[osh] Implement $(( x[0] )) for value.Undef and value.Str

shopt -s strict_arith gives errors for these cases, but we handle them
by default.

Reported by Koiche Murase on Zulip:

    https://oilshell.zulipchat.com/#narrow/stream/121540-oil-discuss/topic/osh.20-c.20'echo.20.24.28.28arr.5B0.5D.29.29'.20fails

osh/sh_expr_eval.py

@@ -709,11 +709,34 @@ def Eval(self, node):
                             key = self.EvalWordToString(node.right)
                             s = left.d.get(key)
 
+                        elif case(value_e.Str):
+                            left = cast(value.Str, UP_left)
+                            if self.exec_opts.strict_arith():
+                                e_die(
+                                    "Value of type Str can't be indexed (strict_arith)",
+                                    node.op)
+                            index = self.EvalToBigInt(node.right)
+                            # s[0] evaluates to s
+                            # s[1] evaluates to Undef
+                            s = left.s if mops.Equal(index,
+                                                     mops.ZERO) else None
+
+                        elif case(value_e.Undef):
+                            if self.exec_opts.strict_arith():
+                                e_die(
+                                    "Value of type Undef can't be indexed (strict_arith)",
+                                    node.op)
+                            s = None  # value.Undef
+
+                            # There isn't a way to distinguish Undef vs. empty
+                            # string, even with set -o nounset?
+                            # s = ''
+
                         else:
                             # TODO: Add error context
                             e_die(
-                                'Expected array or assoc in index expression, got %s'
-                                % ui.ValType(left))
+                                "Value of type %s can't be indexed" %
+                                ui.ValType(left), node.op)
 
                     if s is None:
                         val = value.Undef

spec/arith.test.sh

@@ -759,3 +759,117 @@ echo $((-10 % -3))
 1
 -1
 ## END
+
+#### undef[0]
+case $SH in dash) exit ;; esac
+
+echo ARITH $(( undef[0] ))
+echo status=$?
+echo
+
+(( undef[0] ))
+echo status=$?
+echo
+
+echo UNDEF ${undef[0]}
+echo status=$?
+
+## STDOUT:
+ARITH 0
+status=0
+
+status=1
+
+UNDEF
+status=0
+## END
+## N-I dash STDOUT:
+## END
+
+#### undef[0] with nounset
+case $SH in dash) exit ;; esac
+
+set -o nounset
+echo UNSET $(( undef[0] ))
+echo status=$?
+
+## status: 1
+## STDOUT:
+## END
+
+## N-I dash status: 0
+
+## BUG mksh/zsh status: 0
+## BUG mksh/zsh STDOUT:
+UNSET 0
+status=0
+## END
+
+## N-I dash STDOUT:
+## END
+
+#### s[0] with string abc
+case $SH in dash) exit ;; esac
+
+s='abc'
+echo abc $(( s[0] )) $(( s[1] ))
+echo status=$?
+echo
+
+(( s[0] ))
+echo status=$?
+echo
+
+## STDOUT:
+abc 0 0
+status=0
+
+status=1
+
+## END
+## N-I dash STDOUT:
+## END
+
+#### s[0] with string 42 
+case $SH in dash) exit ;; esac
+
+s='42'
+echo 42 $(( s[0] )) $(( s[1] ))
+echo status=$?
+
+## STDOUT:
+42 42 0
+status=0
+## END
+## N-I dash STDOUT:
+## END
+
+## BUG zsh STDOUT:
+42 0 4
+status=0
+## END
+
+#### s[0] with string '12 34'
+
+s='12 34'
+echo '12 34' $(( s[0] )) $(( s[1] ))
+echo status=$?
+
+## status: 1
+## STDOUT:
+## END
+
+## OK dash status: 2
+
+## BUG zsh status: 0
+## BUG zsh STDOUT:
+12 34 0 1
+status=0
+## END
+
+# bash prints an error, but doesn't fail
+
+## BUG bash status: 0
+## BUG bash STDOUT:
+status=1
+## END

test/runtime-errors.sh

@@ -678,6 +678,12 @@ test-unset_expr() {
   _osh-error-2 'unset -v 1+2'
 }
 
+test-strict-arith() {
+  _osh-error-1 'shopt -s strict_arith; echo $(( undef[0] ))'
+  _osh-error-1 'shopt -s strict_arith; s=abc; echo $(( s[0] ))'
+  _osh-error-1 'shopt -s strict_arith; var i = 42; echo $(( i[0] ))'
+}
+
 # Only dash flags this as an error.
 unquoted-string_to_int_arith() {
   local x='ZZZ'