Report error when calling void function in an expression

Unlike normal functions void functions can not be called as part of an
expression. Trying so will currently hit an internal assert.

Make sure an error is reported instead.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

elab_expr.cc

@@ -1642,6 +1642,9 @@ unsigned PECallFunction::test_width(Design*des, NetScope*scope,
 	    return 0;
       }
 
+      if (def->is_void())
+	    return 0;
+
       NetScope*dscope = def->scope();
       assert(dscope);
 
@@ -2860,6 +2863,14 @@ NetExpr* PECallFunction::elaborate_base_(Design*des, NetScope*scope, NetScope*ds
       if (parm_errors)
             return 0;
 
+      if (def->is_void()) {
+	    cerr << get_fileline() << ": error: void function `"
+		 << dscope->basename() << "` can not be called in an expression."
+		 << endl;
+	    des->errors++;
+	    return nullptr;
+      }
+
 	/* Look for the return value signal for the called
 	   function. This return value is a magic signal in the scope
 	   of the function, that has the name of the function. The