Problem:    Syntax of commands in Vim9 script depends on +eval feature.
Solution:   Use same syntax with and without the +eval feature.

Problem:    Underline color does not work in terminals that don't send a
            termresponse.
Solution:   Do output t_8u if it was set explicitly. (closes #11253)

Problem:    Build error and compiler warnings.
Solution:   Add missing change.  Add type casts.

Problem:    Evaluating "expr" options has more overhead than needed.
Solution:   Use call_simple_func() for 'foldtext', 'includeexpr', 'printexpr',
            "expr" of 'spellsuggest', 'diffexpr', 'patchexpr', 'balloonexpr',
            'formatexpr', 'indentexpr' and 'charconvert'.

Problem:    FEAT_TITLE was removed but is still used.
Solution:   Remove FEAT_TITLE. (Naruhiko Nishino, closes #11256)

Problem:    Calling a function from an "expr" option has too much overhead.
Solution:   Add call_simple_func() and use it for 'foldexpr'

Problem:    Too many delete() calls in tests.
Solution:   Use deferred delete where possible.

Problem:    In Vim9 script a numbered function cannot be called.
Solution:   Do not require "g:" before a numbered function name.
            (closes #11254)

Problem:    Get an error for using const only when executing.
Solution:   Check for const at compile time for filter(), map(), remove(),
            reverse(), sort() and uniq().

Problem:    Coverity warns for not checking return value.
Solution:   Check the return value and simplify the code.