Problem:    Vim9: using filter in compiled command does not work.
Solution:   Generate EXEC including the command modifier.

Problem:    Vim9: substitute expression cannot be a List in a :def function.
Solution:   Use typval2string(). (closes #8330)

Problem:    Vim9: using  does not handle a list of strings.
Solution:   Convert a list to a string and escape each item. (closes #8310)

Problem:    Vim9: "echo Func()" does not give an error for a function without
            a return value.
Solution:   Give an error.  Be more specific about why a value is invalid.

Problem:    Various code lines not covered by tests.
Solution:   Add test cases. (Dominique Pellé, closes #8178)

Problem:    Vim9: still crash when using substitute expression.
Solution:   Put the instruction list in the stack frame. (closes #8154)

Problem:    Vim9: error for missing white space doesn't say where it is
            missing
Solution:   Mention the command. (closes #8149)

Problem:    Vim9: crash when calling a function in a substitute expression.
Solution:   Set the instructions back to the substitute expression
            instrunctions. (closes #8148)

Problem:    Vim9: redir to variable with append does not accept an index.
Solution:   Make the appending work.

Problem:    Vim9: redir to variable does not accept an index.
Solution:   Make the index work.