Do not report function_exists as always-true conditions

[staabm]

Closes phpstan/phpstan#8980

extracted from #5737 which went too far

[staabm]

why did it work before this PR when only a single constant string was given to function_exists?

https://phpstan.org/r/7479cbe6-cdfe-45bf-bada-dab223873fa1

[VincentLanglet]

why did it work before this PR when only a single constant string was given to function_exists?

phpstan.org/r/7479cbe6-cdfe-45bf-bada-dab223873fa1

Either

phpstan-src/src/Analyser/MutatingScope.php

Lines 2159 to 2165 in 1468103

	return $expr instanceof FuncCall
	&& !$expr->isFirstClassCallable()
	&& $expr->name instanceof FullyQualified
	&& $expr->name->toLowerString() === 'function_exists'
	&& isset($expr->getArgs()[0])
	&& count($this->getType($expr->getArgs()[0]->value)->getConstantStrings()) === 1
	&& $type->isTrue()->yes();

or

phpstan-src/src/Type/Php/FunctionExistsFunctionTypeSpecifyingExtension.php

Line 40 in 1468103

if ($argType instanceof ConstantStringType) {

Should it be simplified/worked ?

[staabm]

that was a great hint, thank you.

[staabm]

in this case, the function_exists() will only assure one of the functions to exist.
therefore - to be defensive - I am using only count($constantStrings) === 1 in the src/Type/Php/FunctionExistsFunctionTypeSpecifyingExtension.php so we still expectedly error about function exists

[VincentLanglet]

Might be worth adding it as a coment in this file ?