vim-patch:8.1.0138: negative value of 'softtabstop' not used correctly

Problem:    Negative value of 'softtabstop' not used correctly.
Solution:   Use get_sts_value(). (Tom Ryder)
33d5ab3795

src/nvim/edit.c

@@ -8206,7 +8206,7 @@ static bool ins_bs(int c, int mode, int *inserted_space_p)
         want_vcol = (want_vcol / curbuf->b_p_sw) * curbuf->b_p_sw;
       } else {
         want_vcol = tabstop_start(want_vcol,
-                                  curbuf->b_p_sts,
+                                  get_sts_value(),
                                   curbuf->b_p_vsts_array);
       }
 
@@ -8703,10 +8703,10 @@ static bool ins_tab(void)
     temp = (int)curbuf->b_p_sw;
     temp -= get_nolist_virtcol() % temp;
   } else if (tabstop_count(curbuf->b_p_vsts_array) > 0
-             || curbuf->b_p_sts > 0) {
+             || curbuf->b_p_sts != 0) {
     // use 'softtabstop' when set
     temp = tabstop_padding(get_nolist_virtcol(),
-                           curbuf->b_p_sts,
+                           get_sts_value(),
                            curbuf->b_p_vsts_array);
   } else {
     // otherwise use 'tabstop'

src/nvim/option.c

@@ -7347,7 +7347,7 @@ int get_sw_value(buf_T *buf)
 }
 
 /// Return the effective softtabstop value for the current buffer,
-/// using the effective shiftwidth  value when 'softtabstop' is negative.
+/// using the shiftwidth  value when 'softtabstop' is negative.
 int get_sts_value(void)
 {
   long result = curbuf->b_p_sts < 0 ? get_sw_value(curbuf) : curbuf->b_p_sts;

src/nvim/testdir/test_tab.vim

@@ -1,3 +1,4 @@
+" Various tests for inserting a Tab.
 
 " Tests for "r<Tab>" with 'smarttab' and 'expandtab' set/not set.
 " Also test that dv_ works correctly
@@ -43,3 +44,38 @@ func Test_smarttab()
   enew!
   set expandtab& smartindent& copyindent& ts& sw& sts&
 endfunc
+
+func Test_softtabstop()
+  new
+  set sts=0 sw=0
+  exe "normal ix\<Tab>x\<Esc>"
+  call assert_equal("x\tx", getline(1))
+
+  call setline(1, '')
+  set sts=4
+  exe "normal ix\<Tab>x\<Esc>"
+  call assert_equal("x   x", getline(1))
+
+  call setline(1, '')
+  set sts=-1 sw=4
+  exe "normal ix\<Tab>x\<Esc>"
+  call assert_equal("x   x", getline(1))
+
+  call setline(1, 'x       ')
+  set sts=0 sw=0 backspace=start
+  exe "normal A\<BS>x\<Esc>"
+  call assert_equal("x      x", getline(1))
+
+  call setline(1, 'x       ')
+  set sts=4
+  exe "normal A\<BS>x\<Esc>"
+  call assert_equal("x   x", getline(1))
+
+  call setline(1, 'x       ')
+  set sts=-1 sw=4
+  exe "normal A\<BS>x\<Esc>"
+  call assert_equal("x   x", getline(1))
+
+  set sts=0 sw=0 backspace&
+  bwipe!
+endfunc