vim-patch:8.1.1967: line() only works for the current window

Problem:    Line() only works for the current window.
Solution:   Add an optional argument for the window to use.
8e0a8e7eb7

src/nvim/eval.c

@@ -7617,7 +7617,7 @@ char *save_tv_as_string(typval_T *tv, ptrdiff_t *const len, bool endnl)
 /// @param[out]  ret_fnum  Set to fnum for marks.
 ///
 /// @return Pointer to position or NULL in case of error (e.g. invalid type).
-pos_T *var2fpos(const typval_T *const tv, const int dollar_lnum,
+pos_T *var2fpos(const typval_T *const tv, const bool dollar_lnum,
                 int *const ret_fnum)
   FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_NONNULL_ALL
 {

src/nvim/eval.lua

@@ -217,7 +217,7 @@ return {
     len={args=1},
     libcall={args=3},
     libcallnr={args=3},
-    line={args=1},
+    line={args={1, 2}},
     line2byte={args=1},
     lispindent={args=1},
     list2str={args={1, 2}},

src/nvim/eval/funcs.c

@@ -5540,18 +5540,36 @@ static void f_libcallnr(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   libcall_common(argvars, rettv, VAR_NUMBER);
 }
 
-/*
- * "line(string)" function
- */
+// "line(string, [winid])" function
 static void f_line(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   linenr_T lnum = 0;
-  pos_T       *fp;
+  pos_T *fp = NULL;
   int fnum;
 
-  fp = var2fpos(&argvars[0], TRUE, &fnum);
-  if (fp != NULL)
+  if (argvars[1].v_type != VAR_UNKNOWN) {
+    tabpage_T *tp;
+    win_T *save_curwin;
+    tabpage_T *save_curtab;
+
+    // use window specified in the second argument
+    win_T *wp = win_id2wp_tp(&argvars[1], &tp);
+    if (wp != NULL && tp != NULL) {
+      if (switch_win_noblock(&save_curwin, &save_curtab, wp, tp, true)
+          == OK) {
+        check_cursor();
+        fp = var2fpos(&argvars[0], true, &fnum);
+      }
+      restore_win_noblock(save_curwin, save_curtab, true);
+    }
+  } else {
+    // use current window
+    fp = var2fpos(&argvars[0], true, &fnum);
+  }
+
+  if (fp != NULL) {
     lnum = fp->lnum;
+  }
   rettv->vval.v_number = lnum;
 }