES);  // this would not throw
	// const C3 = \my_function();  // this would throw due to being outside of the whitelist

	// this would throw due to being allowed to resolve to `NS\in_array()`.
	// const C4 = in_array(0, self::VALUES);  
}
</code>

<code php>
<?php
// in the global namespace
class MyClass {
	const VALUES = [1, 0];
	const C1 = count(self::VALUES);  // this would not throw
	const C2 = \COUNT(self::VALUES);  // this would not throw
	const C3 = namespace\count(self::VALUES);  // this (namespace-relative) would not throw
	// const C4 = OtherNS\count();  // this would throw due to being outside of the whitelist
}
</code>

==== Whitelisted Functions ====

Functions with the following properties were chosen.

  * Always return values that can be used in constants (or throw)
  * Impossible to disable in unpatched php builds (e.g. ''json_encode()'' is excluded because ''json'' can be disabled with ''--disable-json'', and because it depends on the ini setting for numeric precision)
  * Reasonably deterministic (e.g. no file/network I/O, not dependent on time zones)
  * Not dependent on locale or ini settings (e.g. ''strtolower'' is different in a Turkish locale, ''(string)$float'' is different in the locale ''de_DE'' due to using a comma instead of a decimal place)
  * Don't expect arbitrary strings as parameters in a way that depends on the way floats implicitly cast to strings without strict_types (due to implicit ''(string)$float'' and ini settings for numeric precision)
    For this reason, ''strlen()'', ''sprintf()'', and ''strpos()'' are omitted from the proposed whitelist. Those may be included in a followup RFC.

The following list of functions is whitelisted.

	abs
	acosh
	acos
	array_chunk
	array_column
	array_count_values
	array_diff_assoc
	array_diff_key
	array_diff
	array_fill_keys
	array_fill
	array_flip
	array_intersect_assoc
	array_intersect_key
	array_intersect
	array_key_exists
	array_key_first
	array_key_last
	array_keys
	array_merge_recursive
	array_merge
	array_pad
	array_product
	array_replace_recursive
	array_replace
	array_reverse
	array_search
	array_slice
	array_sum
	array_unique
	array_values
	asinh
	asin
	atan2
	atanh
	atan
	boolval
	ceil
	checkdate
	chr
	cosh
	cos
	count
	decbin
	dechex
	decoct
	deg2rad
	doubleval
	expm1
	exp
	floatval
	floor
	fmod
	gettype
	gmmktime
	hash_algos
	hypot
	in_array
	intdiv
	intval
	is_array
	is_bool
	is_countable
	is_double
	is_finite
	is_float
	is_infinite
	is_integer
	is_int
	is_iterable
	is_long
	is_nan
	is_null
	is_numeric
	is_object
	is_real
	is_resource
	is_scalar
	is_string
	log10
	log1p
	log
	max
	min
	pi
	pow
	rad2deg
	range
	round
	sinh
	sin
	sizeof
	sqrt
	tanh
	tan

===== Backward Incompatible Changes =====
None

===== Proposed PHP Version(s) =====
8.0

===== RFC Impact =====

==== To Opcache ====
Opcache appears to be unaffected - tests of this RFC are passing. Opcache likely just fails to optimize the constant expressions ahead of time.

In the future, if this gets adopted widely, more aggressive optimizations of calls in constant expressions may be desirable. (e.g. permanently storing parameter defaults if the call is provably deterministic)

Opcache already has the ability to optimize functions such as ''strlen'' at compile time, when using other constants from the same class.

==== New Functions ====

''get_defined_functions_allowed_in_constant_expressions()'' will be added to PHP.

===== Future Scope =====

Future RFCs may expand on this in many ways:

  * Allowing even more expression types in constant expressions, such as static method calls.
  * Adding more groups of functions to the whitelist, or avoiding the whitelist approach.
  * Allowing function calls in the defaults of instance properties.
  * Allowing non-constant defaults for instance properties, such as ''public $x = new Foo();''.

===== Proposed Voting Choices =====

Primary vote: Allow calling global functions that are in the described whitelist (Yes/No, Requires 2/3 majority)

Also see [[https://externals.io/message/108630|Straw poll: Places to allow function calls in constant expressions]]

===== Patches and Tests =====

https://github.com/php/php-src/pull/5139

===== Implementation =====

===== References =====
[[https://externals.io/message/108343|Planning an RFC to allow calls to global functions in constant expressions]]

[[https://externals.io/message/108630|Straw poll: Places to allow function calls in constant expressions]]

https://wiki.php.net/rfc/calls_in_constant_expressions_poll
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