clause.OnConflict is silently ignored

[selaux]

Reproduction Link

Unfortunately I cannot provide a playground link as I am on an ARM system, but here is a link to a reproduction repo.

https://github.com/selaux/gorm-sql-server-upsert-issue

Description

If I use the following code with SQL Server I expect the selected row to be updated when I call this code a second time (this is a short excerpt of the code, for the full code see the example repo above).

type TestModel struct {
	gorm.Model
	Matcher string `gorm:"uniqueIndex;size:512"`
	Data    string
}

result := db.Clauses(clause.OnConflict{
	Columns:   []clause.Column{{Name: "matcher"}},
	DoUpdates: clause.Assignments(map[string]interface{}{"age": 55}),
}).Create(&User{
	Name: "foobar",
	Age:  30,
})

Instead the following error is thown: mssql: Cannot insert duplicate key row in object 'dbo.test_models' with unique index 'idx_test_models_matcher'. The duplicate key value is (foobar).

I am aware that SQL Server does not support the ON CONFLICT clause directly, but there are existing solutions to work around this, e.g. here or here. It would be nice for GORM to either:

• Map the clause.OnConflict clause to one of those implementations
• Return an error that explicitly states that clause.OnConflict is not supported for SQL Server and add this information to GORMs documentation

[jinzhu]

We should support this with merge into now.