cmd/compile: embedded field type cannot be a (pointer to a) type parameter

[Fullstop000]

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

go version devel go1.18-cf51fb5d68 Sun Oct 17 04:27:13 2021 +0000 darwin/amd64

In https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md, the following code should be ok to be compiled:

type Lockable[T any] struct {
	T
	mu sync.Mutex
}

// Get returns the value stored in a Lockable.
func (l *Lockable[T]) Get() T {
	l.mu.Lock()
	defer l.mu.Unlock()
	return l.T
}

// Set sets the value in a Lockable.
func (l *Lockable[T]) Set(v T) {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.T = v
}

What did you expect to see?

compile success

What did you see instead?

~/dev/go2playground ~/goroot/bin/go test
# go2playground [go2playground.test]
./main_test.go:50:2: embedded field type cannot be a (pointer to a) type parameter

[randall77]

The design doc says this should work: https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#embedded-type-parameter

But we checked in a CL to forbid it: https://go-review.googlesource.com/c/go/+/337353

@griesemer

@Fullstop000 You can always name the field, as was done here: https://go-review.googlesource.com/c/go/+/337353/5/test/typeparam/lockable.go

[griesemer]

We tentatively concluded that embedding type parameters may not be permitted initially because of questions re: method promotion. The same questions arise with a pointer to a type parameter. But perhaps we can allow this after all.

Will reconsider.

[griesemer]

We've talked about this again, and we're not going to change this for 1.18. Reasons:

• There are open questions with respect to what the type of such an embedded field would be. Consider also the case:

func f[P constraint]() {
   type S struct{
      P // <<< should this be permitted?
   }
   ...
}

• There are open questions with respect to method promotion.
• We can always add it later, but we won't be able to remove the feature if we make a mistake.
• The implementation is already restricted; making changes such as this shortly before the code freeze is risky.

The fact that the generics proposal still describes this possibility is simply an oversight. Eventually the spec (in progress) will be the final guiding document.

Closing. Once we have gained more experience with type parameters we can reconsider this as a proposal.

[griesemer]

cc: @ianlancetaylor

[szmcdull]

Sad it is not supported:

type (
	Number interface {
		constraints.Integer | constraints.Float
	}

	IntFloat[T Number] struct {
		// embedded field type cannot be a (pointer to a) type parameter
		// https://github.com/golang/go/issues/49030
		T
	}

	INumber interface {
		Add(other INumber) INumber
		Sub(other INumber) INumber
		Mul(other INumber) INumber
		Div(other INumber) INumber
		Comp(other INumber) int
		Eq(other INumber) bool
	}
)

func (me IntFloat[T]) Add(other IntFloat[T]) IntFloat[T] {
	return me.T + other.T
}