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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 types2
import (
// Instantiate instantiates the type typ with the given type arguments targs.
// typ must be a *Named or a *Signature type, and its number of type parameters
// must match the number of provided type arguments. The result is a new,
// instantiated (not parameterized) type of the same kind (either a *Named or a
// *Signature). Any methods attached to a *Named are simply copied; they are
// not instantiated.
// If env is non-nil, it may be used to de-dupe the instance against previous
// instances with the same identity. This functionality is implemented for
// environments with non-nil Checkers.
// If verify is set and constraint satisfaction fails, the returned error may
// be of dynamic type ArgumentError indicating which type argument did not
// satisfy its corresponding type parameter constraint, and why.
// TODO(rfindley): change this function to also return an error if lengths of
// tparams and targs do not match.
func Instantiate(env *Environment, typ Type, targs []Type, validate bool) (Type, error) {
inst := (*Checker)(nil).instance(nopos, typ, targs, env)
var err error
if validate {
var tparams []*TypeParam
switch t := typ.(type) {
case *Named:
tparams = t.TypeParams().list()
case *Signature:
tparams = t.TypeParams().list()
if i, err := (*Checker)(nil).verify(nopos, tparams, targs); err != nil {
return inst, ArgumentError{i, err}
return inst, err
// instantiate creates an instance and defers verification of constraints to
// later in the type checking pass. For Named types the resulting instance will
// be unexpanded.
func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, posList []syntax.Pos) (res Type) {
assert(check != nil)
if check.conf.Trace {
check.trace(pos, "-- instantiating %s with %s", typ, NewTypeList(targs))
defer func() {
var under Type
if res != nil {
// Calling under() here may lead to endless instantiations.
// Test case: type T[P any] T[P]
// TODO(gri) investigate if that's a bug or to be expected.
under = safeUnderlying(res)
check.trace(pos, "=> %s (under = %s)", res, under)
inst := check.instance(pos, typ, targs, check.conf.Environment)
assert(len(posList) <= len(targs))
check.later(func() {
// Collect tparams again because lazily loaded *Named types may not have
// had tparams set up above.
var tparams []*TypeParam
switch t := typ.(type) {
case *Named:
tparams = t.TypeParams().list()
case *Signature:
tparams = t.TypeParams().list()
// Avoid duplicate errors; instantiate will have complained if tparams
// and targs do not have the same length.
if len(tparams) == len(targs) {
if i, err := check.verify(pos, tparams, targs); err != nil {
// best position for error reporting
pos := pos
if i < len(posList) {
pos = posList[i]
check.softErrorf(pos, err.Error())
return inst
// 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 syntax.Pos, typ Type, targs []Type, env *Environment) Type {
switch t := typ.(type) {
case *Named:
var h string
if env != nil {
h = env.TypeHash(t, targs)
// typ may already have been instantiated with identical type arguments. In
// that case, re-use the existing instance.
if named := env.typeForHash(h, nil); named != nil {
return named
tname := NewTypeName(pos, t.obj.pkg,, nil)
named := check.newNamed(tname, t, nil, nil, nil) // methods and tparams are set when named is resolved
named.targs = NewTypeList(targs)
named.resolver = func(env *Environment, n *Named) (*TypeParamList, Type, []*Func) {
return expandNamed(env, n, pos)
if env != nil {
// It's possible that we've lost a race to add named to the environment.
// In this case, use whichever instance is recorded in the environment.
named = env.typeForHash(h, named)
return named
case *Signature:
tparams := t.TypeParams()
if !check.validateTArgLen(pos, tparams.Len(), len(targs)) {
return Typ[Invalid]
if tparams.Len() == 0 {
return typ // nothing to do (minor optimization)
sig := check.subst(pos, typ, makeSubstMap(tparams.list(), targs), env).(*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 == t {
copy := *sig
sig = &copy
// After instantiating a generic signature, it is not generic
// anymore; we need to set tparams to nil.
sig.tparams = nil
return sig
// only types and functions can be generic
panic(fmt.Sprintf("%v: cannot instantiate %v", pos, typ))
// 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 syntax.Pos, ntparams, ntargs int) bool {
if ntargs != ntparams {
// TODO(gri) provide better error message
if check != nil {
check.errorf(pos, "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 syntax.Pos, tparams []*TypeParam, targs []Type) (int, error) {
smap := makeSubstMap(tparams, targs)
for i, tpar := range tparams {
// stop checking bounds after the first failure
if err := check.satisfies(pos, targs[i], tpar, smap); err != nil {
return i, err
return -1, nil
// satisfies reports whether the type argument targ satisfies the constraint of type parameter
// parameter tpar (after any of its type parameters have been substituted through smap).
// A suitable error is reported if the result is false.
// TODO(gri) This should be a method of interfaces or type sets.
func (check *Checker) satisfies(pos syntax.Pos, targ Type, tpar *TypeParam, smap substMap) error {
iface := tpar.iface()
if iface.Empty() {
return nil // no type bound
// TODO(rfindley): it would be great if users could pass in a qualifier here,
// rather than falling back to verbose qualification. Maybe this can be part
// of a the shared environment.
var qf Qualifier
if check != nil {
qf = check.qualifier
errorf := func(format string, args ...interface{}) error {
return errors.New(sprintf(qf, format, args...))
// 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 instantiate
// the parameterized type.
iface = check.subst(pos, iface, smap, nil).(*Interface)
// if iface is comparable, targ must be comparable
// TODO(gri) the error messages needs to be better, here
if iface.IsComparable() && !Comparable(targ) {
if tpar := asTypeParam(targ); tpar != nil && tpar.iface().typeSet().IsAll() {
return errorf("%s has no constraints", targ)
return errorf("%s does not satisfy comparable", targ)
// targ must implement iface (methods)
// - check only if we have methods
if iface.NumMethods() > 0 {
// If the type argument is a pointer to a type parameter, the type argument's
// method set is empty.
// TODO(gri) is this what we want? (spec question)
if base, isPtr := deref(targ); isPtr && asTypeParam(base) != nil {
return errorf("%s has no methods", targ)
if m, wrong := check.missingMethod(targ, iface, true); m != nil {
// TODO(gri) needs to print updated name to avoid major confusion in error message!
// (print warning for now)
// Old warning:
// check.softErrorf(pos, "%s does not satisfy %s (warning: name not updated) = %s (missing method %s)", targ, tpar.bound, iface, m)
if wrong != nil {
// TODO(gri) This can still report uninstantiated types which makes the error message
// more difficult to read then necessary.
return errorf("%s does not satisfy %s: wrong method signature\n\tgot %s\n\twant %s",
targ, tpar.bound, wrong, m,
return errorf("%s does not satisfy %s (missing method %s)", targ, tpar.bound,
// targ's underlying type must also be one of the interface types listed, if any
if !iface.typeSet().hasTerms() {
return nil // nothing to do
// If targ is itself a type parameter, each of its possible types, but at least one, must be in the
// list of iface types (i.e., the targ type list must be a non-empty subset of the iface types).
if targ := asTypeParam(targ); targ != nil {
targBound := targ.iface()
if !targBound.typeSet().hasTerms() {
return errorf("%s does not satisfy %s (%s has no type constraints)", targ, tpar.bound, targ)
if !targBound.typeSet().subsetOf(iface.typeSet()) {
// TODO(gri) need better error message
return errorf("%s does not satisfy %s", targ, tpar.bound)
return nil
// Otherwise, targ's type or underlying type must also be one of the interface types listed, if any.
if !iface.typeSet().includes(targ) {
// TODO(gri) better error message
return errorf("%s does not satisfy %s", targ, tpar.bound)
return nil