vim-patch:8.2.2612: col('.') may get outdated column value

Problem:    col('.') may get outdated column value.
Solution:   Add a note to the help how to make this work and add a test for
            it. (closes vim/vim#7971)
18b7d86d7f

runtime/doc/map.txt

@@ -224,6 +224,20 @@ text before the cursor and start omni completion when some condition is met.
 For abbreviations |v:char| is set to the character that was typed to trigger
 the abbreviation.  You can use this to decide how to expand the {lhs}.  You
 should not either insert or change the v:char.
+ 
+Also, keep in mind that the expression may be evaluated when looking for
+typeahead, before the previous command has been executed.  For example: >
+	func StoreColumn()
+	  let g:column = col('.')
+	  return 'x'
+	endfunc
+	nnoremap <expr> x StoreColumn()
+	nmap ! f!x
+You will notice that g:column has the value from before executing "fx",
+because "z" is evaluated before "fx" is executed.
+This can be solved by inserting <Ignore> before the character that is
+expression-mapped: >
+	nmap ! f!<Ignore>x
 
 Be very careful about side effects!  The expression is evaluated while
 obtaining characters, you may very well make the command dysfunctional.

src/nvim/testdir/test_mapping.vim

@@ -427,6 +427,30 @@ func Test_error_in_map_expr()
   exe buf .. 'bwipe!'
 endfunc
 
+func Test_expr_map_gets_cursor()
+  new
+  call setline(1, ['one', 'some w!rd'])
+  func StoreColumn()
+    let g:exprLine = line('.')
+    let g:exprCol = col('.')
+    return 'x'
+  endfunc
+  nnoremap <expr> x StoreColumn()
+  2
+  nmap ! f!<Ignore>x
+  call feedkeys("!", 'xt')
+  call assert_equal('some wrd', getline(2))
+  call assert_equal(2, g:exprLine)
+  call assert_equal(7, g:exprCol)
+
+  bwipe!
+  unlet g:exprLine
+  unlet g:exprCol
+  delfunc ExprMapped
+  nunmap x
+  nunmap !
+endfunc
+
 " Test for mapping errors
 func Test_map_error()
   call assert_fails('unmap', 'E474:')