charAt LowerBound Unknown

charAt should return a non-negative char. Consider the following test case:

class CharAtTest {
    @NonNegative char test1(@MinLen(1) String s) {
        return s.charAt(0);
    }
}

Output:

CharAtTest.java:6: error: [return.type.incompatible] incompatible types in return.
        return s.charAt(0);
                       ^
  type of expression: @LowerBoundUnknown char
  method return type: @NonNegative char
1 error

Expected: No errors
I'll be working on fixing this issue.

Start by reproducing the CharAtTest example using String.charAt(0) with the @NonNegative return type. Investigate the checker behavior that assigns @LowerBoundUnknown to charAt, then verify that the example compiles without errors and preserves the expected non-negative typing.