1 files changed, 122 insertions, 0 deletions
diff --git a/src/cmd/vendor/golang.org/x/tools/internal/typeparams/coretype.go b/src/cmd/vendor/golang.org/x/tools/internal/typeparams/coretype.go
new file mode 100644
index 0000000000..993135ec90
--- /dev/null
+++ b/src/cmd/vendor/golang.org/x/tools/internal/typeparams/coretype.go
@@ -0,0 +1,122 @@
+// 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 (
+	"go/types"
+)
+
+// CoreType returns the core type of T or nil if T does not have a core type.
+//
+// See https://go.dev/ref/spec#Core_types 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()) {
+			break
+		}
+	}
+
+	if identical == len(terms) {
+		// https://go.dev/ref/spec#Core_types
+		// "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.
+	}
+	// https://go.dev/ref/spec#Core_types
+	// "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) ([]*Term, error) {
+	switch typ := typ.(type) {
+	case *TypeParam:
+		return StructuralTerms(typ)
+	case *Union:
+		return UnionTermSet(typ)
+	case *types.Interface:
+		return InterfaceTermSet(typ)
+	default:
+		return []*Term{NewTerm(false, typ)}, nil
+	}
+}