cmd/compile: Cannot return a defined generic type as a generic type

[godcong]

What version of Go are you using (go version)?

$ go version go1.19.1 windows/amd64

Does this issue reproduce with the latest release?

yes

What did you do?

type Command struct {
	Path string   
	Env  []string
	Cmd  string 
	Args []string
}

type Rename struct {
	Path   string 
	Regexp string 
	Target string 
}

//...

type Ruler interface {
	Command | Replace | Remove | Rename | Copy | Make
}

type Action struct {
        Type      Type  //Enumeration values bound to Ruler types
	Commands []Command 
	Replaces []Replace 
	Removes  []Remove 
	Renames  []Rename 
	Copies   []Copy   
	Makes    []Make   
}


func getRulers[R Ruler](action Action) []R {
	switch action.Type {
	case TypeCommand:
		return action.Commands
	case TypeReplace:
		return action.Replaces
	case TypeRemove:
		return action.Removes
	case TypeRename:
		return action.Renames
	case TypeCopy:
		return action.Copies
	case TypeMake:
		return action.Makes
	}
	return nil
}

What did you expect to see?

If Ruler is the Command type, then the []R I return is []Command
But the compiler does not recognize this

.\action.go:48:10: cannot use action.Commands (variable of type []Command) as type []R in return statement

Now I can only force the type conversion to R:

func castRuleCommand[R Ruler](r []Command) []R {
	return *(*[]R)(unsafe.Pointer(&r))
}

What did you see instead?

The code can be compiled

[seankhliao]

This is working as intended, in your example if you R is instantiated to Command, then all your other return statements are invalid.

See also #45380