Casting Class.forName("java.lang.String") / 'Class.forName("some_str_var")' to Class

This line of code

 Class<String> cls =  (Class<String>) Class.forName("java.lang.String");

Gives compiler warning Type safety: Unchecked cast from Class<capture#1-of ?> to Class<String>

Also, according to API definition it returns returns Class<?>:

Class<?> Class.forName(String arg)

I do not understand why conversion from Class<?> to Class<String> is treated as unchecked (that is compiler cannot check if such cast is possible while compiling).

If it was

Class<String> cls =  (Class<String>) Class.forName("some_str_var");

I could understand that compiler cannot know return type of Class.forName() at compile time (because argument is a variable, not a literal) and in runtime type information is erased.

But Class.forName("java.lang.String") is obviuosly Class<?> during compilation (and cls variable in LHS is still Class<String>), no type information is erased yet (compiler sees source code, not byte-code with erased type-info), and compiler can check everything.