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// Copyright 2022 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package typeparams
import (
// CoreType returns the core type of T or nil if T does not have a core type.
// See for the definition of a core type.
func CoreType(T types.Type) types.Type {
U := T.Underlying()
if _, ok := U.(*types.Interface); !ok {
return U // for non-interface types,
terms, err := _NormalTerms(U)
if len(terms) == 0 || err != nil {
// len(terms) -> empty type set of interface.
// err != nil => U is invalid, exceeds complexity bounds, or has an empty type set.
return nil // no core type.
U = terms[0].Type().Underlying()
var identical int // i in [0,identical) => Identical(U, terms[i].Type().Underlying())
for identical = 1; identical < len(terms); identical++ {
if !types.Identical(U, terms[identical].Type().Underlying()) {
if identical == len(terms) {
// "There is a single type U which is the underlying type of all types in the type set of T"
return U
ch, ok := U.(*types.Chan)
if !ok {
return nil // no core type as identical < len(terms) and U is not a channel.
// "the type chan E if T contains only bidirectional channels, or the type chan<- E or
// <-chan E depending on the direction of the directional channels present."
for chans := identical; chans < len(terms); chans++ {
curr, ok := terms[chans].Type().Underlying().(*types.Chan)
if !ok {
return nil
if !types.Identical(ch.Elem(), curr.Elem()) {
return nil // channel elements are not identical.
if ch.Dir() == types.SendRecv {
// ch is bidirectional. We can safely always use curr's direction.
ch = curr
} else if curr.Dir() != types.SendRecv && ch.Dir() != curr.Dir() {
// ch and curr are not bidirectional and not the same direction.
return nil
return ch
// _NormalTerms returns a slice of terms representing the normalized structural
// type restrictions of a type, if any.
// For all types other than *types.TypeParam, *types.Interface, and
// *types.Union, this is just a single term with Tilde() == false and
// Type() == typ. For *types.TypeParam, *types.Interface, and *types.Union, see
// below.
// Structural type restrictions of a type parameter are created via
// non-interface types embedded in its constraint interface (directly, or via a
// chain of interface embeddings). For example, in the declaration type
// T[P interface{~int; m()}] int the structural restriction of the type
// parameter P is ~int.
// With interface embedding and unions, the specification of structural type
// restrictions may be arbitrarily complex. For example, consider the
// following:
// type A interface{ ~string|~[]byte }
// type B interface{ int|string }
// type C interface { ~string|~int }
// type T[P interface{ A|B; C }] int
// In this example, the structural type restriction of P is ~string|int: A|B
// expands to ~string|~[]byte|int|string, which reduces to ~string|~[]byte|int,
// which when intersected with C (~string|~int) yields ~string|int.
// _NormalTerms computes these expansions and reductions, producing a
// "normalized" form of the embeddings. A structural restriction is normalized
// if it is a single union containing no interface terms, and is minimal in the
// sense that removing any term changes the set of types satisfying the
// constraint. It is left as a proof for the reader that, modulo sorting, there
// is exactly one such normalized form.
// Because the minimal representation always takes this form, _NormalTerms
// returns a slice of tilde terms corresponding to the terms of the union in
// the normalized structural restriction. An error is returned if the type is
// invalid, exceeds complexity bounds, or has an empty type set. In the latter
// case, _NormalTerms returns ErrEmptyTypeSet.
// _NormalTerms makes no guarantees about the order of terms, except that it
// is deterministic.
func _NormalTerms(typ types.Type) ([]*types.Term, error) {
switch typ := typ.(type) {
case *types.TypeParam:
return StructuralTerms(typ)
case *types.Union:
return UnionTermSet(typ)
case *types.Interface:
return InterfaceTermSet(typ)
return []*types.Term{types.NewTerm(false, typ)}, nil