Fix for issue #280 - the condition operator may return a valid enum type.

If the condition expression is 2-state, the result won't be blended, so
the result will be a valid enum value if both true and false expressions
return the same enum type.

(cherry picked from commit 82c8a49573)

netlist.cc

@@ -2443,6 +2443,20 @@ NetETernary::~NetETernary()
       delete false_val_;
 }
 
+const netenum_t* NetETernary::enumeration() const
+{
+	// If the condition can evaluate to an ambiguous value,
+	// the result may be blended, and so is not guaranteed
+	// to be a valid enumeration value.
+      if (cond_->expr_type() != IVL_VT_BOOL)
+	    return 0;
+
+      if (true_val_->enumeration() != false_val_->enumeration())
+	    return 0;
+
+      return true_val_->enumeration();
+}
+
 const NetExpr* NetETernary::cond_expr() const
 {
       return cond_;

netlist.h

@@ -4580,6 +4580,8 @@ class NetETernary  : public NetExpr {
       NetETernary(NetExpr*c, NetExpr*t, NetExpr*f, unsigned wid, bool signed_flag);
       ~NetETernary();
 
+      const netenum_t* enumeration() const;
+
       const NetExpr*cond_expr() const;
       const NetExpr*true_expr() const;
       const NetExpr*false_expr() const;