diff --git a/src/Expr.cc b/src/Expr.cc
index 997c1509e9..ae46abf213 100644
--- a/src/Expr.cc
+++ b/src/Expr.cc
@@ -2830,8 +2830,10 @@ ValPtr IndexSliceAssignExpr::Eval(Frame* f) const
 	return nullptr;
 	}
 
-IndexExpr::IndexExpr(ExprPtr arg_op1, ListExprPtr arg_op2, bool arg_is_slice)
-	: BinaryExpr(EXPR_INDEX, std::move(arg_op1), std::move(arg_op2)), is_slice(arg_is_slice)
+IndexExpr::IndexExpr(ExprPtr arg_op1, ListExprPtr arg_op2, bool arg_is_slice,
+                     bool arg_is_inside_when)
+	: BinaryExpr(EXPR_INDEX, std::move(arg_op1), std::move(arg_op2)), is_slice(arg_is_slice),
+	  is_inside_when(arg_is_inside_when)
 	{
 	if ( IsError() )
 		return;
@@ -4467,8 +4469,8 @@ ValPtr InExpr::Fold(Val* v1, Val* v2) const
 	return val_mgr->Bool(res);
 	}
 
-CallExpr::CallExpr(ExprPtr arg_func, ListExprPtr arg_args, bool in_hook)
-	: Expr(EXPR_CALL), func(std::move(arg_func)), args(std::move(arg_args))
+CallExpr::CallExpr(ExprPtr arg_func, ListExprPtr arg_args, bool in_hook, bool _in_when)
+	: Expr(EXPR_CALL), func(std::move(arg_func)), args(std::move(arg_args)), in_when(_in_when)
 	{
 	if ( func->IsError() || args->IsError() )
 		{
diff --git a/src/Expr.h b/src/Expr.h
index a8b90d42dc..3bb9633cb6 100644
--- a/src/Expr.h
+++ b/src/Expr.h
@@ -1005,7 +1005,7 @@ public:
 class IndexExpr : public BinaryExpr
 	{
 public:
-	IndexExpr(ExprPtr op1, ListExprPtr op2, bool is_slice = false);
+	IndexExpr(ExprPtr op1, ListExprPtr op2, bool is_slice = false, bool is_inside_when = false);
 
 	bool CanAdd() const override;
 	bool CanDel() const override;
@@ -1021,6 +1021,7 @@ public:
 	ValPtr Eval(Frame* f) const override;
 
 	bool IsSlice() const { return is_slice; }
+	bool IsInsideWhen() const { return is_inside_when; }
 
 	// Optimization-related:
 	ExprPtr Duplicate() override;
@@ -1034,6 +1035,7 @@ protected:
 	void ExprDescribe(ODesc* d) const override;
 
 	bool is_slice;
+	bool is_inside_when;
 	};
 
 // The following execute the heart of IndexExpr functionality for
@@ -1059,6 +1061,13 @@ extern VectorValPtr vector_bool_select(VectorTypePtr vt, const VectorVal* v1, co
 // indices to select).
 extern VectorValPtr vector_int_select(VectorTypePtr vt, const VectorVal* v1, const VectorVal* v2);
 
+// The following is used for index expressions that occur inside "when"
+// clauses.  It tracks all the results produced by evaluating indexing
+// aggregates, so that if any of them are Modifiable(), the associated
+// Trigger can register interest in changes to them.
+//
+// One Fine Day we should do the equivalent for accessing fields in records,
+// too.
 class IndexExprWhen final : public IndexExpr
 	{
 public:
@@ -1077,7 +1086,7 @@ public:
 		}
 
 	IndexExprWhen(ExprPtr op1, ListExprPtr op2, bool is_slice = false)
-		: IndexExpr(std::move(op1), std::move(op2), is_slice)
+		: IndexExpr(std::move(op1), std::move(op2), is_slice, true)
 		{
 		}
 
@@ -1398,13 +1407,14 @@ protected:
 class CallExpr final : public Expr
 	{
 public:
-	CallExpr(ExprPtr func, ListExprPtr args, bool in_hook = false);
+	CallExpr(ExprPtr func, ListExprPtr args, bool in_hook = false, bool in_when = false);
 
 	Expr* Func() const { return func.get(); }
 	ListExpr* Args() const { return args.get(); }
 	ListExprPtr ArgsPtr() const { return args; }
 
 	bool IsPure() const override;
+	bool IsInWhen() const { return in_when; }
 
 	ValPtr Eval(Frame* f) const override;
 
@@ -1424,6 +1434,7 @@ protected:
 
 	ExprPtr func;
 	ListExprPtr args;
+	bool in_when;
 	};
 
 /**
diff --git a/src/parse.y b/src/parse.y
index 39046348b1..d2bb078322 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -5,7 +5,7 @@
 // Switching parser table type fixes ambiguity problems.
 %define lr.type ielr
 
-%expect 196
+%expect 195
 
 %token TOK_ADD TOK_ADD_TO TOK_ADDR TOK_ANY
 %token TOK_ATENDIF TOK_ATELSE TOK_ATIF TOK_ATIFDEF TOK_ATIFNDEF
@@ -13,7 +13,8 @@
 %token TOK_CONSTANT TOK_COPY TOK_COUNT TOK_DEFAULT TOK_DELETE
 %token TOK_DOUBLE TOK_ELSE TOK_ENUM TOK_EVENT TOK_EXPORT TOK_FALLTHROUGH
 %token TOK_FILE TOK_FOR TOK_FUNCTION TOK_GLOBAL TOK_HOOK TOK_ID TOK_IF TOK_INT
-%token TOK_INTERVAL TOK_LIST TOK_LOCAL TOK_MODULE
+%token TOK_INTERVAL TOK_LIST TOK_MODULE
+%token TOK_LOCAL TOK_WHEN_LOCAL
 %token TOK_NEXT TOK_OF TOK_OPAQUE TOK_PATTERN TOK_PATTERN_END TOK_PATTERN_TEXT
 %token TOK_PORT TOK_PRINT TOK_RECORD TOK_REDEF
 %token TOK_REMOVE_FROM TOK_RETURN TOK_SCHEDULE TOK_SET
@@ -111,7 +112,7 @@ extern int conditional_epoch; // let's us track embedded conditionals
 // Whether the file we're currently parsing includes @if conditionals.
 extern bool current_file_has_conditionals;
 
-YYLTYPE GetCurrentLocation();
+extern YYLTYPE GetCurrentLocation();
 extern int yyerror(const char[]);
 extern int brolex();
 
@@ -133,7 +134,8 @@ extern Expr* g_curr_debug_expr;
 extern bool in_debug;
 extern const char* g_curr_debug_error;
 
-static int in_when_cond = 0;
+extern int in_when_cond;
+
 static int in_hook = 0;
 int in_init = 0;
 int in_record = 0;
@@ -618,7 +620,7 @@ expr:
 			$$ = get_assign_expr({AdoptRef{}, $1}, {AdoptRef{}, $3}, in_init).release();
 			}
 
-	|	TOK_LOCAL local_id '=' rhs
+	|	TOK_WHEN_LOCAL local_id '=' rhs
 			{
 			set_location(@2, @4);
 			if ( ! locals_at_this_scope.empty() )
@@ -780,7 +782,7 @@ expr:
 				}
 
 			else
-				$$ = new CallExpr({AdoptRef{}, $1}, {AdoptRef{}, $4}, in_hook > 0);
+				$$ = new CallExpr({AdoptRef{}, $1}, {AdoptRef{}, $4}, in_hook > 0, in_when_cond);
 			}
 
 	|	TOK_HOOK { ++in_hook; } expr
diff --git a/src/scan.l b/src/scan.l
index 52d2896692..cca6ddd0f5 100644
--- a/src/scan.l
+++ b/src/scan.l
@@ -68,6 +68,10 @@ std::unordered_set<std::string> files_with_conditionals;
 
 zeek::detail::int_list if_stack;
 
+// Whether we're parsing a "when" conditional, for which we treat
+// the "local" keyword differently.
+int in_when_cond = 0;
+
 int line_number = 1;
 const char* filename = 0; // Absolute path of file currently being parsed.
 const char* last_filename = 0; // Absolute path of last file parsed.
@@ -278,7 +282,7 @@ int	return TOK_INT;
 interval	return TOK_INTERVAL;
 is	return TOK_IS;
 list	return TOK_LIST;
-local	return TOK_LOCAL;
+local	return in_when_cond ? TOK_WHEN_LOCAL : TOK_LOCAL;
 module	return TOK_MODULE;
 next	return TOK_NEXT;
 of	return TOK_OF;
@@ -586,10 +590,11 @@ YYLTYPE zeek::detail::GetCurrentLocation()
 	return currloc;
 	}
 
-void zeek::detail::SetCurrentLocation(YYLTYPE currloc) {
+void zeek::detail::SetCurrentLocation(YYLTYPE currloc)
+	{
 	::filename = currloc.filename;
 	line_number = currloc.first_line;
-}
+	}
 
 static int load_files(const char* orig_file)
 	{
diff --git a/src/script_opt/Expr.cc b/src/script_opt/Expr.cc
index 483b93efe9..b76f4b16bd 100644
--- a/src/script_opt/Expr.cc
+++ b/src/script_opt/Expr.cc
@@ -1863,7 +1863,7 @@ ExprPtr IndexExpr::Duplicate()
 	{
 	auto op1_d = op1->Duplicate();
 	auto op2_l = op2->Duplicate()->AsListExprPtr();
-	return SetSucc(new IndexExpr(op1_d, op2_l, is_slice));
+	return SetSucc(new IndexExpr(op1_d, op2_l, is_slice, is_inside_when));
 	}
 
 bool IndexExpr::HasReducedOps(Reducer* c) const
diff --git a/src/script_opt/ProfileFunc.cc b/src/script_opt/ProfileFunc.cc
index 1b8c31b019..a6057eb4e3 100644
--- a/src/script_opt/ProfileFunc.cc
+++ b/src/script_opt/ProfileFunc.cc
@@ -129,7 +129,15 @@ TraversalCode ProfileFunc::PreStmt(const Stmt* s)
 			return TC_ABORTSTMT;
 
 		case STMT_WHEN:
+			{
 			++num_when_stmts;
+
+			auto w = s->AsWhenStmt();
+			auto wi = w->Info();
+			auto wl = wi ? wi->Lambda() : nullptr;
+			if ( wl )
+				lambdas.push_back(wl.get());
+			}
 			break;
 
 		case STMT_FOR: