expansion: Only clamp ranges when not forcing direction

This caused `$var[2..-1]` to still expand to $var[1] if only one
element was given.

Fixup for #4965.

Fixes #5187.

src/expand.cpp

@@ -219,21 +219,24 @@ static size_t parse_slice(const wchar_t *in, wchar_t **end_ptr, std::vector<long
 
             // debug( 0, L"Push range %d %d", tmp, tmp1 );
             long i2 = tmp1 > -1 ? tmp1 : size + tmp1 + 1;
-            // Clamp to array size, but only when doing a range,
-            // and only when just one is too high.
+            // Skip sequences that are entirely outside.
+            // This means "17..18" expands to nothing if there are less than 17 elements.
             if (i1 > size && i2 > size) {
                 continue;
             }
-            i1 = i1 < size ? i1 : size;
-            i2 = i2 < size ? i2 : size;
-            // debug( 0, L"Push range idx %d %d", i1, i2 );
             short direction = i2 < i1 ? -1 : 1;
             // If only the beginning is negative, always go reverse.
             // If only the end, always go forward.
             // Prevents `[x..-1]` from going reverse if less than x elements are there.
             if (tmp1 > -1 != tmp > -1) {
                 direction = tmp1 > -1 ? -1 : 1;
+            } else {
+                // Clamp to array size when not forcing direction
+                // - otherwise "2..-1" clamps both to 1 and then becomes "1..1".
+                i1 = i1 < size ? i1 : size;
+                i2 = i2 < size ? i2 : size;
             }
+            // debug( 0, L"Push range idx %d %d", i1, i2 );
             for (long jjj = i1; jjj * direction <= i2 * direction; jjj += direction) {
                 // debug(0, L"Expand range [subst]: %i\n", jjj);
                 idx.push_back(jjj);

tests/expansion.in

@@ -71,7 +71,9 @@ expansion $foo[-10..-5]
 expansion (printf '%s\n' $foo)[-5..2]
 expansion (printf '%s\n' $foo)[-2..-1]
 expansion (printf '%s\n' $foo)[-10..-5]
+expansion (echo one)[2..-1]
 
+echo "# foo = "
 set -l foo
 expansion "$foo[1]"
 expansion $foo[1]
@@ -83,11 +85,15 @@ expansion "$foo[1 2]"
 expansion $foo[1 2]
 expansion "$foo[2 1]"
 expansion $foo[2 1]
+echo "# foo = a b c"
 set -l foo a b c
 expansion $foo[17]
 expansion $foo[-17]
 expansion $foo[17..18]
 expansion $foo[4..-2]
+echo "# foo = a"
+set -l foo a
+expansion $foo[2..-1]
 
 echo "$foo[d]"
 echo $foo[d]

tests/expansion.out

@@ -42,6 +42,8 @@
 0
 2  fooest
 0
+0
+# foo = 
 1 
 0
 1 
@@ -52,10 +54,13 @@
 0
 1 
 0
+# foo = a b c
 0
 0
 0
 0
+# foo = a
+0
 
 
 ####################