Carry template argument constraints of if/elseif/else branches ruled out by the condition onto the if result scope - #6827

Summary

With unresolvedTemplateArguments (bleedingEdge), a generic call inside an if branch whose condition is always false got unresolved(*NEVER*) template arguments. The result was a false-positive return.type, or generator.valueType for a yield. The template arguments of such a call come only from the context, for example the declared return type. That context never reached the resolver, because IfHandler dropped the dead branch's whole statement result. Now the constraints of the dead branch are kept.

Changes

• src/Analyser/StmtHandler/IfHandler.php: the scopes of branches that do not take part in the result are collected. This covers an if with an always-false condition, plus elseif/else branches that come after an always-true condition. Their template argument constraints are added to the final scope.
• turbo-ext/src/IfHandler.cpp: the same change in the native mirror.
• Probed and already correct: dead while, for, foreach over an empty array, switch case, match arm, ternary, && right side, dead catch, unreachable statement after return. Dead elseif/else branches already worked in the probed shapes. They are now handled the same way and are covered by the test.

Root cause

InternalStatementResult merges the constraints of exit points and end statements into its scope. IfHandler only kept those for branches the condition does not rule out. A ruled-out branch is still walked, and rules still run on it. Its calls registered template argument sites, but the constraints meant to resolve them, such as the return send from ReturnHandler, were thrown away. So the sites resolved to their initial never. Other handlers already add the constraints of parts that do not flow into their result: TernaryHandler, ForHandler, TryCatchHandler, BooleanAndHandler/BooleanOrHandler. IfHandler was the one that did not.

Test

• ReturnTypeRuleTest::testBug15476 (tests/PHPStan/Rules/Methods/data/bug-15476.php): the issue's reproducer, plus a dead elseif, a dead else, an else after an always-true elseif, and a dead branch nested in a live if.
• YieldTypeRuleTest::testBug15476 (tests/PHPStan/Rules/Generators/data/bug-15476.php): the same pattern with yield in a dead if branch and a dead else.
• Both failed before the fix. They pass with and without the turbo extension loaded.

Fixes phpstan/phpstan#15476

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