proposal: go/types: add HasTypeName interface

[adonovan]

During the types.Alias work, there was a recurring need (7 times in x/tools; @findleyr is adding an eighth) for this interface:

package types // import "go/types"

// HasTypeName abstracts the three kinds of types that have declared names:
// aliases ([*Alias]), defined types ([*Named]), and type parameters ([*TypeParam]).
//
// Note that the Go spec considers built-in types such as string and int to
// be defined types, but this package represents them as [*Basic],
// since they do not have a declaration or [TypeName].
type HasTypeName interface {
    Obj() *TypeName
}

Of course users can easily define it for themselves, but adding it to go/types provides a good place to hang additional documentation.

Related:

• go/types: document that Underlying affects Named, Alias, TypeParam #65774

@findleyr @griesemer

[gopherbot]

Change https://go.dev/cl/604476 mentions this issue: go/types: clarify Named, Alias, TypeName, Object

[griesemer]

I note that we have an unexported function

// hasName reports whether t has a name. This includes
// predeclared types, defined types, and type parameters.
// hasName may be called with types that are not fully set up.
func hasName(t Type) bool {
	switch Unalias(t).(type) {
	case *Basic, *Named, *TypeParam:
		return true
	}
	return false
}

Per the spec: "Predeclared types, defined types, and type parameters are called named types."

Maybe the interface should just be called "HasName". Having theObj method is not quite the same as being a named type (aliases have Obj but may denote type literals - one of the mistakes we made when introducing aliases).

[adonovan]

In the branch where I was playing with this I ended up using the name TypeWithName to make clear that the interface is a restriction of Type; otherwise it's a bit unclear what kind of thing it is.

[findleyr]

My 2 cents: types.HasName seems better than types.TypeWithName

[griesemer]

In the spec, a named type has a very specific meaning; specifically, an alias type may not be a named type. This interface is about whether a type has an Obj method which returns a TypeName. I think HasTypeName or HasObj seem ok names.