[osh-language] Tighten up behavior of assoc arrays inside (( )).

That string keys on the LHS but not the RHS seems like a quirk.  But it
makes sense if you remember that (( )) tries to coerce everything to an
integer.

Related to issue #207.

osh/expr_eval.py

@@ -498,10 +498,6 @@ def Eval(self, node):
 
       rhs = self.Eval(node.right)  # eager evaluation for the rest
 
-      # TODO:
-      # - More type checks
-      # - How do we blame arith_expr?  Really we need the OPERATOR.
-
       if op_id == Id.Arith_LBracket:
         # StrArray or AssocArray
         if isinstance(lhs, list):
@@ -516,15 +512,15 @@ def Eval(self, node):
               # TODO: Should be None for Undef instead?  Or ''?
               return 0
 
+        # Quirk: (( A[$key] = 42 )) works
+        #        (( x = A[$key] )) doesn't work because $key is coerced to
+        #        an integer
+        # We could relax this restriction by using value_t here instead of the
+        # None/int/list representation.
+
         elif isinstance(lhs, dict):
-          try:
-            item = lhs[str(rhs)]  # 0 -> '0'
-          except KeyError:
-            if self.exec_opts.nounset:
-              e_die('Invalid key %r' % rhs)
-            else:
-              # TODO: Should be None for Undef instead?  Or ''?
-              return 0
+          e_die("Can't evaluate associative arrays in arithmetic contexts")
+
         else:
           # TODO: Add error context
           e_die('Expected array in index expression, got %s', lhs)

spec/assoc.test.sh

@@ -331,7 +331,7 @@ echo ${assoc[1]} ${assoc[2]} ${assoc}
 ## N-I osh stdout-json: ""
 ## N-I osh status: 1
 
-#### Associative array expressions inside (( ))
+#### Associative array expressions inside (( )) with keys that look like numbers
 declare -A assoc
 assoc[0]=42
 (( var = ${assoc[0]} ))
@@ -342,6 +342,10 @@ echo $var
 42
 42
 ## END
+## N-I osh status: 1
+## N-I osh STDOUT:
+42
+## END
 
 #### (( A[5] += 42 ))
 declare -A A