fix: Context.Set no such flag

context.go

@@ -3,6 +3,7 @@ package cli
 import (
 	"context"
 	"flag"
+	"fmt"
 	"strings"
 )
 
@@ -46,10 +47,11 @@ func (cCtx *Context) NumFlags() int {
 
 // Set sets a context flag to a value.
 func (cCtx *Context) Set(name, value string) error {
-	if cCtx.flagSet.Lookup(name) == nil {
-		cCtx.onInvalidFlag(name)
+	if fs := cCtx.lookupFlagSet(name); fs != nil {
+		return fs.Set(name, value)
 	}
-	return cCtx.flagSet.Set(name, value)
+
+	return fmt.Errorf("no such flag -%s", name)
 }
 
 // IsSet determines if the flag was actually set

context_test.go

@@ -643,3 +643,19 @@ func TestCheckRequiredFlags(t *testing.T) {
 		})
 	}
 }
+
+func TestContext_ParentContext_Set(t *testing.T) {
+	parentSet := flag.NewFlagSet("parent", flag.ContinueOnError)
+	parentSet.String("Name", "", "")
+
+	context := NewContext(
+		nil,
+		flag.NewFlagSet("child", flag.ContinueOnError),
+		NewContext(nil, parentSet, nil),
+	)
+
+	err := context.Set("Name", "aaa")
+	if err != nil {
+		t.Errorf("expect nil. set parent context flag return err: %s", err)
+	}
+}