Preserve vacuously true conditional expressions during scope merging

Fixes phpstan/phpstan#4173

Author: ondrejmirtes

src/Analyser/MutatingScope.php

@@ -3372,6 +3372,16 @@ public function mergeWith(?self $otherScope): self
 
 		$mergedExpressionTypes = $this->mergeVariableHolders($ourExpressionTypes, $theirExpressionTypes);
 		$conditionalExpressions = $this->intersectConditionalExpressions($otherScope->conditionalExpressions);
+		$conditionalExpressions = $this->preserveVacuousConditionalExpressions(
+			$conditionalExpressions,
+			$this->conditionalExpressions,
+			$theirExpressionTypes,
+		);
+		$conditionalExpressions = $this->preserveVacuousConditionalExpressions(
+			$conditionalExpressions,
+			$otherScope->conditionalExpressions,
+			$ourExpressionTypes,
+		);
 		$conditionalExpressions = $this->createConditionalExpressions(
 			$conditionalExpressions,
 			$ourExpressionTypes,
@@ -3477,6 +3487,43 @@ private function intersectConditionalExpressions(array $otherConditionalExpressi
 		return $newConditionalExpressions;
 	}
 
+	/**
+	 * @param array<string, ConditionalExpressionHolder[]> $currentConditionalExpressions
+	 * @param array<string, ConditionalExpressionHolder[]> $sourceConditionalExpressions
+	 * @param array<string, ExpressionTypeHolder> $otherExpressionTypes
+	 * @return array<string, ConditionalExpressionHolder[]>
+	 */
+	private function preserveVacuousConditionalExpressions(
+		array $currentConditionalExpressions,
+		array $sourceConditionalExpressions,
+		array $otherExpressionTypes,
+	): array
+	{
+		foreach ($sourceConditionalExpressions as $exprString => $holders) {
+			foreach ($holders as $key => $holder) {
+				if (isset($currentConditionalExpressions[$exprString][$key])) {
+					continue;
+				}
+
+				foreach ($holder->getConditionExpressionTypeHolders() as $guardExprString => $guardTypeHolder) {
+					if (!array_key_exists($guardExprString, $otherExpressionTypes)) {
+						continue;
+					}
+
+					$otherType = $otherExpressionTypes[$guardExprString]->getType();
+					$guardType = $guardTypeHolder->getType();
+
+					if ($otherType->isSuperTypeOf($guardType)->no()) {
+						$currentConditionalExpressions[$exprString][$key] = $holder;
+						break;
+					}
+				}
+			}
+		}
+
+		return $currentConditionalExpressions;
+	}
+
 	/**
 	 * @param array<string, ConditionalExpressionHolder[]> $newConditionalExpressions
 	 * @param array<string, ConditionalExpressionHolder[]> $existingConditionalExpressions

tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php

@@ -911,12 +911,7 @@ public function testBug4173(): void
 		$this->polluteScopeWithLoopInitialAssignments = false;
 		$this->checkMaybeUndefinedVariables = true;
 		$this->polluteScopeWithAlwaysIterableForeach = true;
-		$this->analyse([__DIR__ . '/data/bug-4173.php'], [
-			[
-				'Variable $value might not be defined.', // could be fixed
-				30,
-			],
-		]);
+		$this->analyse([__DIR__ . '/data/bug-4173.php'], []);
 	}
 
 	public function testBug5805(): void
@@ -1119,29 +1114,13 @@ public function testDynamicAccess(): void
 				18,
 			],
 			[
-				'Variable $foo might not be defined.',
-				36,
-			],
-			[
-				'Variable $foo might not be defined.',
-				37,
-			],
-			[
-				'Variable $bar might not be defined.',
+				'Undefined variable: $bar',
 				38,
 			],
 			[
-				'Variable $bar might not be defined.',
-				40,
-			],
-			[
-				'Variable $foo might not be defined.',
+				'Undefined variable: $foo',
 				41,
 			],
-			[
-				'Variable $bar might not be defined.',
-				42,
-			],
 			[
 				'Undefined variable: $buz',
 				44,