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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.
// This file implements instantiation of generic types
// through substitution of type parameters by type arguments.
package types
import (
"errors"
"fmt"
"go/token"
)
// Instantiate instantiates the type orig with the given type arguments targs.
// orig must be a *Named or a *Signature type. If there is no error, the
// resulting Type is an instantiated type of the same kind (either a *Named or
// a *Signature). Methods attached to a *Named type are also instantiated, and
// associated with a new *Func that has the same position as the original
// method, but nil function scope.
//
// If ctxt is non-nil, it may be used to de-duplicate the instance against
// previous instances with the same identity. As a special case, generic
// *Signature origin types are only considered identical if they are pointer
// equivalent, so that instantiating distinct (but possibly identical)
// signatures will yield different instances.
//
// If validate is set, Instantiate verifies that the number of type arguments
// and parameters match, and that the type arguments satisfy their
// corresponding type constraints. If verification fails, the resulting error
// may wrap an *ArgumentError indicating which type argument did not satisfy
// its corresponding type parameter constraint, and why.
//
// If validate is not set, Instantiate does not verify the type argument count
// or whether the type arguments satisfy their constraints. Instantiate is
// guaranteed to not return an error, but may panic. Specifically, for
// *Signature types, Instantiate will panic immediately if the type argument
// count is incorrect; for *Named types, a panic may occur later inside the
// *Named API.
func Instantiate(ctxt *Context, orig Type, targs []Type, validate bool) (Type, error) {
if validate {
var tparams []*TypeParam
switch t := orig.(type) {
case *Named:
tparams = t.TypeParams().list()
case *Signature:
tparams = t.TypeParams().list()
}
if len(targs) != len(tparams) {
return nil, fmt.Errorf("got %d type arguments but %s has %d type parameters", len(targs), orig, len(tparams))
}
if i, err := (*Checker)(nil).verify(token.NoPos, tparams, targs); err != nil {
return nil, &ArgumentError{i, err}
}
}
inst := (*Checker)(nil).instance(token.NoPos, orig, targs, ctxt)
return inst, nil
}
// instance creates a type or function instance using the given original type
// typ and arguments targs. For Named types the resulting instance will be
// unexpanded.
func (check *Checker) instance(pos token.Pos, orig Type, targs []Type, ctxt *Context) (res Type) {
var h string
if ctxt != nil {
h = ctxt.instanceHash(orig, targs)
// typ may already have been instantiated with identical type arguments. In
// that case, re-use the existing instance.
if inst := ctxt.lookup(h, orig, targs); inst != nil {
return inst
}
}
switch orig := orig.(type) {
case *Named:
tname := NewTypeName(pos, orig.obj.pkg, orig.obj.name, nil)
named := check.newNamed(tname, orig, nil, nil, nil) // underlying, tparams, and methods are set when named is resolved
named.targs = newTypeList(targs)
named.resolver = func(ctxt *Context, n *Named) (*TypeParamList, Type, *methodList) {
return expandNamed(ctxt, n, pos)
}
res = named
case *Signature:
tparams := orig.TypeParams()
if !check.validateTArgLen(pos, tparams.Len(), len(targs)) {
return Typ[Invalid]
}
if tparams.Len() == 0 {
return orig // nothing to do (minor optimization)
}
sig := check.subst(pos, orig, makeSubstMap(tparams.list(), targs), ctxt).(*Signature)
// If the signature doesn't use its type parameters, subst
// will not make a copy. In that case, make a copy now (so
// we can set tparams to nil w/o causing side-effects).
if sig == orig {
copy := *sig
sig = &copy
}
// After instantiating a generic signature, it is not generic
// anymore; we need to set tparams to nil.
sig.tparams = nil
res = sig
default:
// only types and functions can be generic
panic(fmt.Sprintf("%v: cannot instantiate %v", pos, orig))
}
if ctxt != nil {
// It's possible that we've lost a race to add named to the context.
// In this case, use whichever instance is recorded in the context.
res = ctxt.update(h, orig, targs, res)
}
return res
}
// validateTArgLen verifies that the length of targs and tparams matches,
// reporting an error if not. If validation fails and check is nil,
// validateTArgLen panics.
func (check *Checker) validateTArgLen(pos token.Pos, ntparams, ntargs int) bool {
if ntargs != ntparams {
// TODO(gri) provide better error message
if check != nil {
check.errorf(atPos(pos), _WrongTypeArgCount, "got %d arguments but %d type parameters", ntargs, ntparams)
return false
}
panic(fmt.Sprintf("%v: got %d arguments but %d type parameters", pos, ntargs, ntparams))
}
return true
}
func (check *Checker) verify(pos token.Pos, tparams []*TypeParam, targs []Type) (int, error) {
smap := makeSubstMap(tparams, targs)
for i, tpar := range tparams {
// Ensure that we have a (possibly implicit) interface as type bound (issue #51048).
tpar.iface()
// The type parameter bound is parameterized with the same type parameters
// as the instantiated type; before we can use it for bounds checking we
// need to instantiate it with the type arguments with which we instantiated
// the parameterized type.
bound := check.subst(pos, tpar.bound, smap, nil)
if err := check.implements(targs[i], bound); err != nil {
return i, err
}
}
return -1, nil
}
// implements checks if V implements T and reports an error if it doesn't.
// The receiver may be nil if implements is called through an exported
// API call such as AssignableTo.
func (check *Checker) implements(V, T Type) error {
Vu := under(V)
Tu := under(T)
if Vu == Typ[Invalid] || Tu == Typ[Invalid] {
return nil // avoid follow-on errors
}
if p, _ := Vu.(*Pointer); p != nil && under(p.base) == Typ[Invalid] {
return nil // avoid follow-on errors (see issue #49541 for an example)
}
errorf := func(format string, args ...any) error {
return errors.New(check.sprintf(format, args...))
}
Ti, _ := Tu.(*Interface)
if Ti == nil {
var cause string
if isInterfacePtr(Tu) {
cause = check.sprintf("type %s is pointer to interface, not interface", T)
} else {
cause = check.sprintf("%s is not an interface", T)
}
return errorf("%s does not implement %s (%s)", V, T, cause)
}
// Every type satisfies the empty interface.
if Ti.Empty() {
return nil
}
// T is not the empty interface (i.e., the type set of T is restricted)
// An interface V with an empty type set satisfies any interface.
// (The empty set is a subset of any set.)
Vi, _ := Vu.(*Interface)
if Vi != nil && Vi.typeSet().IsEmpty() {
return nil
}
// type set of V is not empty
// No type with non-empty type set satisfies the empty type set.
if Ti.typeSet().IsEmpty() {
return errorf("cannot implement %s (empty type set)", T)
}
// V must implement T's methods, if any.
if m, wrong := check.missingMethod(V, Ti, true); m != nil /* !Implements(V, Ti) */ {
return errorf("%s does not implement %s %s", V, T, check.missingMethodReason(V, T, m, wrong))
}
// If T is comparable, V must be comparable.
// Remember as a pending error and report only if we don't have a more specific error.
var pending error
if Ti.IsComparable() && !comparable(V, false, nil, nil) {
pending = errorf("%s does not implement comparable", V)
}
// V must also be in the set of types of T, if any.
// Constraints with empty type sets were already excluded above.
if !Ti.typeSet().hasTerms() {
return pending // nothing to do
}
// If V is itself an interface, each of its possible types must be in the set
// of T types (i.e., the V type set must be a subset of the T type set).
// Interfaces V with empty type sets were already excluded above.
if Vi != nil {
if !Vi.typeSet().subsetOf(Ti.typeSet()) {
// TODO(gri) report which type is missing
return errorf("%s does not implement %s", V, T)
}
return pending
}
// Otherwise, V's type must be included in the iface type set.
var alt Type
if Ti.typeSet().is(func(t *term) bool {
if !t.includes(V) {
// If V ∉ t.typ but V ∈ ~t.typ then remember this type
// so we can suggest it as an alternative in the error
// message.
if alt == nil && !t.tilde && Identical(t.typ, under(t.typ)) {
tt := *t
tt.tilde = true
if tt.includes(V) {
alt = t.typ
}
}
return true
}
return false
}) {
if alt != nil {
return errorf("%s does not implement %s (possibly missing ~ for %s in constraint %s)", V, T, alt, T)
} else {
return errorf("%s does not implement %s", V, T)
}
}
return pending
}