Fix phpstan/phpstan#14411: Incorrect type narrowing with dependent types

[phpstan-bot]

Summary

When variables are assigned in different branches of an if/else and then one is checked for null, PHPStan could incorrectly narrow the type of a dependent variable from the other branch. For example, checking $b !== null after $b was assigned in both branches would incorrectly narrow $a to non-null, even though $b could be non-null from the else branch where $a IS null.

Changes

• Added a type disjointness check in src/Analyser/MutatingScope.php in the createConditionalExpressions() method
• A variable is now only used as a type guard when its types from the two branches are disjoint (no overlapping values)
• This prevents creating conditional expressions where the guard type can't actually discriminate which branch was taken

Root cause

The createConditionalExpressions() method creates conditional type relationships like "if $b has type X, then $a has type Y". These are used to propagate type narrowing across dependent variables. However, the method didn't check whether the guard variable's types from both branches were disjoint. When they overlapped (e.g., mixed~null from the if-branch and int|null from the else-branch share int), narrowing the merged type could produce the exact branch type even for values originating from the other branch, causing incorrect type narrowing.

The fix adds a check: $holder->getType()->isSuperTypeOf($theirExpressionTypes[$exprString]->getType())->no() — ensuring that the guard variable's types from both branches don't overlap before using it as a discriminator.

Test

Added tests/PHPStan/Analyser/nsrt/bug-14411.php which reproduces the original issue: $a = get_mixed(), then $b assigned conditionally from $a or get_optional_int(), then checking $b !== null should not narrow $a to non-null.

Fixes phpstan/phpstan#14411

[ondrejmirtes]

1. Please add regression tests for phpstan/phpstan#10843, phpstan/phpstan#14211, phpstan/phpstan#11328, phpstan/phpstan#10085

2. You broke Sylius:

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  Line   Sylius/Bundle/ShopBundle/EventListener/OrderCustomerIpListener.php                                                                                            
 ------ -------------------------------------------------------------------------------------------------------------------------------------------------------------- 
  52     Variable $order might not be defined.                                                                                                                         
         🪪  variable.undefined                                                                                                                                        
         ✏️  https://github.com/Sylius/Sylius/blob/da59129e4c249cdc3d4b57840c86880008315592/src/Sylius/Bundle/ShopBundle/EventListener/OrderCustomerIpListener.php#L52  
 ------ -------------------------------------------------------------------------------------------------------------------------------------------------------------- 

Please reproduce the problem (a test that passes without changes to src/ but fails with current src/ changes) and then fix it too.

[phpstan-bot]

Both commits have been pushed. Here's a summary of what was done:

Fix (commit 1): The original isSuperTypeOf check was too broad - it rejected type guards whenever isSuperTypeOf returned Maybe, which incorrectly prevented object type guards like Event vs OrderInterface from working. The fix now:

• Still skips guards when isSuperTypeOf returns yes (complete type containment)
• Still skips guards when isSuperTypeOf returns maybe AND at least one type is NOT an object type (genuine value overlap for arrays, mixed types, etc.)
• Preserves guards when isSuperTypeOf returns maybe for two object types (theoretical class/interface overlap that shouldn't invalidate the guard)

This fixes the Sylius regression where $order was reported as "might not be defined" while keeping the original #14411 fix intact.

Regression tests (commit 2): Added NSRT tests for:

• #10843 - match(true) with instanceof (pre-existing issue, test documents current behavior)
• #14211 - repeated if ($m) with isset-based bool (passes)
• #11328 - backed enum in_array conditional narrowing (passes)
• #10085 - list type incorrectly inferred as non-empty (passes)