Fix phpstan/phpstan#4090: Array detected as empty while it cannot

src/Analyser/MutatingScope.php

@@ -2144,6 +2144,36 @@
 		);
 	}
 
+	private function conditionalExpressionHolderMatches(ExpressionTypeHolder $specified, ExpressionTypeHolder $condition): bool
+	{
+		if ($specified->equals($condition)) {
+			return true;
+		}
+
+		if (!$specified->getCertainty()->equals($condition->getCertainty())) {
+			return false;
+		}
+
+		$conditionType = $condition->getType();
+
+		// Only use isSuperTypeOf for non-constant integer types (i.e. IntegerRangeType).
+		// This is needed for count()-in-variable patterns: when $count = count($a) creates
+		// a condition on int<1, max>, and $count is later narrowed to int<2, max> or 1,
+		// the subtype relationship correctly preserves the array narrowing.
+		//
+		// We cannot safely extend this to all types because filterBySpecifiedTypes()
+		// intersects results from ALL matching conditional expressions (line ~3251).
+		// For types like bool, isSuperTypeOf would match both a 'false' condition AND
+		// a 'bool' condition simultaneously, and intersecting their (potentially conflicting)
+		// results produces *NEVER*. Integer ranges don't have this problem because
+		// count() creates non-overlapping conditions (e.g. int<1, max> vs 0).
+		if (!$conditionType->isInteger()->yes() || $conditionType->isConstantScalarValue()->yes()) {
+			return false;
+		}
+
+		return $conditionType->isSuperTypeOf($specified->getType())->yes();
+	}
+
 	private function expressionTypeIsUnchangeable(ExpressionTypeHolder $typeHolder): bool
 	{
 		$expr = $typeHolder->getExpr();
@@ -3218,7 +3248,7 @@
 				}
 				foreach ($conditionalExpressions as $conditionalExpression) {
 					foreach ($conditionalExpression->getConditionExpressionTypeHolders() as $holderExprString => $conditionalTypeHolder) {
-						if (!array_key_exists($holderExprString, $specifiedExpressions) || !$specifiedExpressions[$holderExprString]->equals($conditionalTypeHolder)) {
+						if (!array_key_exists($holderExprString, $specifiedExpressions) || !$this->conditionalExpressionHolderMatches($specifiedExpressions[$holderExprString], $conditionalTypeHolder)) {
 							continue 2;
 						}
 					}

tests/PHPStan/Analyser/nsrt/bug-4090.php

@@ -0,0 +1,50 @@
+<?php declare(strict_types = 1);
+
+namespace Bug4090;
+
+use function PHPStan\Testing\assertType;
+
+/** @param string[] $a */
+function foo(array $a): void
+{
+	if (count($a) > 1) {
+		assertType('non-empty-array<string>', $a);
+		echo implode(',', $a);
+	} elseif (count($a) === 1) {
+		assertType('non-empty-array<string>', $a);
+		echo trim(current($a));
+	}
+}
+
+
+/** @param string[] $a */
+function bar(array $a): void
+{
+	$count = count($a);
+	if ($count > 1) {
+		assertType('non-empty-array<string>', $a);
+		echo implode(',', $a);
+	} elseif ($count === 1) {
+		assertType('non-empty-array<string>', $a);
+		echo trim(current($a));
+	}
+}
+
+
+/** @param string[] $a */
+function qux(array $a): void
+{
+	switch (count($a)) {
+		case 0:
+			assertType('array{}', $a);
+			break;
+		case 1:
+			assertType('non-empty-array<string>', $a);
+			echo trim(current($a));
+			break;
+		default:
+			assertType('non-empty-array<string>', $a);
+			echo implode(',', $a);
+			break;
+	}
+}