Infer numeric-string for DateInterval::format('%a') when interval comes from diff()

[phpstan-bot]

Summary

DateInterval::format('%a') was incorrectly inferred as non-empty-string (not numeric) even when the DateInterval came from DateTimeInterface::diff(). This caused false positives when using format('%a') in arithmetic operations like $interval->format('%a') * 60.

The root cause: the extension evaluated format specifiers against new DateInterval('P0D'), where %a returns (unknown) because days is false. But after diff(), days is narrowed to int and %a returns a numeric string.

Changes

• Extracted shared type inference logic from DateIntervalFormatDynamicReturnTypeExtension into a new DateIntervalFormatReturnTypeHelper (src/Type/Php/DateIntervalFormatReturnTypeHelper.php)
• The helper checks if the interval's days property is int (indicating it came from diff() or date_diff())
• When days is int, a diff-created DateInterval is used for evaluating %a, producing correct numeric-string inference
• Added DateIntervalFormatFunctionReturnTypeExtension (src/Type/Php/DateIntervalFormatFunctionReturnTypeExtension.php) for the procedural date_interval_format() function, which previously had no dynamic return type inference at all
• Simplified DateIntervalFormatDynamicReturnTypeExtension to delegate to the helper

Root cause

The DateIntervalFormatDynamicReturnTypeExtension tested all format specifiers against a fresh new DateInterval('P0D'). For %a (total days), this produces (unknown) since the days property is false when the interval was not created via diff(). However, PHPStan's stubs already narrow diff() to return DateInterval&object{days:int}, which means the %a specifier will always produce a numeric string in that context.

Test

• Added tests/PHPStan/Analyser/nsrt/bug-1452.php testing:
  • DateTimeImmutable::diff() + format('%a') → numeric-string
  • DateTime::diff() + format('%a') → numeric-string
  • DateTimeInterface::diff() + format('%a') → numeric-string
  • date_diff() + format('%a') → numeric-string
  • %R%a combination → numeric-string (since +0 is numeric)
  • Plain DateInterval (not from diff) + format('%a') → non-empty-string (not numeric)
  • date_interval_format() function with both diff and non-diff intervals
  • Arithmetic on format('%a') result produces float|int (no error)

Fixes phpstan/phpstan#1452