diff: add TestBug using structs from real code

diff_test.go

@@ -372,6 +372,54 @@ func TestInfLoop(t *testing.T) {
 	diff.Test(t, t.Errorf, a, b, equal)
 }
 
+func TestBug(t *testing.T) {
+	type Mode string
+
+	type Usage struct {
+		ProviderID string `json:"-"`
+	}
+
+	type Tier struct {
+		Upto  int `json:"upto,omitempty"`
+		Price int `json:"price,omitempty"`
+		Base  int `json:"base,omitempty"`
+	}
+
+	type Feature struct {
+		ProviderID string `json:"-"`
+
+		Title string `json:"title,omitempty"`
+
+		Mode      Mode   `json:"type,omitempty"`
+		Reset     string `json:"reset,omitempty"`
+		Aggregate string `json:"-,omitempty"`
+		Tiers     []Tier `json:"tiers,omitempty"`
+
+		// Set in LookupSchedule and later reporters of usage
+		Usage Usage `json:"-"`
+	}
+
+	type Features map[string]Feature
+
+	type Schedule struct {
+		Features Features
+	}
+
+	got := Schedule{
+		Features: Features{
+			"foo": Feature{},
+		},
+	}
+
+	want := Schedule{
+		Features: Features{
+			"foo": Feature{},
+		},
+	}
+
+	diff.Test(t, t.Errorf, got, want)
+}
+
 func testUnequal(t *testing.T, a, b any) {
 	t.Helper()
 	equal := true