Allow nested square brackets again

Code like echo $list[$var[1]] was producing an error because of
nested square brackets. Allow these brackets again.

Fixes #5362

src/tokenizer.cpp

@@ -205,17 +205,8 @@ tok_t tokenizer_t::read_string() {
             expecting.pop_back();
         } else if (c == L'[') {
             if (this->buff != buff_start) {
-                if ((mode & tok_modes::array_brackets) == tok_modes::array_brackets) {
-                    // Nested brackets should not overwrite the existing slice_offset
-                    // mqudsi: TOK_ILLEGAL_SLICE is the right error here, but the shell
-                    // prints an error message with the caret pointing at token_start,
-                    // not err_loc, making the TOK_ILLEGAL_SLICE message misleading.
-                    // return call_error(TOK_ILLEGAL_SLICE, buff_start, this->buff);
-                    return this->call_error(tokenizer_error_t::unterminated_slice, this->start,
-                                            this->buff);
-                }
-                slice_offset = this->buff - this->start;
                 mode |= tok_modes::array_brackets;
+                slice_offset = this->buff - this->start;
             } else {
                 // This is actually allowed so the test operator `[` can be used as the head of a
                 // command

tests/expansion.err

@@ -1,3 +1,6 @@
+
+####################
+# Slices
 fish: Invalid index value
 echo "$foo[d]"
            ^

tests/expansion.in

@@ -60,6 +60,8 @@ set -l fooer ''
 expansion $$foo
 expansion prefix$$foo
 
+logmsg Slices
+
 set -l foo bar '' fooest
 expansion "$$foo"
 expansion $$foo
@@ -101,6 +103,11 @@ echo $foo[d]
 echo ()[1]
 echo ()[d]
 
+set -l outer out1 out2
+set -l inner 1 2
+echo $outer[$inner[2]] # out2
+echo $outer[$inner[2..1]] # out2 out1
+
 logmsg Percent self
 echo %selfNOT NOT%self \%self "%self" '%self'
 echo %self | string match -qr '^\\d+$'

tests/expansion.out

@@ -32,6 +32,9 @@
 2 prefixbaz prefixquux
 3 baz quux 
 3 prefixbaz prefixquux prefix
+
+####################
+# Slices
 1 baz quux  fooest
 2 baz quux
 1 prefixbaz quux  fooest
@@ -62,6 +65,8 @@
 # foo = a
 0
 
+out2
+out2 out1
 
 ####################
 # Percent self