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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 typeparams contains common utilities for writing tools that interact
// with generic Go code, as introduced with Go 1.18.
// Many of the types and functions in this package are proxies for the new APIs
// introduced in the standard library with Go 1.18. For example, the
// typeparams.Union type is an alias for go/types.Union, and the ForTypeSpec
// function returns the value of the go/ast.TypeSpec.TypeParams field. At Go
// versions older than 1.18 these helpers are implemented as stubs, allowing
// users of this package to write code that handles generic constructs inline,
// even if the Go version being used to compile does not support generics.
// Additionally, this package contains common utilities for working with the
// new generic constructs, to supplement the standard library APIs. Notably,
// the StructuralTerms API computes a minimal representation of the structural
// restrictions on a type parameter.
// An external version of these APIs is available in the
// module.
package typeparams
import (
// UnpackIndexExpr extracts data from AST nodes that represent index
// expressions.
// For an ast.IndexExpr, the resulting indices slice will contain exactly one
// index expression. For an ast.IndexListExpr (go1.18+), it may have a variable
// number of index expressions.
// For nodes that don't represent index expressions, the first return value of
// UnpackIndexExpr will be nil.
func UnpackIndexExpr(n ast.Node) (x ast.Expr, lbrack token.Pos, indices []ast.Expr, rbrack token.Pos) {
switch e := n.(type) {
case *ast.IndexExpr:
return e.X, e.Lbrack, []ast.Expr{e.Index}, e.Rbrack
case *ast.IndexListExpr:
return e.X, e.Lbrack, e.Indices, e.Rbrack
return nil, token.NoPos, nil, token.NoPos
// PackIndexExpr returns an *ast.IndexExpr or *ast.IndexListExpr, depending on
// the cardinality of indices. Calling PackIndexExpr with len(indices) == 0
// will panic.
func PackIndexExpr(x ast.Expr, lbrack token.Pos, indices []ast.Expr, rbrack token.Pos) ast.Expr {
switch len(indices) {
case 0:
panic("empty indices")
case 1:
return &ast.IndexExpr{
X: x,
Lbrack: lbrack,
Index: indices[0],
Rbrack: rbrack,
return &ast.IndexListExpr{
X: x,
Lbrack: lbrack,
Indices: indices,
Rbrack: rbrack,
// IsTypeParam reports whether t is a type parameter.
func IsTypeParam(t types.Type) bool {
_, ok := t.(*types.TypeParam)
return ok
// OriginMethod returns the origin method associated with the method fn.
// For methods on a non-generic receiver base type, this is just
// fn. However, for methods with a generic receiver, OriginMethod returns the
// corresponding method in the method set of the origin type.
// As a special case, if fn is not a method (has no receiver), OriginMethod
// returns fn.
func OriginMethod(fn *types.Func) *types.Func {
recv := fn.Type().(*types.Signature).Recv()
if recv == nil {
return fn
base := recv.Type()
p, isPtr := base.(*types.Pointer)
if isPtr {
base = p.Elem()
named, isNamed := base.(*types.Named)
if !isNamed {
// Receiver is a *types.Interface.
return fn
if named.TypeParams().Len() == 0 {
// Receiver base has no type parameters, so we can avoid the lookup below.
return fn
orig := named.Origin()
gfn, _, _ := types.LookupFieldOrMethod(orig, true, fn.Pkg(), fn.Name())
// This is a fix for a gopls crash (#60628) due to a go/types bug (#60634). In:
// package p
// type T *int
// func (*T) f() {}
// LookupFieldOrMethod(T, true, p, f)=nil, but NewMethodSet(*T)={(*T).f}.
// Here we make them consistent by force.
// (The go/types bug is general, but this workaround is reached only
// for generic T thanks to the early return above.)
if gfn == nil {
mset := types.NewMethodSet(types.NewPointer(orig))
for i := 0; i < mset.Len(); i++ {
m := mset.At(i)
if m.Obj().Id() == fn.Id() {
gfn = m.Obj()
// In golang/go#61196, we observe another crash, this time inexplicable.
if gfn == nil {
panic(fmt.Sprintf("missing origin method for %s.%s; named == origin: %t, named.NumMethods(): %d, origin.NumMethods(): %d", named, fn, named == orig, named.NumMethods(), orig.NumMethods()))
return gfn.(*types.Func)
// GenericAssignableTo is a generalization of types.AssignableTo that
// implements the following rule for uninstantiated generic types:
// If V and T are generic named types, then V is considered assignable to T if,
// for every possible instantation of V[A_1, ..., A_N], the instantiation
// T[A_1, ..., A_N] is valid and V[A_1, ..., A_N] implements T[A_1, ..., A_N].
// If T has structural constraints, they must be satisfied by V.
// For example, consider the following type declarations:
// type Interface[T any] interface {
// Accept(T)
// }
// type Container[T any] struct {
// Element T
// }
// func (c Container[T]) Accept(t T) { c.Element = t }
// In this case, GenericAssignableTo reports that instantiations of Container
// are assignable to the corresponding instantiation of Interface.
func GenericAssignableTo(ctxt *types.Context, V, T types.Type) bool {
// If V and T are not both named, or do not have matching non-empty type
// parameter lists, fall back on types.AssignableTo.
VN, Vnamed := V.(*types.Named)
TN, Tnamed := T.(*types.Named)
if !Vnamed || !Tnamed {
return types.AssignableTo(V, T)
vtparams := VN.TypeParams()
ttparams := TN.TypeParams()
if vtparams.Len() == 0 || vtparams.Len() != ttparams.Len() || VN.TypeArgs().Len() != 0 || TN.TypeArgs().Len() != 0 {
return types.AssignableTo(V, T)
// V and T have the same (non-zero) number of type params. Instantiate both
// with the type parameters of V. This must always succeed for V, and will
// succeed for T if and only if the type set of each type parameter of V is a
// subset of the type set of the corresponding type parameter of T, meaning
// that every instantiation of V corresponds to a valid instantiation of T.
// Minor optimization: ensure we share a context across the two
// instantiations below.
if ctxt == nil {
ctxt = types.NewContext()
var targs []types.Type
for i := 0; i < vtparams.Len(); i++ {
targs = append(targs, vtparams.At(i))
vinst, err := types.Instantiate(ctxt, V, targs, true)
if err != nil {
panic("type parameters should satisfy their own constraints")
tinst, err := types.Instantiate(ctxt, T, targs, true)
if err != nil {
return false
return types.AssignableTo(vinst, tinst)