diff --git a/src/go/types/infer.go b/src/go/types/infer.go
new file mode 100644
index 0000000000..c0b1a4b71a
--- /dev/null
+++ b/src/go/types/infer.go
@@ -0,0 +1,359 @@
+// Copyright 2018 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 type parameter inference given
+// a list of concrete arguments and a parameter list.
+
+package types
+
+import (
+	"go/token"
+	"strings"
+)
+
+// infer returns the list of actual type arguments for the given list of type parameters tparams
+// by inferring them from the actual arguments args for the parameters params. If type inference
+// is impossible because unification fails, an error is reported and the resulting types list is
+// nil, and index is 0. Otherwise, types is the list of inferred type arguments, and index is
+// the index of the first type argument in that list that couldn't be inferred (and thus is nil).
+// If all type arguments were inferred successfully, index is < 0.
+func (check *Checker) infer(tparams []*TypeName, params *Tuple, args []*operand) (types []Type, index int) {
+	assert(params.Len() == len(args))
+
+	u := newUnifier(check, false)
+	u.x.init(tparams)
+
+	errorf := func(kind string, tpar, targ Type, arg *operand) {
+		// provide a better error message if we can
+		targs, failed := u.x.types()
+		if failed == 0 {
+			// The first type parameter couldn't be inferred.
+			// If none of them could be inferred, don't try
+			// to provide the inferred type in the error msg.
+			allFailed := true
+			for _, targ := range targs {
+				if targ != nil {
+					allFailed = false
+					break
+				}
+			}
+			if allFailed {
+				check.errorf(arg, 0, "%s %s of %s does not match %s (cannot infer %s)", kind, targ, arg.expr, tpar, typeNamesString(tparams))
+				return
+			}
+		}
+		smap := makeSubstMap(tparams, targs)
+		// TODO(rFindley): pass a positioner here, rather than arg.Pos().
+		inferred := check.subst(arg.Pos(), tpar, smap)
+		if inferred != tpar {
+			check.errorf(arg, 0, "%s %s of %s does not match inferred type %s for %s", kind, targ, arg.expr, inferred, tpar)
+		} else {
+			check.errorf(arg, 0, "%s %s of %s does not match %s", kind, targ, arg.expr, tpar)
+		}
+	}
+
+	// Terminology: generic parameter = function parameter with a type-parameterized type
+
+	// 1st pass: Unify parameter and argument types for generic parameters with typed arguments
+	//           and collect the indices of generic parameters with untyped arguments.
+	var indices []int
+	for i, arg := range args {
+		par := params.At(i)
+		// If we permit bidirectional unification, this conditional code needs to be
+		// executed even if par.typ is not parameterized since the argument may be a
+		// generic function (for which we want to infer // its type arguments).
+		if isParameterized(tparams, par.typ) {
+			if arg.mode == invalid {
+				// An error was reported earlier. Ignore this targ
+				// and continue, we may still be able to infer all
+				// targs resulting in fewer follon-on errors.
+				continue
+			}
+			if targ := arg.typ; isTyped(targ) {
+				// If we permit bidirectional unification, and targ is
+				// a generic function, we need to initialize u.y with
+				// the respective type parameters of targ.
+				if !u.unify(par.typ, targ) {
+					errorf("type", par.typ, targ, arg)
+					return nil, 0
+				}
+			} else {
+				indices = append(indices, i)
+			}
+		}
+	}
+
+	// Some generic parameters with untyped arguments may have been given a type
+	// indirectly through another generic parameter with a typed argument; we can
+	// ignore those now. (This only means that we know the types for those generic
+	// parameters; it doesn't mean untyped arguments can be passed safely. We still
+	// need to verify that assignment of those arguments is valid when we check
+	// function parameter passing external to infer.)
+	j := 0
+	for _, i := range indices {
+		par := params.At(i)
+		// Since untyped types are all basic (i.e., non-composite) types, an
+		// untyped argument will never match a composite parameter type; the
+		// only parameter type it can possibly match against is a *TypeParam.
+		// Thus, only keep the indices of generic parameters that are not of
+		// composite types and which don't have a type inferred yet.
+		if tpar, _ := par.typ.(*TypeParam); tpar != nil && u.x.at(tpar.index) == nil {
+			indices[j] = i
+			j++
+		}
+	}
+	indices = indices[:j]
+
+	// 2nd pass: Unify parameter and default argument types for remaining generic parameters.
+	for _, i := range indices {
+		par := params.At(i)
+		arg := args[i]
+		targ := Default(arg.typ)
+		// The default type for an untyped nil is untyped nil. We must not
+		// infer an untyped nil type as type parameter type. Ignore untyped
+		// nil by making sure all default argument types are typed.
+		if isTyped(targ) && !u.unify(par.typ, targ) {
+			errorf("default type", par.typ, targ, arg)
+			return nil, 0
+		}
+	}
+
+	return u.x.types()
+}
+
+// typeNamesString produces a string containing all the
+// type names in list suitable for human consumption.
+func typeNamesString(list []*TypeName) string {
+	// common cases
+	n := len(list)
+	switch n {
+	case 0:
+		return ""
+	case 1:
+		return list[0].name
+	case 2:
+		return list[0].name + " and " + list[1].name
+	}
+
+	// general case (n > 2)
+	var b strings.Builder
+	for i, tname := range list[:n-1] {
+		if i > 0 {
+			b.WriteString(", ")
+		}
+		b.WriteString(tname.name)
+	}
+	b.WriteString(", and ")
+	b.WriteString(list[n-1].name)
+	return b.String()
+}
+
+// IsParameterized reports whether typ contains any of the type parameters of tparams.
+func isParameterized(tparams []*TypeName, typ Type) bool {
+	w := tpWalker{
+		seen:    make(map[Type]bool),
+		tparams: tparams,
+	}
+	return w.isParameterized(typ)
+}
+
+type tpWalker struct {
+	seen    map[Type]bool
+	tparams []*TypeName
+}
+
+func (w *tpWalker) isParameterized(typ Type) (res bool) {
+	// detect cycles
+	if x, ok := w.seen[typ]; ok {
+		return x
+	}
+	w.seen[typ] = false
+	defer func() {
+		w.seen[typ] = res
+	}()
+
+	switch t := typ.(type) {
+	case nil, *Basic: // TODO(gri) should nil be handled here?
+		break
+
+	case *Array:
+		return w.isParameterized(t.elem)
+
+	case *Slice:
+		return w.isParameterized(t.elem)
+
+	case *Struct:
+		for _, fld := range t.fields {
+			if w.isParameterized(fld.typ) {
+				return true
+			}
+		}
+
+	case *Pointer:
+		return w.isParameterized(t.base)
+
+	case *Tuple:
+		n := t.Len()
+		for i := 0; i < n; i++ {
+			if w.isParameterized(t.At(i).typ) {
+				return true
+			}
+		}
+
+	case *Sum:
+		return w.isParameterizedList(t.types)
+
+	case *Signature:
+		// t.tparams may not be nil if we are looking at a signature
+		// of a generic function type (or an interface method) that is
+		// part of the type we're testing. We don't care about these type
+		// parameters.
+		// Similarly, the receiver of a method may declare (rather then
+		// use) type parameters, we don't care about those either.
+		// Thus, we only need to look at the input and result parameters.
+		return w.isParameterized(t.params) || w.isParameterized(t.results)
+
+	case *Interface:
+		if t.allMethods != nil {
+			// TODO(rFindley) at some point we should enforce completeness here
+			for _, m := range t.allMethods {
+				if w.isParameterized(m.typ) {
+					return true
+				}
+			}
+			return w.isParameterizedList(unpackType(t.allTypes))
+		}
+
+		return t.iterate(func(t *Interface) bool {
+			for _, m := range t.methods {
+				if w.isParameterized(m.typ) {
+					return true
+				}
+			}
+			return w.isParameterizedList(unpackType(t.types))
+		}, nil)
+
+	case *Map:
+		return w.isParameterized(t.key) || w.isParameterized(t.elem)
+
+	case *Chan:
+		return w.isParameterized(t.elem)
+
+	case *Named:
+		return w.isParameterizedList(t.targs)
+
+	case *TypeParam:
+		// t must be one of w.tparams
+		return t.index < len(w.tparams) && w.tparams[t.index].typ == t
+
+	case *instance:
+		return w.isParameterizedList(t.targs)
+
+	default:
+		unreachable()
+	}
+
+	return false
+}
+
+func (w *tpWalker) isParameterizedList(list []Type) bool {
+	for _, t := range list {
+		if w.isParameterized(t) {
+			return true
+		}
+	}
+	return false
+}
+
+// inferB returns the list of actual type arguments inferred from the type parameters'
+// bounds and an initial set of type arguments. If type inference is impossible because
+// unification fails, an error is reported, the resulting types list is nil, and index is 0.
+// Otherwise, types is the list of inferred type arguments, and index is the index of the
+// first type argument in that list that couldn't be inferred (and thus is nil). If all
+// type arguments where inferred successfully, index is < 0. The number of type arguments
+// provided may be less than the number of type parameters, but there must be at least one.
+func (check *Checker) inferB(tparams []*TypeName, targs []Type) (types []Type, index int) {
+	assert(len(tparams) >= len(targs) && len(targs) > 0)
+
+	// Setup bidirectional unification between those structural bounds
+	// and the corresponding type arguments (which may be nil!).
+	u := newUnifier(check, false)
+	u.x.init(tparams)
+	u.y = u.x // type parameters between LHS and RHS of unification are identical
+
+	// Set the type arguments which we know already.
+	for i, targ := range targs {
+		if targ != nil {
+			u.x.set(i, targ)
+		}
+	}
+
+	// Unify type parameters with their structural constraints, if any.
+	for _, tpar := range tparams {
+		typ := tpar.typ.(*TypeParam)
+		sbound := check.structuralType(typ.bound)
+		if sbound != nil {
+			if !u.unify(typ, sbound) {
+				check.errorf(tpar, 0, "%s does not match %s", tpar, sbound)
+				return nil, 0
+			}
+		}
+	}
+
+	// u.x.types() now contains the incoming type arguments plus any additional type
+	// arguments for which there were structural constraints. The newly inferred non-
+	// nil entries may still contain references to other type parameters. For instance,
+	// for [A any, B interface{type []C}, C interface{type *A}], if A == int
+	// was given, unification produced the type list [int, []C, *A]. We eliminate the
+	// remaining type parameters by substituting the type parameters in this type list
+	// until nothing changes anymore.
+	types, index = u.x.types()
+	if debug {
+		for i, targ := range targs {
+			assert(targ == nil || types[i] == targ)
+		}
+	}
+
+	// dirty tracks the indices of all types that may still contain type parameters.
+	// We know that nil type entries and entries corresponding to provided (non-nil)
+	// type arguments are clean, so exclude them from the start.
+	var dirty []int
+	for i, typ := range types {
+		if typ != nil && (i >= len(targs) || targs[i] == nil) {
+			dirty = append(dirty, i)
+		}
+	}
+
+	for len(dirty) > 0 {
+		// TODO(gri) Instead of creating a new substMap for each iteration,
+		// provide an update operation for substMaps and only change when
+		// needed. Optimization.
+		smap := makeSubstMap(tparams, types)
+		n := 0
+		for _, index := range dirty {
+			t0 := types[index]
+			if t1 := check.subst(token.NoPos, t0, smap); t1 != t0 {
+				types[index] = t1
+				dirty[n] = index
+				n++
+			}
+		}
+		dirty = dirty[:n]
+	}
+
+	return
+}
+
+// structuralType returns the structural type of a constraint, if any.
+func (check *Checker) structuralType(constraint Type) Type {
+	if iface, _ := under(constraint).(*Interface); iface != nil {
+		check.completeInterface(token.NoPos, iface)
+		types := unpackType(iface.allTypes)
+		if len(types) == 1 {
+			return types[0]
+		}
+		return nil
+	}
+	return constraint
+}
diff --git a/src/go/types/unify.go b/src/go/types/unify.go
new file mode 100644
index 0000000000..ab18febbdf
--- /dev/null
+++ b/src/go/types/unify.go
@@ -0,0 +1,452 @@
+// Copyright 2020 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 type unification.
+
+package types
+
+import (
+	"go/token"
+	"sort"
+)
+
+// The unifier maintains two separate sets of type parameters x and y
+// which are used to resolve type parameters in the x and y arguments
+// provided to the unify call. For unidirectional unification, only
+// one of these sets (say x) is provided, and then type parameters are
+// only resolved for the x argument passed to unify, not the y argument
+// (even if that also contains possibly the same type parameters). This
+// is crucial to infer the type parameters of self-recursive calls:
+//
+//	func f[P any](a P) { f(a) }
+//
+// For the call f(a) we want to infer that the type argument for P is P.
+// During unification, the parameter type P must be resolved to the type
+// parameter P ("x" side), but the argument type P must be left alone so
+// that unification resolves the type parameter P to P.
+//
+// For bidirection unification, both sets are provided. This enables
+// unification to go from argument to parameter type and vice versa.
+// For constraint type inference, we use bidirectional unification
+// where both the x and y type parameters are identical. This is done
+// by setting up one of them (using init) and then assigning its value
+// to the other.
+
+// A unifier maintains the current type parameters for x and y
+// and the respective types inferred for each type parameter.
+// A unifier is created by calling newUnifier.
+type unifier struct {
+	check *Checker
+	exact bool
+	x, y  tparamsList // x and y must initialized via tparamsList.init
+	types []Type      // inferred types, shared by x and y
+}
+
+// newUnifier returns a new unifier.
+// If exact is set, unification requires unified types to match
+// exactly. If exact is not set, a named type's underlying type
+// is considered if unification would fail otherwise, and the
+// direction of channels is ignored.
+func newUnifier(check *Checker, exact bool) *unifier {
+	u := &unifier{check: check, exact: exact}
+	u.x.unifier = u
+	u.y.unifier = u
+	return u
+}
+
+// unify attempts to unify x and y and reports whether it succeeded.
+func (u *unifier) unify(x, y Type) bool {
+	return u.nify(x, y, nil)
+}
+
+// A tparamsList describes a list of type parameters and the types inferred for them.
+type tparamsList struct {
+	unifier *unifier
+	tparams []*TypeName
+	// For each tparams element, there is a corresponding type slot index in indices.
+	// index  < 0: unifier.types[-index-1] == nil
+	// index == 0: no type slot allocated yet
+	// index  > 0: unifier.types[index-1] == typ
+	// Joined tparams elements share the same type slot and thus have the same index.
+	// By using a negative index for nil types we don't need to check unifier.types
+	// to see if we have a type or not.
+	indices []int // len(d.indices) == len(d.tparams)
+}
+
+// init initializes d with the given type parameters.
+// The type parameters must be in the order in which they appear in their declaration
+// (this ensures that the tparams indices match the respective type parameter index).
+func (d *tparamsList) init(tparams []*TypeName) {
+	if len(tparams) == 0 {
+		return
+	}
+	if debug {
+		for i, tpar := range tparams {
+			assert(i == tpar.typ.(*TypeParam).index)
+		}
+	}
+	d.tparams = tparams
+	d.indices = make([]int, len(tparams))
+}
+
+// join unifies the i'th type parameter of x with the j'th type parameter of y.
+// If both type parameters already have a type associated with them and they are
+// not joined, join fails and return false.
+func (u *unifier) join(i, j int) bool {
+	ti := u.x.indices[i]
+	tj := u.y.indices[j]
+	switch {
+	case ti == 0 && tj == 0:
+		// Neither type parameter has a type slot associated with them.
+		// Allocate a new joined nil type slot (negative index).
+		u.types = append(u.types, nil)
+		u.x.indices[i] = -len(u.types)
+		u.y.indices[j] = -len(u.types)
+	case ti == 0:
+		// The type parameter for x has no type slot yet. Use slot of y.
+		u.x.indices[i] = tj
+	case tj == 0:
+		// The type parameter for y has no type slot yet. Use slot of x.
+		u.y.indices[j] = ti
+
+	// Both type parameters have a slot: ti != 0 && tj != 0.
+	case ti == tj:
+		// Both type parameters already share the same slot. Nothing to do.
+		break
+	case ti > 0 && tj > 0:
+		// Both type parameters have (possibly different) inferred types. Cannot join.
+		return false
+	case ti > 0:
+		// Only the type parameter for x has an inferred type. Use x slot for y.
+		u.y.setIndex(j, ti)
+	default:
+		// Either the type parameter for y has an inferred type, or neither type
+		// parameter has an inferred type. In either case, use y slot for x.
+		u.x.setIndex(i, tj)
+	}
+	return true
+}
+
+// If typ is a type parameter of d, index returns the type parameter index.
+// Otherwise, the result is < 0.
+func (d *tparamsList) index(typ Type) int {
+	if t, ok := typ.(*TypeParam); ok {
+		if i := t.index; i < len(d.tparams) && d.tparams[i].typ == t {
+			return i
+		}
+	}
+	return -1
+}
+
+// setIndex sets the type slot index for the i'th type parameter
+// (and all its joined parameters) to tj. The type parameter
+// must have a (possibly nil) type slot associated with it.
+func (d *tparamsList) setIndex(i, tj int) {
+	ti := d.indices[i]
+	assert(ti != 0 && tj != 0)
+	for k, tk := range d.indices {
+		if tk == ti {
+			d.indices[k] = tj
+		}
+	}
+}
+
+// at returns the type set for the i'th type parameter; or nil.
+func (d *tparamsList) at(i int) Type {
+	if ti := d.indices[i]; ti > 0 {
+		return d.unifier.types[ti-1]
+	}
+	return nil
+}
+
+// set sets the type typ for the i'th type parameter;
+// typ must not be nil and it must not have been set before.
+func (d *tparamsList) set(i int, typ Type) {
+	assert(typ != nil)
+	u := d.unifier
+	switch ti := d.indices[i]; {
+	case ti < 0:
+		u.types[-ti-1] = typ
+		d.setIndex(i, -ti)
+	case ti == 0:
+		u.types = append(u.types, typ)
+		d.indices[i] = len(u.types)
+	default:
+		panic("type already set")
+	}
+}
+
+// types returns the list of inferred types (via unification) for the type parameters
+// described by d, and an index. If all types were inferred, the returned index is < 0.
+// Otherwise, it is the index of the first type parameter which couldn't be inferred;
+// i.e., for which list[index] is nil.
+func (d *tparamsList) types() (list []Type, index int) {
+	list = make([]Type, len(d.tparams))
+	index = -1
+	for i := range d.tparams {
+		t := d.at(i)
+		list[i] = t
+		if index < 0 && t == nil {
+			index = i
+		}
+	}
+	return
+}
+
+func (u *unifier) nifyEq(x, y Type, p *ifacePair) bool {
+	return x == y || u.nify(x, y, p)
+}
+
+// nify implements the core unification algorithm which is an
+// adapted version of Checker.identical0. For changes to that
+// code the corresponding changes should be made here.
+// Must not be called directly from outside the unifier.
+func (u *unifier) nify(x, y Type, p *ifacePair) bool {
+	// types must be expanded for comparison
+	x = expand(x)
+	y = expand(y)
+
+	if !u.exact {
+		// If exact unification is known to fail because we attempt to
+		// match a type name against an unnamed type literal, consider
+		// the underlying type of the named type.
+		// (Subtle: We use isNamed to include any type with a name (incl.
+		// basic types and type parameters. We use asNamed() because we only
+		// want *Named types.)
+		switch {
+		case !isNamed(x) && y != nil && asNamed(y) != nil:
+			return u.nify(x, under(y), p)
+		case x != nil && asNamed(x) != nil && !isNamed(y):
+			return u.nify(under(x), y, p)
+		}
+	}
+
+	// Cases where at least one of x or y is a type parameter.
+	switch i, j := u.x.index(x), u.y.index(y); {
+	case i >= 0 && j >= 0:
+		// both x and y are type parameters
+		if u.join(i, j) {
+			return true
+		}
+		// both x and y have an inferred type - they must match
+		return u.nifyEq(u.x.at(i), u.y.at(j), p)
+
+	case i >= 0:
+		// x is a type parameter, y is not
+		if tx := u.x.at(i); tx != nil {
+			return u.nifyEq(tx, y, p)
+		}
+		// otherwise, infer type from y
+		u.x.set(i, y)
+		return true
+
+	case j >= 0:
+		// y is a type parameter, x is not
+		if ty := u.y.at(j); ty != nil {
+			return u.nifyEq(x, ty, p)
+		}
+		// otherwise, infer type from x
+		u.y.set(j, x)
+		return true
+	}
+
+	// For type unification, do not shortcut (x == y) for identical
+	// types. Instead keep comparing them element-wise to unify the
+	// matching (and equal type parameter types). A simple test case
+	// where this matters is: func f[P any](a P) { f(a) } .
+
+	switch x := x.(type) {
+	case *Basic:
+		// Basic types are singletons except for the rune and byte
+		// aliases, thus we cannot solely rely on the x == y check
+		// above. See also comment in TypeName.IsAlias.
+		if y, ok := y.(*Basic); ok {
+			return x.kind == y.kind
+		}
+
+	case *Array:
+		// Two array types are identical if they have identical element types
+		// and the same array length.
+		if y, ok := y.(*Array); ok {
+			// If one or both array lengths are unknown (< 0) due to some error,
+			// assume they are the same to avoid spurious follow-on errors.
+			return (x.len < 0 || y.len < 0 || x.len == y.len) && u.nify(x.elem, y.elem, p)
+		}
+
+	case *Slice:
+		// Two slice types are identical if they have identical element types.
+		if y, ok := y.(*Slice); ok {
+			return u.nify(x.elem, y.elem, p)
+		}
+
+	case *Struct:
+		// Two struct types are identical if they have the same sequence of fields,
+		// and if corresponding fields have the same names, and identical types,
+		// and identical tags. Two embedded fields are considered to have the same
+		// name. Lower-case field names from different packages are always different.
+		if y, ok := y.(*Struct); ok {
+			if x.NumFields() == y.NumFields() {
+				for i, f := range x.fields {
+					g := y.fields[i]
+					if f.embedded != g.embedded ||
+						x.Tag(i) != y.Tag(i) ||
+						!f.sameId(g.pkg, g.name) ||
+						!u.nify(f.typ, g.typ, p) {
+						return false
+					}
+				}
+				return true
+			}
+		}
+
+	case *Pointer:
+		// Two pointer types are identical if they have identical base types.
+		if y, ok := y.(*Pointer); ok {
+			return u.nify(x.base, y.base, p)
+		}
+
+	case *Tuple:
+		// Two tuples types are identical if they have the same number of elements
+		// and corresponding elements have identical types.
+		if y, ok := y.(*Tuple); ok {
+			if x.Len() == y.Len() {
+				if x != nil {
+					for i, v := range x.vars {
+						w := y.vars[i]
+						if !u.nify(v.typ, w.typ, p) {
+							return false
+						}
+					}
+				}
+				return true
+			}
+		}
+
+	case *Signature:
+		// Two function types are identical if they have the same number of parameters
+		// and result values, corresponding parameter and result types are identical,
+		// and either both functions are variadic or neither is. Parameter and result
+		// names are not required to match.
+		// TODO(gri) handle type parameters or document why we can ignore them.
+		if y, ok := y.(*Signature); ok {
+			return x.variadic == y.variadic &&
+				u.nify(x.params, y.params, p) &&
+				u.nify(x.results, y.results, p)
+		}
+
+	case *Sum:
+		// This should not happen with the current internal use of sum types.
+		panic("type inference across sum types not implemented")
+
+	case *Interface:
+		// Two interface types are identical if they have the same set of methods with
+		// the same names and identical function types. Lower-case method names from
+		// different packages are always different. The order of the methods is irrelevant.
+		if y, ok := y.(*Interface); ok {
+			// If identical0 is called (indirectly) via an external API entry point
+			// (such as Identical, IdenticalIgnoreTags, etc.), check is nil. But in
+			// that case, interfaces are expected to be complete and lazy completion
+			// here is not needed.
+			if u.check != nil {
+				u.check.completeInterface(token.NoPos, x)
+				u.check.completeInterface(token.NoPos, y)
+			}
+			a := x.allMethods
+			b := y.allMethods
+			if len(a) == len(b) {
+				// Interface types are the only types where cycles can occur
+				// that are not "terminated" via named types; and such cycles
+				// can only be created via method parameter types that are
+				// anonymous interfaces (directly or indirectly) embedding
+				// the current interface. Example:
+				//
+				//    type T interface {
+				//        m() interface{T}
+				//    }
+				//
+				// If two such (differently named) interfaces are compared,
+				// endless recursion occurs if the cycle is not detected.
+				//
+				// If x and y were compared before, they must be equal
+				// (if they were not, the recursion would have stopped);
+				// search the ifacePair stack for the same pair.
+				//
+				// This is a quadratic algorithm, but in practice these stacks
+				// are extremely short (bounded by the nesting depth of interface
+				// type declarations that recur via parameter types, an extremely
+				// rare occurrence). An alternative implementation might use a
+				// "visited" map, but that is probably less efficient overall.
+				q := &ifacePair{x, y, p}
+				for p != nil {
+					if p.identical(q) {
+						return true // same pair was compared before
+					}
+					p = p.prev
+				}
+				if debug {
+					assert(sort.IsSorted(byUniqueMethodName(a)))
+					assert(sort.IsSorted(byUniqueMethodName(b)))
+				}
+				for i, f := range a {
+					g := b[i]
+					if f.Id() != g.Id() || !u.nify(f.typ, g.typ, q) {
+						return false
+					}
+				}
+				return true
+			}
+		}
+
+	case *Map:
+		// Two map types are identical if they have identical key and value types.
+		if y, ok := y.(*Map); ok {
+			return u.nify(x.key, y.key, p) && u.nify(x.elem, y.elem, p)
+		}
+
+	case *Chan:
+		// Two channel types are identical if they have identical value types.
+		if y, ok := y.(*Chan); ok {
+			return (!u.exact || x.dir == y.dir) && u.nify(x.elem, y.elem, p)
+		}
+
+	case *Named:
+		// Two named types are identical if their type names originate
+		// in the same type declaration.
+		// if y, ok := y.(*Named); ok {
+		// 	return x.obj == y.obj
+		// }
+		if y, ok := y.(*Named); ok {
+			// TODO(gri) This is not always correct: two types may have the same names
+			//           in the same package if one of them is nested in a function.
+			//           Extremely unlikely but we need an always correct solution.
+			if x.obj.pkg == y.obj.pkg && x.obj.name == y.obj.name {
+				assert(len(x.targs) == len(y.targs))
+				for i, x := range x.targs {
+					if !u.nify(x, y.targs[i], p) {
+						return false
+					}
+				}
+				return true
+			}
+		}
+
+	case *TypeParam:
+		// Two type parameters (which are not part of the type parameters of the
+		// enclosing type as those are handled in the beginning of this function)
+		// are identical if they originate in the same declaration.
+		return x == y
+
+	// case *instance:
+	//	unreachable since types are expanded
+
+	case nil:
+		// avoid a crash in case of nil type
+
+	default:
+		u.check.dump("### u.nify(%s, %s), u.x.tparams = %s", x, y, u.x.tparams)
+		unreachable()
+	}
+
+	return false
+}