json.Unmarshal([]byte{}, &m) returns a bare io.ErrUnexpectedEOF, not a jsontext.SyntacticError.

This results in the v1 wrapper not converting the ErrUnexpectedEOF into the old-style error "unexpected end of JSON input", but I think the problem is in the v2 Unmarshal not returning the right error type here; it should consistently return a SyntacticError or SemanticError.