diff --git a/src/Rules/FunctionCallParametersCheck.php b/src/Rules/FunctionCallParametersCheck.php
index 57ae028018..211a34ef80 100644
--- a/src/Rules/FunctionCallParametersCheck.php
+++ b/src/Rules/FunctionCallParametersCheck.php
@@ -100,6 +100,10 @@ public function check(
 			$functionParametersMaxCount = -1;
 		}
 
+		if (!$scope instanceof MutatingScope) {
+			throw new ShouldNotHappenException();
+		}
+
 		/** @var array<int, array{Expr, Type|null, bool, string|null, int}> $arguments */
 		$arguments = [];
 		/** @var array<int, Node\Arg> $args */
@@ -114,6 +118,10 @@ public function check(
 				$argumentName = $arg->name->toString();
 			}
 
+			if ($arg->value instanceof Expr\Assign) {
+				$scope = $scope->assignExpression($arg->value->var, $scope->getType($arg->value->expr), $scope->getNativeType($arg->value->expr));
+			}
+
 			if ($hasNamedArguments && $arg->unpack) {
 				$errors[] = RuleErrorBuilder::message('Named argument cannot be followed by an unpacked (...) argument.')
 					->identifier('argument.unpackAfterNamed')
@@ -318,14 +326,9 @@ public function check(
 			}
 
 			if ($argumentValueType === null) {
-				if ($scope instanceof MutatingScope) {
-					$scope = $scope->pushInFunctionCall(null, $parameter);
-				}
+				$scope = $scope->pushInFunctionCall(null, $parameter);
 				$argumentValueType = $scope->getType($argumentValue);
-
-				if ($scope instanceof MutatingScope) {
-					$scope = $scope->popInFunctionCall();
-				}
+				$scope = $scope->popInFunctionCall();
 			}
 
 			if (!$acceptsNamedArguments->yes()) {
diff --git a/tests/PHPStan/Rules/Functions/CallToFunctionParametersRuleTest.php b/tests/PHPStan/Rules/Functions/CallToFunctionParametersRuleTest.php
index 9a9cc2f6e5..a7e89bc83d 100644
--- a/tests/PHPStan/Rules/Functions/CallToFunctionParametersRuleTest.php
+++ b/tests/PHPStan/Rules/Functions/CallToFunctionParametersRuleTest.php
@@ -2317,4 +2317,11 @@ public function testBug12317(): void
 		]);
 	}
 
+	public function testBug12234(): void
+	{
+		$this->checkExplicitMixed = true;
+		$this->checkImplicitMixed = true;
+		$this->analyse([__DIR__ . '/data/bug-12234.php'], []);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Functions/data/bug-12234.php b/tests/PHPStan/Rules/Functions/data/bug-12234.php
new file mode 100644
index 0000000000..1afa1da92a
--- /dev/null
+++ b/tests/PHPStan/Rules/Functions/data/bug-12234.php
@@ -0,0 +1,18 @@
+<?php declare(strict_types = 1);
+
+namespace Bug12234;
+
+class Foo {
+	public function getSize(): int {
+		return 0;
+	}
+}
+
+function bar(Foo $foo, int $b): true {
+	return true;
+}
+
+$test = bar(
+	$foo = new Foo(),
+	$foo->getSize(),
+);
diff --git a/tests/PHPStan/Rules/Methods/CallMethodsRuleTest.php b/tests/PHPStan/Rules/Methods/CallMethodsRuleTest.php
index 649a505d5b..f3ad8d2a84 100644
--- a/tests/PHPStan/Rules/Methods/CallMethodsRuleTest.php
+++ b/tests/PHPStan/Rules/Methods/CallMethodsRuleTest.php
@@ -3656,4 +3656,26 @@ public function testBug3396(): void
 		$this->analyse([__DIR__ . '/data/bug-3396.php'], []);
 	}
 
+	public function testBug12735(): void
+	{
+		$this->checkThisOnly = false;
+		$this->checkNullables = true;
+		$this->checkUnionTypes = true;
+		$this->checkExplicitMixed = true;
+		$this->checkImplicitMixed = true;
+
+		$this->analyse([__DIR__ . '/data/bug-12735.php'], []);
+	}
+
+	public function testBug12735b(): void
+	{
+		$this->checkThisOnly = false;
+		$this->checkNullables = true;
+		$this->checkUnionTypes = true;
+		$this->checkExplicitMixed = true;
+		$this->checkImplicitMixed = true;
+
+		$this->analyse([__DIR__ . '/data/bug-12735b.php'], []);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Methods/data/bug-12735.php b/tests/PHPStan/Rules/Methods/data/bug-12735.php
new file mode 100644
index 0000000000..97ea2d8a58
--- /dev/null
+++ b/tests/PHPStan/Rules/Methods/data/bug-12735.php
@@ -0,0 +1,18 @@
+<?php
+
+namespace Bug12735;
+
+class HelloWorld
+{
+	public function test(): void
+	{
+		$this->foo(
+			$now = new DateTimeImmutable(),
+			$now,
+		);
+
+		$this->foo($now, $now);
+	}
+
+	private function foo(DateTimeImmutable $a, DateTimeImmutable $b): void {}
+}
diff --git a/tests/PHPStan/Rules/Methods/data/bug-12735b.php b/tests/PHPStan/Rules/Methods/data/bug-12735b.php
new file mode 100644
index 0000000000..f7c2f08eb9
--- /dev/null
+++ b/tests/PHPStan/Rules/Methods/data/bug-12735b.php
@@ -0,0 +1,25 @@
+<?php
+
+namespace Bug12735b;
+
+use DateTimeImmutable;
+
+class HelloWorld
+{
+	public function test(): void
+	{
+		$now = null;
+		if (rand(0,1)) {
+			$now = new DateTimeImmutable();
+		}
+
+		$this->foo(
+			$now ??= new DateTimeImmutable(),
+			$now,
+		);
+
+		$this->foo($now, $now);
+	}
+
+	private function foo(DateTimeImmutable $a, DateTimeImmutable $b): void {}
+}