Summary

When $key = array_key_first($list) is assigned and then checked with $key !== null, PHPStan was not recognizing that $list[$key] is a valid offset access, producing a false positive "Offset int<0, max> might not exist on list."

This PR adds conditional expression holders so that after the null check, the array is narrowed to non-empty and the dim fetch $arr[$key] is recognized as valid.

Changes

• src/Analyser/NodeScopeResolver.php: Added handling for array_key_first/array_key_last assignments to create conditional expression holders that link the non-null state of the key variable to (1) the array being non-empty and (2) the dim fetch $arr[$key] having the array's value type
• tests/PHPStan/Rules/Arrays/data/bug-14081.php: New regression test data file with test cases for list, array map, and reversed null checks
• tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php: Added testBug14081 method
• tests/PHPStan/Analyser/nsrt/bug-14081.php: New NSRT test verifying type narrowing for array, key, and dim fetch
• tests/PHPStan/Analyser/nsrt/bug-13546.php: Updated existing assertion from list<string> to non-empty-list<string> to reflect the improved narrowing (the old test even had a comment noting this could be improved)

Root cause

When $key = array_key_first($list) is a standalone assignment (not inside a condition), the existing TypeSpecifier code at lines 758-780 only registers the dim fetch expression type when the array is already known to be non-empty (isIterableAtLeastOnce()->yes()). For possibly-empty arrays, no type information was registered, so a subsequent $key !== null check would narrow $key to int<0, max> but not establish that $list[$key] is valid.

The fix uses PHPStan's conditional expression holder mechanism (the same system used for $count = count($arr); if ($count > 0) patterns) to create holders during the assignment that say: "if $key has its non-null type, then $arr is non-empty and $arr[$key] has the value type." When $key !== null is later checked, these holders fire and the type narrowing is applied.

Test

• Rule test (testBug14081): Verifies no errors for $list[$key] after array_key_first/array_key_last with null check, covering list<string>, array<string, int>, and reversed null check patterns
• NSRT test: Verifies that after the null check, the array is narrowed to non-empty, the key type is correct, and the dim fetch type is the value type

Fixes phpstan/phpstan#14081