Filter right side of null coalesce assign with === null

src/Analyser/NodeScopeResolver.php

@@ -1932,7 +1932,28 @@ function (MutatingScope $scope) use ($expr, $nodeCallback, $context): Expression
 				$expr,
 				$nodeCallback,
 				$context,
-				fn (MutatingScope $scope): ExpressionResult => $this->processExprNode($expr->expr, $scope, $nodeCallback, $context->enterDeep()),
+				function (MutatingScope $scope) use ($expr, $nodeCallback, $context): ExpressionResult {
+					$originalScope = $scope;
+					if ($expr instanceof Expr\AssignOp\Coalesce) {
+						$scope = $scope->filterByFalseyValue(
+							new BinaryOp\NotIdentical($expr->var, new ConstFetch(new Name('null'))),
+						);
+					}
+
+					$result = $this->processExprNode($expr->expr, $scope, $nodeCallback, $context->enterDeep());
+					if ($expr instanceof Expr\AssignOp\Coalesce) {
+						return new ExpressionResult(
+							$result->getScope()->mergeWith(
+								$this->processExprNode($expr->expr, $originalScope, static function (): void {
+								}, $context->enterDeep())->getScope(),
+							),
+							$result->hasYield(),
+							$result->getThrowPoints(),
+						);
+					}
+
+					return $result;
+				},
 				$expr instanceof Expr\AssignOp\Coalesce,
 			);
 			$scope = $result->getScope();

tests/PHPStan/Rules/Classes/InstantiationRuleTest.php

@@ -456,4 +456,9 @@ public function testBug7574(): void
 		$this->analyse([__DIR__ . '/data/bug-7574.php'], []);
 	}
 
+	public function testBug9946(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-9946.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Classes/data/bug-9946.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Bug9946;
+
+class Foo
+{
+
+	function test(?\DateTimeImmutable $a, ?string $b): string
+	{
+		if (!$a && !$b) {
+			throw new \LogicException('Either a or b MUST be set');
+		}
+		if (!$a) {
+			$c = new \DateTimeImmutable($b);
+		}
+		$a ??= new \DateTimeImmutable($b);
+
+		return $a->format('c');
+	}
+
+}