patch 9.0.0090: no error when assigning bool to a string option
Problem: No error when assigning bool to a string option with setwinvar().
Solution: Give an error (closes #10766)
diff --git a/src/proto/option.pro b/src/proto/option.pro
index c64672d..ca150b6 100644
--- a/src/proto/option.pro
+++ b/src/proto/option.pro
@@ -43,6 +43,7 @@
char_u *get_highlight_default(void);
char_u *get_encoding_default(void);
int is_option_allocated(char *name);
+int is_string_option(char_u *name);
int makeset(FILE *fd, int opt_flags, int local_only);
int makefoldset(FILE *fd);
void clear_termoptions(void);