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// Copyright 2021 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 types2
import "cmd/compile/internal/syntax"
// ----------------------------------------------------------------------------
// API
// A Union represents a union of terms embedded in an interface.
type Union struct {
terms []*Term // list of syntactical terms (not a canonicalized termlist)
tset *TypeSet // type set described by this union, computed lazily
}
// NewUnion returns a new Union type with the given terms.
// It is an error to create an empty union; they are syntactically not possible.
func NewUnion(terms []*Term) *Union {
if len(terms) == 0 {
panic("empty union")
}
return &Union{terms, nil}
}
func (u *Union) Len() int { return len(u.terms) }
func (u *Union) Term(i int) *Term { return u.terms[i] }
func (u *Union) Underlying() Type { return u }
func (u *Union) String() string { return TypeString(u, nil) }
// A Term represents a term in a Union.
type Term term
// NewTerm returns a new union term.
func NewTerm(tilde bool, typ Type) *Term { return &Term{tilde, typ} }
func (t *Term) Tilde() bool { return t.tilde }
func (t *Term) Type() Type { return t.typ }
func (t *Term) String() string { return (*term)(t).String() }
// ----------------------------------------------------------------------------
// Implementation
// Avoid excessive type-checking times due to quadratic termlist operations.
const maxTermCount = 100
// parseUnion parses the given list of type expressions tlist as a union of
// those expressions. The result is a Union type, or Typ[Invalid] for some
// errors.
func parseUnion(check *Checker, tlist []syntax.Expr) Type {
var terms []*Term
for _, x := range tlist {
tilde, typ := parseTilde(check, x)
if len(tlist) == 1 && !tilde {
return typ // single type (optimization)
}
if len(terms) >= maxTermCount {
check.errorf(x, "cannot handle more than %d union terms (implementation limitation)", maxTermCount)
return Typ[Invalid]
}
terms = append(terms, NewTerm(tilde, typ))
}
// Check validity of terms.
// Do this check later because it requires types to be set up.
// Note: This is a quadratic algorithm, but unions tend to be short.
check.later(func() {
for i, t := range terms {
typ := expand(t.typ)
if typ == Typ[Invalid] {
continue
}
x := tlist[i]
pos := syntax.StartPos(x)
// We may not know the position of x if it was a typechecker-
// introduced ~T term for a type list entry T. Use the position
// of T instead.
// TODO(gri) remove this test once we don't support type lists anymore
if !pos.IsKnown() {
if op, _ := x.(*syntax.Operation); op != nil {
pos = syntax.StartPos(op.X)
}
}
u := under(typ)
f, _ := u.(*Interface)
if t.tilde {
if f != nil {
check.errorf(x, "invalid use of ~ (%s is an interface)", typ)
continue // don't report another error for t
}
if !Identical(u, typ) {
check.errorf(x, "invalid use of ~ (underlying type of %s is %s)", typ, u)
continue // don't report another error for t
}
}
// Stand-alone embedded interfaces are ok and are handled by the single-type case
// in the beginning. Embedded interfaces with tilde are excluded above. If we reach
// here, we must have at least two terms in the union.
if f != nil && !f.typeSet().IsTypeSet() {
check.errorf(pos, "cannot use %s in union (interface contains methods)", t)
continue // don't report another error for t
}
// Report overlapping (non-disjoint) terms such as
// a|a, a|~a, ~a|~a, and ~a|A (where under(A) == a).
if j := overlappingTerm(terms[:i], t); j >= 0 {
check.softErrorf(pos, "overlapping terms %s and %s", t, terms[j])
}
}
})
return &Union{terms, nil}
}
func parseTilde(check *Checker, x syntax.Expr) (tilde bool, typ Type) {
if op, _ := x.(*syntax.Operation); op != nil && op.Op == syntax.Tilde {
x = op.X
tilde = true
}
typ = check.anyType(x)
// embedding stand-alone type parameters is not permitted (issue #47127).
if _, ok := under(typ).(*TypeParam); ok {
check.error(x, "cannot embed a type parameter")
typ = Typ[Invalid]
}
return
}
// overlappingTerm reports the index of the term x in terms which is
// overlapping (not disjoint) from y. The result is < 0 if there is no
// such term.
func overlappingTerm(terms []*Term, y *Term) int {
for i, x := range terms {
// disjoint requires non-nil, non-top arguments
if debug {
if x == nil || x.typ == nil || y == nil || y.typ == nil {
panic("empty or top union term")
}
}
if !(*term)(x).disjoint((*term)(y)) {
return i
}
}
return -1
}
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