Fix phpstan/phpstan#14384: Result of function_exists() can't be "cached"

[phpstan-bot]

Summary

When storing the result of function_exists() in a variable and using that variable as a condition later, PHPStan reported function.notFound because the function existence narrowing was not propagated through the variable assignment.

Changes

• Added FuncCall to the allowed expression types in processSureTypesForConditionalExpressionsAfterAssign() and processSureNotTypesForConditionalExpressionsAfterAssign() in src/Analyser/ExprHandler/AssignHandler.php
• Added regression test in tests/PHPStan/Rules/Functions/data/bug-14384.php

Root cause

FunctionExistsFunctionTypeSpecifyingExtension creates a synthetic FuncCall expression as the target of type narrowing (e.g., function_exists('some_func') mapped to ConstantBooleanType(true)). When assigning $var = function_exists('some_func'), the AssignHandler processes the specified types to create ConditionalExpressionHolder objects that link the variable's truthiness to the narrowed expressions. However, the filter in processSureTypesForConditionalExpressionsAfterAssign only allowed Variable, PropertyFetch, and ArrayDimFetch expressions through, silently dropping FuncCall expressions. This meant the function existence narrowing was never stored as a conditional expression, so if ($var) didn't know the function existed.

The same fix also applies to class_exists() / interface_exists() / trait_exists() / enum_exists() which use the same pattern with synthetic FuncCall expressions.

Test

Added tests/PHPStan/Rules/Functions/data/bug-14384.php which stores function_exists() result in a variable and calls the function conditionally. The test expects no errors (false positive fix).

Fixes phpstan/phpstan#14384

[VincentLanglet]

I wonder if this wasn't done on purpose.
I also wonder if there is more Expr to not skip (method call ? Static call ?).

But i have nothing against this bugfix...

[thg2k]

I believe i recently ran into a similar issue with file_exists

[staabm]

@thg2k if you find a problem please reproduce on the playground and file an issue

[thg2k]

I know the drill :-) too much work, also cause i'm still on 1.12 and didn't have time to check first if it was fixed in 2.x.