go/ssa/subst.go - tools - Git at Google

 // 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 ssa

 import (
 	"fmt"
 	"go/types"

 	"golang.org/x/tools/internal/typeparams"
 )

 // Type substituter for a fixed set of replacement types.
 //
 // A nil *subster is an valid, empty substitution map. It always acts as
 // the identity function. This allows for treating parameterized and
 // non-parameterized functions identically while compiling to ssa.
 //
 // Not concurrency-safe.
 type subster struct {
 	replacements map[*typeparams.TypeParam]types.Type // values should contain no type params
 	cache        map[types.Type]types.Type            // cache of subst results
 	ctxt         *typeparams.Context
 	debug        bool // perform extra debugging checks
 	// TODO(taking): consider adding Pos
 }

 // Returns a subster that replaces tparams[i] with targs[i]. Uses ctxt as a cache.
 // targs should not contain any types in tparams.
 func makeSubster(ctxt *typeparams.Context, tparams []*typeparams.TypeParam, targs []types.Type, debug bool) *subster {
 	assert(len(tparams) == len(targs), "makeSubster argument count must match")

 	subst := &subster{
 		replacements: make(map[*typeparams.TypeParam]types.Type, len(tparams)),
 		cache:        make(map[types.Type]types.Type),
 		ctxt:         ctxt,
 		debug:        debug,
 	}
 	for i, tpar := range tparams {
 		subst.replacements[tpar] = targs[i]
 	}
 	if subst.debug {
 		if err := subst.wellFormed(); err != nil {
 			panic(err)
 		}
 	}
 	return subst
 }

 // wellFormed returns an error if subst was not properly initialized.
 func (subst *subster) wellFormed() error {
 	if subst == nil || len(subst.replacements) == 0 {
 		return nil
 	}
 	// Check that all of the type params do not appear in the arguments.
 	s := make(map[types.Type]bool, len(subst.replacements))
 	for tparam := range subst.replacements {
 		s[tparam] = true
 	}
 	for _, r := range subst.replacements {
 		if reaches(r, s) {
 			return fmt.Errorf("\n‰r %s s %v replacements %v\n", r, s, subst.replacements)
 		}
 	}
 	return nil
 }

 // typ returns the type of t with the type parameter tparams[i] substituted
 // for the type targs[i] where subst was created using tparams and targs.
 func (subst *subster) typ(t types.Type) (res types.Type) {
 	if subst == nil {
 		return t // A nil subst is type preserving.
 	}
 	if r, ok := subst.cache[t]; ok {
 		return r
 	}
 	defer func() {
 		subst.cache[t] = res
 	}()

 	// fall through if result r will be identical to t, types.Identical(r, t).
 	switch t := t.(type) {
 	case *typeparams.TypeParam:
 		r := subst.replacements[t]
 		assert(r != nil, "type param without replacement encountered")
 		return r

 	case *types.Basic:
 		return t

 	case *types.Array:
 		if r := subst.typ(t.Elem()); r != t.Elem() {
 			return types.NewArray(r, t.Len())
 		}
 		return t

 	case *types.Slice:
 		if r := subst.typ(t.Elem()); r != t.Elem() {
 			return types.NewSlice(r)
 		}
 		return t

 	case *types.Pointer:
 		if r := subst.typ(t.Elem()); r != t.Elem() {
 			return types.NewPointer(r)
 		}
 		return t

 	case *types.Tuple:
 		return subst.tuple(t)

 	case *types.Struct:
 		return subst.struct_(t)

 	case *types.Map:
 		key := subst.typ(t.Key())
 		elem := subst.typ(t.Elem())
 		if key != t.Key() || elem != t.Elem() {
 			return types.NewMap(key, elem)
 		}
 		return t

 	case *types.Chan:
 		if elem := subst.typ(t.Elem()); elem != t.Elem() {
 			return types.NewChan(t.Dir(), elem)
 		}
 		return t

 	case *types.Signature:
 		return subst.signature(t)

 	case *typeparams.Union:
 		return subst.union(t)

 	case *types.Interface:
 		return subst.interface_(t)

 	case *types.Named:
 		return subst.named(t)

 	default:
 		panic("unreachable")
 	}
 }

 // types returns the result of {subst.typ(ts[i])}.
 func (subst *subster) types(ts []types.Type) []types.Type {
 	res := make([]types.Type, len(ts))
 	for i := range ts {
 		res[i] = subst.typ(ts[i])
 	}
 	return res
 }

 func (subst *subster) tuple(t *types.Tuple) *types.Tuple {
 	if t != nil {
 		if vars := subst.varlist(t); vars != nil {
 			return types.NewTuple(vars...)
 		}
 	}
 	return t
 }

 type varlist interface {
 	At(i int) *types.Var
 	Len() int
 }

 // fieldlist is an adapter for structs for the varlist interface.
 type fieldlist struct {
 	str *types.Struct
 }

 func (fl fieldlist) At(i int) *types.Var { return fl.str.Field(i) }
 func (fl fieldlist) Len() int            { return fl.str.NumFields() }

 func (subst *subster) struct_(t *types.Struct) *types.Struct {
 	if t != nil {
 		if fields := subst.varlist(fieldlist{t}); fields != nil {
 			tags := make([]string, t.NumFields())
 			for i, n := 0, t.NumFields(); i < n; i++ {
 				tags[i] = t.Tag(i)
 			}
 			return types.NewStruct(fields, tags)
 		}
 	}
 	return t
 }

 // varlist reutrns subst(in[i]) or return nils if subst(v[i]) == v[i] for all i.
 func (subst *subster) varlist(in varlist) []*types.Var {
 	var out []*types.Var // nil => no updates
 	for i, n := 0, in.Len(); i < n; i++ {
 		v := in.At(i)
 		w := subst.var_(v)
 		if v != w && out == nil {
 			out = make([]*types.Var, n)
 			for j := 0; j < i; j++ {
 				out[j] = in.At(j)
 			}
 		}
 		if out != nil {
 			out[i] = w
 		}
 	}
 	return out
 }

 func (subst *subster) var_(v *types.Var) *types.Var {
 	if v != nil {
 		if typ := subst.typ(v.Type()); typ != v.Type() {
 			if v.IsField() {
 				return types.NewField(v.Pos(), v.Pkg(), v.Name(), typ, v.Embedded())
 			}
 			return types.NewVar(v.Pos(), v.Pkg(), v.Name(), typ)
 		}
 	}
 	return v
 }

 func (subst *subster) union(u *typeparams.Union) *typeparams.Union {
 	var out []*typeparams.Term // nil => no updates

 	for i, n := 0, u.Len(); i < n; i++ {
 		t := u.Term(i)
 		r := subst.typ(t.Type())
 		if r != t.Type() && out == nil {
 			out = make([]*typeparams.Term, n)
 			for j := 0; j < i; j++ {
 				out[j] = u.Term(j)
 			}
 		}
 		if out != nil {
 			out[i] = typeparams.NewTerm(t.Tilde(), r)
 		}
 	}

 	if out != nil {
 		return typeparams.NewUnion(out)
 	}
 	return u
 }

 func (subst *subster) interface_(iface *types.Interface) *types.Interface {
 	if iface == nil {
 		return nil
 	}

 	// methods for the interface. Initially nil if there is no known change needed.
 	// Signatures for the method where recv is nil. NewInterfaceType fills in the recievers.
 	var methods []*types.Func
 	initMethods := func(n int) { // copy first n explicit methods
 		methods = make([]*types.Func, iface.NumExplicitMethods())
 		for i := 0; i < n; i++ {
 			f := iface.ExplicitMethod(i)
 			norecv := changeRecv(f.Type().(*types.Signature), nil)
 			methods[i] = types.NewFunc(f.Pos(), f.Pkg(), f.Name(), norecv)
 		}
 	}
 	for i := 0; i < iface.NumExplicitMethods(); i++ {
 		f := iface.ExplicitMethod(i)
 		// On interfaces, we need to cycle break on anonymous interface types
 		// being in a cycle with their signatures being in cycles with their recievers
 		// that do not go through a Named.
 		norecv := changeRecv(f.Type().(*types.Signature), nil)
 		sig := subst.typ(norecv)
 		if sig != norecv && methods == nil {
 			initMethods(i)
 		}
 		if methods != nil {
 			methods[i] = types.NewFunc(f.Pos(), f.Pkg(), f.Name(), sig.(*types.Signature))
 		}
 	}

 	var embeds []types.Type
 	initEmbeds := func(n int) { // copy first n embedded types
 		embeds = make([]types.Type, iface.NumEmbeddeds())
 		for i := 0; i < n; i++ {
 			embeds[i] = iface.EmbeddedType(i)
 		}
 	}
 	for i := 0; i < iface.NumEmbeddeds(); i++ {
 		e := iface.EmbeddedType(i)
 		r := subst.typ(e)
 		if e != r && embeds == nil {
 			initEmbeds(i)
 		}
 		if embeds != nil {
 			embeds[i] = r
 		}
 	}

 	if methods == nil && embeds == nil {
 		return iface
 	}
 	if methods == nil {
 		initMethods(iface.NumExplicitMethods())
 	}
 	if embeds == nil {
 		initEmbeds(iface.NumEmbeddeds())
 	}
 	return types.NewInterfaceType(methods, embeds).Complete()
 }

 func (subst *subster) named(t *types.Named) types.Type {
 	// A name type may be:
 	// (1) ordinary (no type parameters, no type arguments),
 	// (2) generic (type parameters but no type arguments), or
 	// (3) instantiated (type parameters and type arguments).
 	tparams := typeparams.ForNamed(t)
 	if tparams.Len() == 0 {
 		// case (1) ordinary

 		// Note: If Go allows for local type declarations in generic
 		// functions we may need to descend into underlying as well.
 		return t
 	}
 	targs := typeparams.NamedTypeArgs(t)

 	// insts are arguments to instantiate using.
 	insts := make([]types.Type, tparams.Len())

 	// case (2) generic ==> targs.Len() == 0
 	// Instantiating a generic with no type arguments should be unreachable.
 	// Please report a bug if you encounter this.
 	assert(targs.Len() != 0, "substition into a generic Named type is currently unsupported")

 	// case (3) instantiated.
 	// Substitute into the type arguments and instantiate the replacements/
 	// Example:
 	//    type N[A any] func() A
 	//    func Foo[T](g N[T]) {}
 	//  To instantiate Foo[string], one goes through {T->string}. To get the type of g
 	//  one subsitutes T with string in {N with TypeArgs == {T} and TypeParams == {A} }
 	//  to get {N with TypeArgs == {string} and TypeParams == {A} }.
 	assert(targs.Len() == tparams.Len(), "TypeArgs().Len() must match TypeParams().Len() if present")
 	for i, n := 0, targs.Len(); i < n; i++ {
 		inst := subst.typ(targs.At(i)) // TODO(generic): Check with rfindley for mutual recursion
 		insts[i] = inst
 	}
 	r, err := typeparams.Instantiate(subst.ctxt, typeparams.NamedTypeOrigin(t), insts, false)
 	assert(err == nil, "failed to Instantiate Named type")
 	return r
 }

 func (subst *subster) signature(t *types.Signature) types.Type {
 	tparams := typeparams.ForSignature(t)

 	// We are choosing not to support tparams.Len() > 0 until a need has been observed in practice.
 	//
 	// There are some known usages for types.Types coming from types.{Eval,CheckExpr}.
 	// To support tparams.Len() > 0, we just need to do the following [psuedocode]:
 	//   targs := {subst.replacements[tparams[i]]]}; Instantiate(ctxt, t, targs, false)

 	assert(tparams.Len() == 0, "Substituting types.Signatures with generic functions are currently unsupported.")

 	// Either:
 	// (1)non-generic function.
 	//    no type params to substitute
 	// (2)generic method and recv needs to be substituted.

 	// Recievers can be either:
 	// named
 	// pointer to named
 	// interface
 	// nil
 	// interface is the problematic case. We need to cycle break there!
 	recv := subst.var_(t.Recv())
 	params := subst.tuple(t.Params())
 	results := subst.tuple(t.Results())
 	if recv != t.Recv() || params != t.Params() || results != t.Results() {
 		return typeparams.NewSignatureType(recv, nil, nil, params, results, t.Variadic())
 	}
 	return t
 }

 // reaches returns true if a type t reaches any type t' s.t. c[t'] == true.
 // Updates c to cache results.
 func reaches(t types.Type, c map[types.Type]bool) (res bool) {
 	if c, ok := c[t]; ok {
 		return c
 	}
 	c[t] = false // prevent cycles
 	defer func() {
 		c[t] = res
 	}()

 	switch t := t.(type) {
 	case *typeparams.TypeParam, *types.Basic:
 		// no-op => c == false
 	case *types.Array:
 		return reaches(t.Elem(), c)
 	case *types.Slice:
 		return reaches(t.Elem(), c)
 	case *types.Pointer:
 		return reaches(t.Elem(), c)
 	case *types.Tuple:
 		for i := 0; i < t.Len(); i++ {
 			if reaches(t.At(i).Type(), c) {
 				return true
 			}
 		}
 	case *types.Struct:
 		for i := 0; i < t.NumFields(); i++ {
 			if reaches(t.Field(i).Type(), c) {
 				return true
 			}
 		}
 	case *types.Map:
 		return reaches(t.Key(), c) || reaches(t.Elem(), c)
 	case *types.Chan:
 		return reaches(t.Elem(), c)
 	case *types.Signature:
 		if t.Recv() != nil && reaches(t.Recv().Type(), c) {
 			return true
 		}
 		return reaches(t.Params(), c) || reaches(t.Results(), c)
 	case *typeparams.Union:
 		for i := 0; i < t.Len(); i++ {
 			if reaches(t.Term(i).Type(), c) {
 				return true
 			}
 		}
 	case *types.Interface:
 		for i := 0; i < t.NumEmbeddeds(); i++ {
 			if reaches(t.Embedded(i), c) {
 				return true
 			}
 		}
 		for i := 0; i < t.NumExplicitMethods(); i++ {
 			if reaches(t.ExplicitMethod(i).Type(), c) {
 				return true
 			}
 		}
 	case *types.Named:
 		return reaches(t.Underlying(), c)
 	default:
 		panic("unreachable")
 	}
 	return false
 }
	// 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 ssa

	import (
	"fmt"
	"go/types"

	"golang.org/x/tools/internal/typeparams"
	)

	// Type substituter for a fixed set of replacement types.
	//
	// A nil *subster is an valid, empty substitution map. It always acts as
	// the identity function. This allows for treating parameterized and
	// non-parameterized functions identically while compiling to ssa.
	//
	// Not concurrency-safe.
	type subster struct {
	replacements map[*typeparams.TypeParam]types.Type // values should contain no type params
	cache map[types.Type]types.Type // cache of subst results
	ctxt *typeparams.Context
	debug bool // perform extra debugging checks
	// TODO(taking): consider adding Pos
	}

	// Returns a subster that replaces tparams[i] with targs[i]. Uses ctxt as a cache.
	// targs should not contain any types in tparams.
	func makeSubster(ctxt typeparams.Context, tparams []typeparams.TypeParam, targs []types.Type, debug bool) *subster {
	assert(len(tparams) == len(targs), "makeSubster argument count must match")

	subst := &subster{
	replacements: make(map[*typeparams.TypeParam]types.Type, len(tparams)),
	cache: make(map[types.Type]types.Type),
	ctxt: ctxt,
	debug: debug,
	}
	for i, tpar := range tparams {
	subst.replacements[tpar] = targs[i]
	}
	if subst.debug {
	if err := subst.wellFormed(); err != nil {
	panic(err)
	}
	}
	return subst
	}

	// wellFormed returns an error if subst was not properly initialized.
	func (subst *subster) wellFormed() error {
	if subst == nil \|\| len(subst.replacements) == 0 {
	return nil
	}
	// Check that all of the type params do not appear in the arguments.
	s := make(map[types.Type]bool, len(subst.replacements))
	for tparam := range subst.replacements {
	s[tparam] = true
	}
	for _, r := range subst.replacements {
	if reaches(r, s) {
	return fmt.Errorf("\n‰r %s s %v replacements %v\n", r, s, subst.replacements)
	}
	}
	return nil
	}

	// typ returns the type of t with the type parameter tparams[i] substituted
	// for the type targs[i] where subst was created using tparams and targs.
	func (subst *subster) typ(t types.Type) (res types.Type) {
	if subst == nil {
	return t // A nil subst is type preserving.
	}
	if r, ok := subst.cache[t]; ok {
	return r
	}
	defer func() {
	subst.cache[t] = res
	}()

	// fall through if result r will be identical to t, types.Identical(r, t).
	switch t := t.(type) {
	case *typeparams.TypeParam:
	r := subst.replacements[t]
	assert(r != nil, "type param without replacement encountered")
	return r

	case *types.Basic:
	return t

	case *types.Array:
	if r := subst.typ(t.Elem()); r != t.Elem() {
	return types.NewArray(r, t.Len())
	}
	return t

	case *types.Slice:
	if r := subst.typ(t.Elem()); r != t.Elem() {
	return types.NewSlice(r)
	}
	return t

	case *types.Pointer:
	if r := subst.typ(t.Elem()); r != t.Elem() {
	return types.NewPointer(r)
	}
	return t

	case *types.Tuple:
	return subst.tuple(t)

	case *types.Struct:
	return subst.struct_(t)

	case *types.Map:
	key := subst.typ(t.Key())
	elem := subst.typ(t.Elem())
	if key != t.Key() \|\| elem != t.Elem() {
	return types.NewMap(key, elem)
	}
	return t

	case *types.Chan:
	if elem := subst.typ(t.Elem()); elem != t.Elem() {
	return types.NewChan(t.Dir(), elem)
	}
	return t

	case *types.Signature:
	return subst.signature(t)

	case *typeparams.Union:
	return subst.union(t)

	case *types.Interface:
	return subst.interface_(t)

	case *types.Named:
	return subst.named(t)

	default:
	panic("unreachable")
	}
	}

	// types returns the result of {subst.typ(ts[i])}.
	func (subst *subster) types(ts []types.Type) []types.Type {
	res := make([]types.Type, len(ts))
	for i := range ts {
	res[i] = subst.typ(ts[i])
	}
	return res
	}

	func (subst subster) tuple(t types.Tuple) *types.Tuple {
	if t != nil {
	if vars := subst.varlist(t); vars != nil {
	return types.NewTuple(vars...)
	}
	}
	return t
	}

	type varlist interface {
	At(i int) *types.Var
	Len() int
	}

	// fieldlist is an adapter for structs for the varlist interface.
	type fieldlist struct {
	str *types.Struct
	}

	func (fl fieldlist) At(i int) *types.Var { return fl.str.Field(i) }
	func (fl fieldlist) Len() int { return fl.str.NumFields() }

	func (subst subster) struct_(t types.Struct) *types.Struct {
	if t != nil {
	if fields := subst.varlist(fieldlist{t}); fields != nil {
	tags := make([]string, t.NumFields())
	for i, n := 0, t.NumFields(); i < n; i++ {
	tags[i] = t.Tag(i)
	}
	return types.NewStruct(fields, tags)
	}
	}
	return t
	}

	// varlist reutrns subst(in[i]) or return nils if subst(v[i]) == v[i] for all i.
	func (subst subster) varlist(in varlist) []types.Var {
	var out []*types.Var // nil => no updates
	for i, n := 0, in.Len(); i < n; i++ {
	v := in.At(i)
	w := subst.var_(v)
	if v != w && out == nil {
	out = make([]*types.Var, n)
	for j := 0; j < i; j++ {
	out[j] = in.At(j)
	}
	}
	if out != nil {
	out[i] = w
	}
	}
	return out
	}

	func (subst subster) var_(v types.Var) *types.Var {
	if v != nil {
	if typ := subst.typ(v.Type()); typ != v.Type() {
	if v.IsField() {
	return types.NewField(v.Pos(), v.Pkg(), v.Name(), typ, v.Embedded())
	}
	return types.NewVar(v.Pos(), v.Pkg(), v.Name(), typ)
	}
	}
	return v
	}

	func (subst subster) union(u typeparams.Union) *typeparams.Union {
	var out []*typeparams.Term // nil => no updates

	for i, n := 0, u.Len(); i < n; i++ {
	t := u.Term(i)
	r := subst.typ(t.Type())
	if r != t.Type() && out == nil {
	out = make([]*typeparams.Term, n)
	for j := 0; j < i; j++ {
	out[j] = u.Term(j)
	}
	}
	if out != nil {
	out[i] = typeparams.NewTerm(t.Tilde(), r)
	}
	}

	if out != nil {
	return typeparams.NewUnion(out)
	}
	return u
	}

	func (subst subster) interface_(iface types.Interface) *types.Interface {
	if iface == nil {
	return nil
	}

	// methods for the interface. Initially nil if there is no known change needed.
	// Signatures for the method where recv is nil. NewInterfaceType fills in the recievers.
	var methods []*types.Func
	initMethods := func(n int) { // copy first n explicit methods
	methods = make([]*types.Func, iface.NumExplicitMethods())
	for i := 0; i < n; i++ {
	f := iface.ExplicitMethod(i)
	norecv := changeRecv(f.Type().(*types.Signature), nil)
	methods[i] = types.NewFunc(f.Pos(), f.Pkg(), f.Name(), norecv)
	}
	}
	for i := 0; i < iface.NumExplicitMethods(); i++ {
	f := iface.ExplicitMethod(i)
	// On interfaces, we need to cycle break on anonymous interface types
	// being in a cycle with their signatures being in cycles with their recievers
	// that do not go through a Named.
	norecv := changeRecv(f.Type().(*types.Signature), nil)
	sig := subst.typ(norecv)
	if sig != norecv && methods == nil {
	initMethods(i)
	}
	if methods != nil {
	methods[i] = types.NewFunc(f.Pos(), f.Pkg(), f.Name(), sig.(*types.Signature))
	}
	}

	var embeds []types.Type
	initEmbeds := func(n int) { // copy first n embedded types
	embeds = make([]types.Type, iface.NumEmbeddeds())
	for i := 0; i < n; i++ {
	embeds[i] = iface.EmbeddedType(i)
	}
	}
	for i := 0; i < iface.NumEmbeddeds(); i++ {
	e := iface.EmbeddedType(i)
	r := subst.typ(e)
	if e != r && embeds == nil {
	initEmbeds(i)
	}
	if embeds != nil {
	embeds[i] = r
	}
	}

	if methods == nil && embeds == nil {
	return iface
	}
	if methods == nil {
	initMethods(iface.NumExplicitMethods())
	}
	if embeds == nil {
	initEmbeds(iface.NumEmbeddeds())
	}
	return types.NewInterfaceType(methods, embeds).Complete()
	}

	func (subst subster) named(t types.Named) types.Type {
	// A name type may be:
	// (1) ordinary (no type parameters, no type arguments),
	// (2) generic (type parameters but no type arguments), or
	// (3) instantiated (type parameters and type arguments).
	tparams := typeparams.ForNamed(t)
	if tparams.Len() == 0 {
	// case (1) ordinary

	// Note: If Go allows for local type declarations in generic
	// functions we may need to descend into underlying as well.
	return t
	}
	targs := typeparams.NamedTypeArgs(t)

	// insts are arguments to instantiate using.
	insts := make([]types.Type, tparams.Len())

	// case (2) generic ==> targs.Len() == 0
	// Instantiating a generic with no type arguments should be unreachable.
	// Please report a bug if you encounter this.
	assert(targs.Len() != 0, "substition into a generic Named type is currently unsupported")

	// case (3) instantiated.
	// Substitute into the type arguments and instantiate the replacements/
	// Example:
	// type N[A any] func() A
	// func Foo[T](g N[T]) {}
	// To instantiate Foo[string], one goes through {T->string}. To get the type of g
	// one subsitutes T with string in {N with TypeArgs == {T} and TypeParams == {A} }
	// to get {N with TypeArgs == {string} and TypeParams == {A} }.
	assert(targs.Len() == tparams.Len(), "TypeArgs().Len() must match TypeParams().Len() if present")
	for i, n := 0, targs.Len(); i < n; i++ {
	inst := subst.typ(targs.At(i)) // TODO(generic): Check with rfindley for mutual recursion
	insts[i] = inst
	}
	r, err := typeparams.Instantiate(subst.ctxt, typeparams.NamedTypeOrigin(t), insts, false)
	assert(err == nil, "failed to Instantiate Named type")
	return r
	}

	func (subst subster) signature(t types.Signature) types.Type {
	tparams := typeparams.ForSignature(t)

	// We are choosing not to support tparams.Len() > 0 until a need has been observed in practice.
	//
	// There are some known usages for types.Types coming from types.{Eval,CheckExpr}.
	// To support tparams.Len() > 0, we just need to do the following [psuedocode]:
	// targs := {subst.replacements[tparams[i]]]}; Instantiate(ctxt, t, targs, false)

	assert(tparams.Len() == 0, "Substituting types.Signatures with generic functions are currently unsupported.")

	// Either:
	// (1)non-generic function.
	// no type params to substitute
	// (2)generic method and recv needs to be substituted.

	// Recievers can be either:
	// named
	// pointer to named
	// interface
	// nil
	// interface is the problematic case. We need to cycle break there!
	recv := subst.var_(t.Recv())
	params := subst.tuple(t.Params())
	results := subst.tuple(t.Results())
	if recv != t.Recv() \|\| params != t.Params() \|\| results != t.Results() {
	return typeparams.NewSignatureType(recv, nil, nil, params, results, t.Variadic())
	}
	return t
	}

	// reaches returns true if a type t reaches any type t' s.t. c[t'] == true.
	// Updates c to cache results.
	func reaches(t types.Type, c map[types.Type]bool) (res bool) {
	if c, ok := c[t]; ok {
	return c
	}
	c[t] = false // prevent cycles
	defer func() {
	c[t] = res
	}()

	switch t := t.(type) {
	case typeparams.TypeParam, types.Basic:
	// no-op => c == false
	case *types.Array:
	return reaches(t.Elem(), c)
	case *types.Slice:
	return reaches(t.Elem(), c)
	case *types.Pointer:
	return reaches(t.Elem(), c)
	case *types.Tuple:
	for i := 0; i < t.Len(); i++ {
	if reaches(t.At(i).Type(), c) {
	return true
	}
	}
	case *types.Struct:
	for i := 0; i < t.NumFields(); i++ {
	if reaches(t.Field(i).Type(), c) {
	return true
	}
	}
	case *types.Map:
	return reaches(t.Key(), c) \|\| reaches(t.Elem(), c)
	case *types.Chan:
	return reaches(t.Elem(), c)
	case *types.Signature:
	if t.Recv() != nil && reaches(t.Recv().Type(), c) {
	return true
	}
	return reaches(t.Params(), c) \|\| reaches(t.Results(), c)
	case *typeparams.Union:
	for i := 0; i < t.Len(); i++ {
	if reaches(t.Term(i).Type(), c) {
	return true
	}
	}
	case *types.Interface:
	for i := 0; i < t.NumEmbeddeds(); i++ {
	if reaches(t.Embedded(i), c) {
	return true
	}
	}
	for i := 0; i < t.NumExplicitMethods(); i++ {
	if reaches(t.ExplicitMethod(i).Type(), c) {
	return true
	}
	}
	case *types.Named:
	return reaches(t.Underlying(), c)
	default:
	panic("unreachable")
	}
	return false
	}