Report error for class tasks used as expressions

Class object method calls in expression context call func_def() without first
checking that the resolved class method is a function. If the method is a task,
func_def() triggers an assert instead of reporting a normal elaboration error.

Check the method scope type before accessing the function definition and report
an error for tasks.

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

elab_expr.cc

@@ -3410,6 +3410,14 @@ NetExpr* PECallFunction::elaborate_expr_method_(Design*des, NetScope*scope,
 		  return 0;
 	    }
 
+	    if (method->type() != NetScope::FUNC) {
+		  cerr << get_fileline() << ": error: Method " << method_name
+		       << " of class " << class_type->get_name()
+		       << " is not a function." << endl;
+		  des->errors++;
+		  return nullptr;
+	    }
+
 	    const NetFuncDef*def = method->func_def();
 	    ivl_assert(*this, def);