Do not re-wrap NeverType as TemplateMixedType in TemplateUnionType::filterTypes()#5500
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ondrejmirtes merged 1 commit intophpstan:2.1.xfrom Apr 21, 2026
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…pe::filterTypes()` - When TemplateUnionType::filterTypes() filters out all inner types, the parent returns NeverType. The override was re-wrapping it via TemplateTypeFactory::create(), which has no NeverType branch and falls through to TemplateMixedType. - This caused MixedType::hasMethod() to return Yes for __toString, leading to a false "Possibly impure call to method stdClass::__toString()" on pure functions casting template types like T of int|string to string. - Applied the same fix to TemplateBenevolentUnionType::filterTypes(). - The fix covers all callers of filterTypes: filterTypeWithMethod (string cast, concatenation, interpolation, echo, print), filterTypeWithProperty, filterTypeWithConstant, and filterTypeWhenIterable.
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Summary
When a pure function casts a template type
T of int|stringto string, PHPStan incorrectly reports "Possibly impure call to method stdClass::__toString()". This happens becauseTemplateUnionType::filterTypes()wraps aNeverTyperesult (no matching types) back intoTemplateMixedTypeviaTemplateTypeFactory::create(), andMixedTypereports that it has every method, including__toString.Changes
src/Type/Generic/TemplateUnionType.php: ReturnNeverTypedirectly fromfilterTypes()instead of re-wrapping it throughTemplateTypeFactory::create()(which falls through toTemplateMixedType)src/Type/Generic/TemplateBenevolentUnionType.php: Same fix applied to the benevolent union variantRoot cause
TemplateUnionType::filterTypes()callsparent::filterTypes()which returnsNeverTypewhen all union members are filtered out. The override then passes thisNeverTypetoTemplateTypeFactory::create(), which has no branch forNeverTypeand falls through to the catch-all:new TemplateMixedType(...). SinceMixedType::hasMethod()returnsYesfor any method name,filterTypeWithMethod()finds a__toStringmethod on the resulting type and creates aDummyMethodReflectionwithstdClassas its declaring class — producing the false positive purity error.The fix is at the
filterTypes()level, so it covers all callers:filterTypeWithMethod(string cast, concatenation, interpolation, echo, print,.=)filterTypeWithProperty,filterTypeWithConstant,filterTypeWhenIterableAnalogous cases probed:
TemplateBenevolentUnionType— same bug pattern, fixed with same approachImplicitToStringCallHelperpath)T of int|float|bool) — confirmed fixedTest
tests/PHPStan/Rules/Pure/data/bug-14504.php— tests(string)cast, string concatenation, string interpolation, and non-string scalar union bounds (int|float|bool) in pure functions with template typestests/PHPStan/Rules/Pure/data/bug-14504-method.php— tests the same pattern in a pure method contextFixes phpstan/phpstan#14504