Free-standing binary op exressions should be treated as equivalent "if" expressions

[muglug]

function maybeString(): ?string {
    return rand(0, 10) > 4 ? "test" : null;
}

function test(): string {
    $foo = maybeString();
    ($foo !== null) || ($foo = "");
    return $foo;
}

is directly equivalent to

function maybeString(): ?string {
    return rand(0, 10) > 4 ? "test" : null;
}

function test(): string {
    $foo = maybeString();
    if (!($foo !== null)) {
        $foo = "";
    }
    return $foo;
}

Similarly,

function maybeString(): ?string {
    return rand(0, 10) > 4 ? "test" : null;
}

function test(): string {
    $foo = maybeString();
    ($foo === null) && ($foo = "");
    return $foo;
}

is directly equivalent to

function maybeString(): ?string {
    return rand(0, 10) > 4 ? "test" : null;
}

function test(): string {
    $foo = maybeString();
    if ($foo === null) {
        $foo = "";
    }
    return $foo;
}

https://phpstan.org/r/de46736e-f6da-495a-ad3d-a1484626dfeb and
https://phpstan.org/r/ae3b7244-1720-4bf5-8bf3-5222caf2dacf

[muglug]

Associated Psalm fix: vimeo/psalm@bf67c03#diff-210d9c6ab20da8a7e5f6796f7090a6e7R82

[phpstan-bot]

@muglug PHPStan now reports different result with your code snippet:

@@ @@
+PHP 7.2 – 8.0 (2 errors)
+==========
+
+ 9: Right side of || is always false.
+10: Function test() should return string but returns string|null.
+
+PHP 7.1 (3 errors)
+==========
+
+ 9: Only booleans are allowed in ||, string given on the right side.
  9: Right side of || is always false.
 10: Function test() should return string but returns string|null.

Full report

PHP 7.2 – 8.0 (2 errors)

Line	Error
9	Function test() should return string but returns string
10	Function test() should return string but returns string

PHP 7.1 (3 errors)

Line	Error
9	Function test() should return string but returns string
9	Function test() should return string but returns string
10	Function test() should return string but returns string

[phpstan-bot]

@muglug PHPStan now reports different result with your code snippet:

@@ @@
+PHP 7.4 – 8.0 (3 errors)
+==========
+
+ 9: Only booleans are allowed in &&, string given on the right side.
+ 9: Right side of && is always false.
+10: Function test() should return string but returns string|null.
+
+PHP 7.1 – 7.3 (2 errors)
+==========
+
  9: Right side of && is always false.
 10: Function test() should return string but returns string|null.

Full report

PHP 7.4 – 8.0 (3 errors)

Line	Error
9	Function test() should return string but returns string
9	Function test() should return string but returns string
10	Function test() should return string but returns string

PHP 7.1 – 7.3 (2 errors)

Line	Error
9	Function test() should return string but returns string
10	Function test() should return string but returns string

[ondrejmirtes]

Fixed: phpstan/phpstan-src@f9374e7

[github-actions]

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