lang: ast: Clear env in ExprVar.Graph()

Fix the bug in ExprVar.Graph() described in the previous commit.

lang/ast/structs.go

@@ -8460,9 +8460,29 @@ func (obj *ExprVar) Unify() ([]interfaces.Invariant, error) {
 // to avoid duplicating production of the incoming input value from the bound
 // expression.
 func (obj *ExprVar) Graph(env map[string]interfaces.Func) (*pgraph.Graph, interfaces.Func, error) {
-	// Delegate to the target.
-	target := obj.scope.Variables[obj.Name]
-	graph, varFunc, err := target.Graph(env)
+	// Delegate to the targetExpr.
+	targetExpr := obj.scope.Variables[obj.Name]
+	if _, isParam := targetExpr.(*ExprParam); isParam {
+		// The variable points to a function parameter. We should be able to find
+		// this parameter in the environment.
+		targetFunc, exists := env[obj.Name]
+		if !exists {
+			return nil, nil, fmt.Errorf("param `%s` is not in the environment", obj.Name)
+		}
+
+		graph, err := pgraph.NewGraph("ExprParam")
+		if err != nil {
+			return nil, nil, err
+		}
+		graph.AddVertex(targetFunc)
+		return graph, targetFunc, nil
+	}
+
+	// The variable points to a top-level expression. The parameters which are
+	// visible at this use site must not be visible at the definition site, so
+	// we pass an empty environment.
+	emptyEnv := map[string]interfaces.Func{}
+	graph, varFunc, err := targetExpr.Graph(emptyEnv)
 	return graph, varFunc, err
 }
 
@@ -8620,21 +8640,7 @@ func (obj *ExprParam) Unify() ([]interfaces.Invariant, error) {
 // that fulfill the Stmt interface do not produces vertices, where as their
 // children might.
 func (obj *ExprParam) Graph(env map[string]interfaces.Func) (*pgraph.Graph, interfaces.Func, error) {
-	// Since ExprParam represents a function parameter, we want to receive values
-	// from the arguments passed to the function. The caller of ExprParam.Graph()
-	// should already know what those arguments are, so we can simply look up the
-	// argument by name in the environment.
-	paramFunc, exists := env[obj.Name]
-	if !exists {
-		return nil, nil, fmt.Errorf("param `%s` is not in the environment", obj.Name)
-	}
-
-	graph, err := pgraph.NewGraph("ExprParam")
-	if err != nil {
-		return nil, nil, err
-	}
-	graph.AddVertex(paramFunc)
-	return graph, paramFunc, nil
+	panic("ExprParam.Graph(): should not happen, ExprVar.Graph() should handle the case where the ExprVar points to an ExprParam")
 }
 
 // SetValue here is a no-op, because algorithmically when this is called from