ListAssignment: define RHS kind explicitly for all expression types

It's easy to introduce bug by adding a new expression and not defining
its ListAssignment RHS kind. Add all expression types explicitly and
remove default case so that it becomes compile time error if the list is
not updated.

hphp/compiler/expression/list_assignment.cpp

@@ -73,6 +73,7 @@ static ListAssignment::RHSKind GetRHSKind(ExpressionPtr rhs) {
     case Expression::KindOfObjectMethodExpression:
     case Expression::KindOfNewObjectExpression:
     case Expression::KindOfAssignmentExpression:
+    case Expression::KindOfExpressionList:
     case Expression::KindOfIncludeExpression:
       return ListAssignment::Regular;
 
@@ -106,9 +107,24 @@ static ListAssignment::RHSKind GetRHSKind(ExpressionPtr rhs) {
     case Expression::KindOfQOpExpression:
       return ListAssignment::Checked;
 
-    default: break;
+    // invalid context
+    case Expression::KindOfArrayPairExpression:
+    case Expression::KindOfParameterExpression:
+    case Expression::KindOfModifierExpression:
+    case Expression::KindOfUserAttribute:
+      always_assert(false);
+
+    // non-arrays
+    case Expression::KindOfScalarExpression:
+    case Expression::KindOfConstantExpression:
+    case Expression::KindOfClassConstantExpression:
+    case Expression::KindOfEncapsListExpression:
+    case Expression::KindOfClosureExpression:
+      return ListAssignment::Null;
   }
-  return ListAssignment::Null;
+
+  // unreachable for known expression kinds
+  always_assert(false);
 }
 
 static bool AssignmentCouldSet(ExpressionListPtr vars, ExpressionPtr var) {