database/sql: rewrite Null[T].Value method, update doc for Null[T]

Update doc for Null[T] to clarify that T should be one of the types
accepted by driver.Value.

Modify the Value() method of Null[T]:
1) recognize T implementing driver.Valuer interface and invoke it.
2) use the DefaultParameterConverter to convert native types that
are not directly supported as driver.Value types.

Fixes #69728
Fixes #69837

Change-Id: Iba782c878b2bde168125f5390abf319b88424149
GitHub-Last-Rev: 3df182d23d
GitHub-Pull-Request: golang/go#69938
Reviewed-on: https://go-review.googlesource.com/c/go/+/620858
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/database/sql/sql.go

@@ -410,6 +410,8 @@ func (n NullTime) Value() (driver.Value, error) {
 //	} else {
 //	   // NULL value
 //	}
+//
+// T should be one of the types accepted by [driver.Value].
 type Null[T any] struct {
 	V     T
 	Valid bool
@@ -428,7 +430,17 @@ func (n Null[T]) Value() (driver.Value, error) {
 	if !n.Valid {
 		return nil, nil
 	}
-	return n.V, nil
+	v := any(n.V)
+	// See issue 69728.
+	if valuer, ok := v.(driver.Valuer); ok {
+		val, err := callValuerValue(valuer)
+		if err != nil {
+			return val, err
+		}
+		v = val
+	}
+	// See issue 69837.
+	return driver.DefaultParameterConverter.ConvertValue(v)
 }
 
 // Scanner is an interface used by [Rows.Scan].

src/database/sql/sql_test.go

@@ -4957,3 +4957,50 @@ func BenchmarkConnRequestSet(b *testing.B) {
 		}
 	}
 }
+
+func TestIssue69837(t *testing.T) {
+	u := Null[uint]{V: 1, Valid: true}
+	val, err := driver.DefaultParameterConverter.ConvertValue(u)
+	if err != nil {
+		t.Errorf("ConvertValue() error = %v, want nil", err)
+	}
+
+	if v, ok := val.(int64); !ok {
+		t.Errorf("val.(type): got %T, expected int64", val)
+	} else if v != 1 {
+		t.Errorf("val: got %d, expected 1", v)
+	}
+}
+
+type issue69728Type struct {
+	ID   int
+	Name string
+}
+
+func (t issue69728Type) Value() (driver.Value, error) {
+	return []byte(fmt.Sprintf("%d, %s", t.ID, t.Name)), nil
+}
+
+func TestIssue69728(t *testing.T) {
+	forValue := Null[issue69728Type]{
+		Valid: true,
+		V: issue69728Type{
+			ID:   42,
+			Name: "foobar",
+		},
+	}
+
+	v1, err := forValue.Value()
+	if err != nil {
+		t.Errorf("forValue.Value() error = %v, want nil", err)
+	}
+
+	v2, err := forValue.V.Value()
+	if err != nil {
+		t.Errorf("forValue.V.Value() error = %v, want nil", err)
+	}
+
+	if !reflect.DeepEqual(v1, v2) {
+		t.Errorf("not equal; v1 = %v, v2 = %v", v1, v2)
+	}
+}