[ysh] Add parse error for splat expr ...x

Fix crash

This is issue #2237

Author: Andy C

test/ysh-parse-errors.sh

@@ -1703,6 +1703,13 @@ test-int-overflow() {
   _ysh-parse-error '= 0x123456789_123456789_123456789'
 }
 
+test-splat-expr() {
+  _ysh-parse-error '= ...y'
+  _ysh-parse-error '= [42, ...y]'
+  _ysh-parse-error '= (...x for x in [1,2,3])'
+  _ysh-parse-error '= [...x for x in [1,2,3]]'
+}
+
 #
 # Entry Points
 #

ysh/expr_to_ast.py

@@ -324,7 +324,11 @@ def _TestlistComp(self, parent, p_node, id0):
 
         n = p_node.NumChildren()
         if n > 1 and p_node.GetChild(1).typ == grammar_nt.comp_for:
-            elt = self.Expr(p_node.GetChild(0))
+            child0 = p_node.GetChild(0)
+            if child0.typ == grammar_nt.splat_expr:
+                p_die('Splat not currently supported', child0.tok)
+            elt = self.Expr(child0)
+
             comp = self._CompFor(p_node.GetChild(1))
             if id0 == Id.Op_LParen:  # (x+1 for x in y)
                 return expr.GeneratorExp(elt, [comp])
@@ -346,7 +350,10 @@ def _TestlistComp(self, parent, p_node, id0):
         if id0 == Id.Op_LBracket:  # List [1,2,3]
             elts = []  # type: List[expr_t]
             for i in xrange(0, n, 2):  # skip commas
-                elts.append(self.Expr(p_node.GetChild(i)))
+                child = p_node.GetChild(i)
+                if child.typ == grammar_nt.splat_expr:
+                    p_die('Splat not currently supported', child.tok)
+                elts.append(self.Expr(child))
 
             return expr.List(parent.tok, elts,
                              expr_context_e.Store)  # unused expr_context_e

ysh/grammar.pgen2

@@ -80,6 +80,10 @@ factor: ('+'|'-'|'~') factor | power
 # YSH: removed Python 3 'await'
 power: atom trailer* ['**' factor]
 
+# Note: I think splat_expr is not for list comprehensions, it's only for
+# literals like [42, *x] in Python, or [42, ...x] in YSH.  This is new in
+# Python 3.
+# I think splat_expr expressed awkwardly because of pgen limitations.
 testlist_comp: (test|splat_expr) ( comp_for | (',' (test|splat_expr))* [','] )
 
 atom: (