Fix AccessoryLiteralStringType::setOffsetValueType()

Author: ondrejmirtes

src/Type/Accessory/AccessoryLiteralStringType.php

@@ -153,7 +153,11 @@ public function getOffsetValueType(Type $offsetType): Type
 
 	public function setOffsetValueType(?Type $offsetType, Type $valueType, bool $unionValues = true): Type
 	{
-		return $this;
+		if ($valueType->isLiteralString()->yes()) {
+			return $this;
+		}
+
+		return new StringType();
 	}
 
 	public function setExistingOffsetValueType(Type $offsetType, Type $valueType): Type

tests/PHPStan/Analyser/nsrt/bug-3981.php

@@ -27,4 +27,17 @@ public function doFoo(string $s, string $nonEmptyString): void
 		assertType('non-empty-string', $s);
 	}
 
+	/**
+	 * @param literal-string $literalString
+	 * @param literal-string $anotherLiteralString
+	 */
+	public function doBar(string $literalString, string $generalString): void
+	{
+		$literalString[0] = 'a';
+		assertType('literal-string&non-empty-string', $literalString);
+
+		$literalString[1] = $generalString;
+		assertType('non-empty-string', $literalString);
+	}
+
 }