Add warning for ineffective &default arguments in handlers

For event/hook handlers that had a previous declaration, any &default
arguments are ineffective.  Only &default uses in the initial
prototype's arguments have an effect (that includes if the handler
is actually the site at which the declaration occurs).

doc

@@ -1 +1 @@
-Subproject commit 9e77250acc4b5eaf03c0535e8ea00a0a4d19cae0
+Subproject commit d8ddfa04683d6b062cbb22b1198163748a45aade

scripts/base/frameworks/netcontrol/non-cluster.zeek

@@ -28,7 +28,7 @@ event rule_expire(r: Rule, p: PluginState) &priority=-5
 	rule_expire_impl(r, p);
 	}
 
-event rule_exists(r: Rule, p: PluginState, msg: string &default="") &priority=5
+event rule_exists(r: Rule, p: PluginState, msg: string) &priority=5
 	{
 	rule_added_impl(r, p, T, msg);
 
@@ -36,7 +36,7 @@ event rule_exists(r: Rule, p: PluginState, msg: string &default="") &priority=5
 		schedule r$expire { rule_expire(r, p) };
 	}
 
-event rule_added(r: Rule, p: PluginState, msg: string &default="") &priority=5
+event rule_added(r: Rule, p: PluginState, msg: string) &priority=5
 	{
 	rule_added_impl(r, p, F, msg);
 
@@ -44,7 +44,7 @@ event rule_added(r: Rule, p: PluginState, msg: string &default="") &priority=5
 		schedule r$expire { rule_expire(r, p) };
 	}
 
-event rule_removed(r: Rule, p: PluginState, msg: string &default="") &priority=-5
+event rule_removed(r: Rule, p: PluginState, msg: string) &priority=-5
 	{
 	rule_removed_impl(r, p, msg);
 	}
@@ -54,7 +54,7 @@ event rule_timeout(r: Rule, i: FlowInfo, p: PluginState) &priority=-5
 	rule_timeout_impl(r, i, p);
 	}
 
-event rule_error(r: Rule, p: PluginState, msg: string &default="") &priority=-5
+event rule_error(r: Rule, p: PluginState, msg: string) &priority=-5
 	{
 	rule_error_impl(r, p, msg);
 	}

scripts/policy/frameworks/netcontrol/catch-and-release.zeek

@@ -259,7 +259,7 @@ function cr_check_rule(r: Rule): bool
 
 @if ( ! Cluster::is_enabled() || ( Cluster::is_enabled() && Cluster::local_node_type() == Cluster::MANAGER ) )
 
-event rule_added(r: Rule, p: PluginState, msg: string &default="")
+event rule_added(r: Rule, p: PluginState, msg: string)
 	{
 	if ( !cr_check_rule(r) )
 		return;

src/Var.cc

@@ -471,22 +471,46 @@ void begin_func(ID* id, const char* module_name, function_flavor flavor,
 
 	if ( id->Type() )
 		{
-		prototype = func_type_check(id->Type()->AsFuncType(), t.get());
+		auto decl = id->Type()->AsFuncType();
+		prototype = func_type_check(decl, t.get());
 
 		if ( prototype )
 			{
-			// If a previous declaration of the function had &default params,
-			// automatically transfer any that are missing (convenience so that
-			// implementations don't need to specify the &default expression again).
-			transfer_arg_defaults(prototype->args.get(), t->Args());
+			if ( decl->Flavor() == FUNC_FLAVOR_FUNCTION )
+				{
+				// If a previous declaration of the function had &default
+				// params, automatically transfer any that are missing
+				// (convenience so that implementations don't need to specify
+				// the &default expression again).
+				transfer_arg_defaults(prototype->args.get(), t->Args());
+				}
+			else
+				{
+				// Warn for trying to use &default parameters in hook/event
+				// handler body when it already has a declaration since only
+				// &default in the declaration has any effect.
+				auto args = t->Args();
+
+				for ( int i = 0; i < args->NumFields(); ++i )
+					{
+					auto f = args->FieldDecl(i);
+
+					if ( f->attrs && f->attrs->FindAttr(ATTR_DEFAULT) )
+						{
+						reporter->PushLocation(args->GetLocationInfo());
+						reporter->Warning(
+						    "&default on parameter '%s' has no effect (not a %s declaration)",
+						    args->FieldName(i), t->FlavorString().data());
+						reporter->PopLocation();
+						}
+					}
+				}
 
 			if ( prototype->deprecated )
 				t->Warn("use of deprecated prototype", id);
 			}
 		else
 			{
-			auto decl = id->Type()->AsFuncType();
-
 			// Allow renaming arguments, but only for the canonical
 			// prototypes of hooks/events.
 			if ( canonical_arg_types_match(decl, t.get()) )

testing/btest/Baseline/language.ineffective-default-args/out

@@ -0,0 +1,8 @@
+warning in /home/jon/pro/zeek/zeek/testing/btest/.tmp/language.ineffective-default-args/ineffective-default-args.zeek, line 9: &default on parameter 'b' has no effect (not a event declaration)
+warning in /home/jon/pro/zeek/zeek/testing/btest/.tmp/language.ineffective-default-args/ineffective-default-args.zeek, line 19: &default on parameter 'c' has no effect (not a event declaration)
+bar, A, B
+baz, A, B
+qux, Q, Q
+grault, A, B
+corge, C
+foo c, C

testing/btest/language/ineffective-default-args.zeek

@@ -0,0 +1,54 @@
+# @TEST-EXEC: zeek -b %INPUT >out 2>&1
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff out
+
+global grault: event(a: string, b: string &default="B");
+
+global foo: event(a: string, b: string, c: string &default="C");
+global foo: event(c: string);
+
+event grault(a: string, b: string &default="G")
+	{
+	print "grault", a, b;
+	}
+
+event corge(c: string &default="C")
+	{
+	print "corge", c;
+	}
+
+event foo(c: string &default="CCCC")
+	{
+	print "foo c", c;
+	}
+
+global bar: function(a: string, b: string);
+
+function bar(a: string &default="A", b: string &default="B")
+	{
+	print "bar", a, b;
+	}
+
+global baz: function(a: string &default="A", b: string &default="B");
+
+function baz(a: string, b: string)
+	{
+	print "baz", a, b;
+	}
+
+global qux: function(a: string &default="A", b: string &default="B");
+
+function qux(a: string &default="Q", b: string &default="Q")
+	{
+	print "qux", a, b;
+	}
+
+event zeek_init()
+	{
+	bar();
+	baz();
+	qux();
+	event grault("A");
+	event corge();
+	event foo("A", "B");
+	}
+