fix(:let): fix error when applying operator to boolean option (#24030)

2 files changed, 18 insertions, 5 deletions

diff --git a/src/nvim/eval/vars.c b/src/nvim/eval/vars.c
index 35ae558006..9b6427fef7 100644
--- a/src/nvim/eval/vars.c
+++ b/src/nvim/eval/vars.c
@@ -817,12 +817,11 @@ static char *ex_let_option(char *arg, typval_T *const tv, const bool is_const,
         new_n = num_modulus(cur_n, new_n); break;
       }
 
-      // clamp boolean values
-      if (newval.type == kOptValTypeBoolean && (new_n > 1 || new_n < -1)) {
-        new_n = (new_n > 1) ? 1 : -1;
+      if (curval.type == kOptValTypeNumber) {
+        newval = NUMBER_OPTVAL(new_n);
+      } else {
+        newval = BOOLEAN_OPTVAL(new_n == 0 ? kFalse : (new_n >= 1 ? kTrue : kNone));
       }
-
-      newval = kOptValTypeNumber ? NUMBER_OPTVAL(new_n) : BOOLEAN_OPTVAL((TriState)new_n);
     } else if (!hidden && is_string
                && curval.data.string.data != NULL && newval.data.string.data != NULL) {  // string
       OptVal newval_old = newval;
diff --git a/test/functional/vimscript/let_spec.lua b/test/functional/vimscript/let_spec.lua
index 164fa86452..11417c5846 100644
--- a/test/functional/vimscript/let_spec.lua
+++ b/test/functional/vimscript/let_spec.lua
@@ -92,6 +92,20 @@ describe(':let', function()
     ]])
     eq(1, eval('1'))
   end)
+
+  it('can apply operator to boolean option', function()
+    eq(true, meths.get_option_value('equalalways', {}))
+    command('let &equalalways -= 1')
+    eq(false, meths.get_option_value('equalalways', {}))
+    command('let &equalalways += 1')
+    eq(true, meths.get_option_value('equalalways', {}))
+    command('let &equalalways *= 1')
+    eq(true, meths.get_option_value('equalalways', {}))
+    command('let &equalalways /= 1')
+    eq(true, meths.get_option_value('equalalways', {}))
+    command('let &equalalways %= 1')
+    eq(false, meths.get_option_value('equalalways', {}))
+  end)
 end)
 
 describe(':let and :const', function()