go/types: add Alias parameter methods

[griesemer]

For the purposes of generic Alias types we propose the following additional API.
As with other generic types, instances are created with types.Instantiate.
These methods match the corresponding methods for Named types.

package types

// TypeParams returns the type parameters of the alias type a, or nil.
// A generic Alias and its instances have the same type parameters.
func (a *Alias) TypeParams() *TypeParamList

// SetTypeParams sets the type parameters of the alias type a.
// a must not have type arguments.
func (a *Alias) SetTypeParams(tparams []*TypeParam)

// TypeArgs returns the type arguments used to instantiate the Alias type.
// If a is not an instance of a generic alias, the result is nil.
func (a *Alias) TypeArgs() *TypeList

// Origin returns the generic Alias type of which a is an instance.
// If a is not an instance of a generic alias, Origin returns a.
func (a *Alias) Origin() *Alias

[gopherbot]

Change https://go.dev/cl/583757 mentions this issue: go/types, types2: add Alias.{TypeParams, SetTypeParams, TypeArgs, Origin}

[rsc]

These are the same methods that types.Named already has, so this seems fine.

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

The proposal is #67143 (comment).

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group

The proposal is #67143 (comment).