Fix/specifier bug

src/Analyser/MutatingScope.php

@@ -3401,7 +3401,7 @@ public function unsetExpression(Expr $expr): self
 				[],
 				$this->afterExtractCall,
 				$this->parentScope,
-			);
+			)->invalidateExpression($expr);
 		} elseif ($expr instanceof Expr\ArrayDimFetch && $expr->dim !== null) {
 			return $this->specifyExpressionType(
 				$expr->var,
@@ -3499,7 +3499,7 @@ public function specifyExpressionType(Expr $expr, Type $type, ?Type $nativeType
 				$this->inFunctionCallsStack,
 				$this->afterExtractCall,
 				$this->parentScope,
-			);
+			)->invalidateExpression($expr);
 		} elseif ($expr instanceof Expr\ArrayDimFetch && $expr->dim !== null) {
 			$constantArrays = TypeUtils::getOldConstantArrays($this->getType($expr->var));
 			if (count($constantArrays) > 0) {

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -920,6 +920,7 @@ public function dataFileAsserts(): iterable
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/remember-possibly-impure-function-values.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/emptyiterator.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/collected-data.php');
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-1396.php');
 	}
 
 	/**

tests/PHPStan/Analyser/data/bug-1396.php

@@ -0,0 +1,25 @@
+<?php declare(strict_types = 1);
+
+namespace Bug1396;
+
+use PHPStan\Type\Constant\ConstantStringType;
+use PHPStan\Type\ConstantScalarType;
+use function PHPStan\Testing\assertType;
+
+class HelloWorld
+{
+	public function doFoo(
+		ConstantScalarType $constantType
+	): SpecifiedTypes
+	{
+		if ($constantType->getValue() === null) {
+			return new SpecifiedTypes();
+		}
+
+		if (
+			$constantType instanceof ConstantStringType
+		) {
+			assertType('string', $constantType->getValue());
+		}
+	}
+}