diff --git a/src/go/types/builtins.go b/src/go/types/builtins.go
index 2a2d54da88..99122dfe7c 100644
--- a/src/go/types/builtins.go
+++ b/src/go/types/builtins.go
@@ -783,7 +783,7 @@ func (check *Checker) applyTypeFunc(f func(Type) Type, x Type) Type {
 		tpar := NewTypeName(token.NoPos, nil /* = Universe pkg */, "<type parameter>", nil)
 		ptyp := check.newTypeParam(tpar, 0, &emptyInterface) // assigns type to tpar as a side-effect
 		tsum := _NewSum(rtypes)
-		ptyp.bound = &Interface{types: tsum, allMethods: markComplete, allTypes: tsum}
+		ptyp.bound = &Interface{allMethods: markComplete, allTypes: tsum}
 
 		return ptyp
 	}
diff --git a/src/go/types/errorcodes.go b/src/go/types/errorcodes.go
index 3d24da7b53..2afb6a383c 100644
--- a/src/go/types/errorcodes.go
+++ b/src/go/types/errorcodes.go
@@ -281,16 +281,7 @@ const (
 	_IncomparableMapKey
 
 	// _InvalidIfaceEmbed occurs when a non-interface type is embedded in an
-	// interface.
-	//
-	// Example:
-	//  type T struct {}
-	//
-	//  func (T) m()
-	//
-	//  type I interface {
-	//  	T
-	//  }
+	// interface (for go 1.17 or earlier).
 	_InvalidIfaceEmbed
 
 	// _InvalidPtrEmbed occurs when an embedded field is of the pointer form *T,
diff --git a/src/go/types/infer.go b/src/go/types/infer.go
index 5d49351e1f..951c6b8cbd 100644
--- a/src/go/types/infer.go
+++ b/src/go/types/infer.go
@@ -315,6 +315,9 @@ func (w *tpWalker) isParameterized(typ Type) (res bool) {
 		// Thus, we only need to look at the input and result parameters.
 		return w.isParameterized(t.params) || w.isParameterized(t.results)
 
+	case *Union:
+		panic("unimplemented")
+
 	case *Interface:
 		if t.allMethods != nil {
 			// TODO(rFindley) at some point we should enforce completeness here
@@ -332,7 +335,7 @@ func (w *tpWalker) isParameterized(typ Type) (res bool) {
 					return true
 				}
 			}
-			return w.isParameterizedList(unpackType(t.types))
+			return w.isParameterizedList(t.embeddeds)
 		}, nil)
 
 	case *Map:
diff --git a/src/go/types/interface.go b/src/go/types/interface.go
index fd3fe0ef91..611f387046 100644
--- a/src/go/types/interface.go
+++ b/src/go/types/interface.go
@@ -13,74 +13,84 @@ import (
 )
 
 func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, def *Named) {
-	var tlist *ast.Ident // "type" name of first entry in a type list declaration
-	var types []ast.Expr
+	var tlist []ast.Expr
+	var tname *ast.Ident // "type" name of first entry in a type list declaration
+
 	for _, f := range iface.Methods.List {
-		if len(f.Names) > 0 {
-			// We have a method with name f.Names[0], or a type
-			// of a type list (name.Name == "type").
-			// (The parser ensures that there's only one method
-			// and we don't care if a constructed AST has more.)
-			name := f.Names[0]
-			if name.Name == "_" {
-				check.errorf(name, _BlankIfaceMethod, "invalid method name _")
-				continue // ignore
-			}
-
-			if name.Name == "type" {
-				// Always collect all type list entries, even from
-				// different type lists, under the assumption that
-				// the author intended to include all types.
-				types = append(types, f.Type)
-				if tlist != nil && tlist != name {
-					check.errorf(name, _Todo, "cannot have multiple type lists in an interface")
-				}
-				tlist = name
-				continue
-			}
-
-			typ := check.typ(f.Type)
-			sig, _ := typ.(*Signature)
-			if sig == nil {
-				if typ != Typ[Invalid] {
-					check.invalidAST(f.Type, "%s is not a method signature", typ)
-				}
-				continue // ignore
-			}
-
-			// Always type-check method type parameters but complain if they are not enabled.
-			// (This extra check is needed here because interface method signatures don't have
-			// a receiver specification.)
-			if sig.tparams != nil {
-				var at positioner = f.Type
-				if tparams := typeparams.Get(f.Type); tparams != nil {
-					at = tparams
-				}
-				check.errorf(at, _Todo, "methods cannot have type parameters")
-			}
-
-			// use named receiver type if available (for better error messages)
-			var recvTyp Type = ityp
-			if def != nil {
-				recvTyp = def
-			}
-			sig.recv = NewVar(name.Pos(), check.pkg, "", recvTyp)
-
-			m := NewFunc(name.Pos(), check.pkg, name.Name, sig)
-			check.recordDef(name, m)
-			ityp.methods = append(ityp.methods, m)
-		} else {
-			// We have an embedded type. completeInterface will
-			// eventually verify that we have an interface.
-			ityp.embeddeds = append(ityp.embeddeds, check.typ(f.Type))
+		if len(f.Names) == 0 {
+			// We have an embedded type; possibly a union of types.
+			ityp.embeddeds = append(ityp.embeddeds, parseUnion(check, flattenUnion(nil, f.Type)))
 			check.posMap[ityp] = append(check.posMap[ityp], f.Type.Pos())
+			continue
 		}
+
+		// We have a method with name f.Names[0], or a type
+		// of a type list (name.Name == "type").
+		// (The parser ensures that there's only one method
+		// and we don't care if a constructed AST has more.)
+		name := f.Names[0]
+		if name.Name == "_" {
+			check.errorf(name, _BlankIfaceMethod, "invalid method name _")
+			continue // ignore
+		}
+
+		if name.Name == "type" {
+			// For now, collect all type list entries as if it
+			// were a single union, where each union element is
+			// of the form ~T.
+			// TODO(rfindley) remove once we disallow type lists
+			op := new(ast.UnaryExpr)
+			op.Op = token.TILDE
+			op.X = f.Type
+			tlist = append(tlist, op)
+			if tname != nil && tname != name {
+				check.errorf(name, _Todo, "cannot have multiple type lists in an interface")
+			}
+			tname = name
+			continue
+		}
+
+		typ := check.typ(f.Type)
+		sig, _ := typ.(*Signature)
+		if sig == nil {
+			if typ != Typ[Invalid] {
+				check.invalidAST(f.Type, "%s is not a method signature", typ)
+			}
+			continue // ignore
+		}
+
+		// Always type-check method type parameters but complain if they are not enabled.
+		// (This extra check is needed here because interface method signatures don't have
+		// a receiver specification.)
+		if sig.tparams != nil {
+			var at positioner = f.Type
+			if tparams := typeparams.Get(f.Type); tparams != nil {
+				at = tparams
+			}
+			check.errorf(at, _Todo, "methods cannot have type parameters")
+		}
+
+		// use named receiver type if available (for better error messages)
+		var recvTyp Type = ityp
+		if def != nil {
+			recvTyp = def
+		}
+		sig.recv = NewVar(name.Pos(), check.pkg, "", recvTyp)
+
+		m := NewFunc(name.Pos(), check.pkg, name.Name, sig)
+		check.recordDef(name, m)
+		ityp.methods = append(ityp.methods, m)
 	}
 
 	// type constraints
-	ityp.types = _NewSum(check.collectTypeConstraints(iface.Pos(), types))
+	if tlist != nil {
+		ityp.embeddeds = append(ityp.embeddeds, parseUnion(check, tlist))
+		// Types T in a type list are added as ~T expressions but we don't
+		// have the position of the '~'. Use the first type position instead.
+		check.posMap[ityp] = append(check.posMap[ityp], tlist[0].(*ast.UnaryExpr).X.Pos())
+	}
 
-	if len(ityp.methods) == 0 && ityp.types == nil && len(ityp.embeddeds) == 0 {
+	if len(ityp.methods) == 0 && len(ityp.embeddeds) == 0 {
 		// empty interface
 		ityp.allMethods = markComplete
 		return
@@ -93,32 +103,12 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 	check.later(func() { check.completeInterface(iface.Pos(), ityp) })
 }
 
-func (check *Checker) collectTypeConstraints(pos token.Pos, types []ast.Expr) []Type {
-	list := make([]Type, 0, len(types)) // assume all types are correct
-	for _, texpr := range types {
-		if texpr == nil {
-			check.invalidAST(atPos(pos), "missing type constraint")
-			continue
-		}
-		list = append(list, check.varType(texpr))
+func flattenUnion(list []ast.Expr, x ast.Expr) []ast.Expr {
+	if o, _ := x.(*ast.BinaryExpr); o != nil && o.Op == token.OR {
+		list = flattenUnion(list, o.X)
+		x = o.Y
 	}
-
-	// Ensure that each type is only present once in the type list.  Types may be
-	// interfaces, which may not be complete yet. It's ok to do this check at the
-	// end because it's not a requirement for correctness of the code.
-	// Note: This is a quadratic algorithm, but type lists tend to be short.
-	check.later(func() {
-		for i, t := range list {
-			if t := asInterface(t); t != nil {
-				check.completeInterface(types[i].Pos(), t)
-			}
-			if includes(list[:i], t) {
-				check.softErrorf(types[i], _Todo, "duplicate type %s in type list", t)
-			}
-		}
-	})
-
-	return list
+	return append(list, x)
 }
 
 // includes reports whether typ is in list.
@@ -146,6 +136,7 @@ func (check *Checker) completeInterface(pos token.Pos, ityp *Interface) {
 	completeInterface(check, pos, ityp)
 }
 
+// completeInterface may be called with check == nil.
 func completeInterface(check *Checker, pos token.Pos, ityp *Interface) {
 	assert(ityp.allMethods == nil)
 
@@ -198,6 +189,7 @@ func completeInterface(check *Checker, pos token.Pos, ityp *Interface) {
 			if check == nil {
 				panic(fmt.Sprintf("%v: duplicate method %s", m.pos, m.name))
 			}
+			// check != nil
 			check.errorf(atPos(pos), _DuplicateDecl, "duplicate method %s", m.name)
 			check.errorf(atPos(mpos[other.(*Func)]), _DuplicateDecl, "\tother declaration of %s", m.name) // secondary error, \t indented
 		default:
@@ -211,6 +203,7 @@ func completeInterface(check *Checker, pos token.Pos, ityp *Interface) {
 				todo = append(todo, m, other.(*Func))
 				break
 			}
+			// check != nil
 			check.later(func() {
 				if !check.allowVersion(m.pkg, 1, 14) || !check.identical(m.typ, other.Type()) {
 					check.errorf(atPos(pos), _DuplicateDecl, "duplicate method %s", m.name)
@@ -224,9 +217,8 @@ func completeInterface(check *Checker, pos token.Pos, ityp *Interface) {
 		addMethod(m.pos, m, true)
 	}
 
-	// collect types
-	allTypes := ityp.types
-
+	// collect embedded elements
+	var allTypes Type
 	var posList []token.Pos
 	if check != nil {
 		posList = check.posMap[ityp]
@@ -236,32 +228,36 @@ func completeInterface(check *Checker, pos token.Pos, ityp *Interface) {
 		if posList != nil {
 			pos = posList[i]
 		}
-		utyp := under(typ)
-		etyp := asInterface(utyp)
-		if etyp == nil {
-			if utyp != Typ[Invalid] {
-				var format string
-				if _, ok := utyp.(*_TypeParam); ok {
-					format = "%s is a type parameter, not an interface"
-				} else {
-					format = "%s is not an interface"
-				}
-				if check != nil {
-					// TODO: correct error code.
-					check.errorf(atPos(pos), _InvalidIfaceEmbed, format, typ)
-				} else {
-					panic(fmt.Sprintf(format, typ))
-				}
+		var types Type
+		switch t := under(typ).(type) {
+		case *Interface:
+			if t.allMethods == nil {
+				completeInterface(check, pos, t)
 			}
-			continue
+			for _, m := range t.allMethods {
+				addMethod(pos, m, false) // use embedding position pos rather than m.pos
+
+			}
+			types = t.allTypes
+		case *Union:
+			types = NewSum(t.terms)
+		case *TypeParam:
+			if check != nil && !check.allowVersion(check.pkg, 1, 18) {
+				check.errorf(atPos(pos), _InvalidIfaceEmbed, "%s is a type parameter, not an interface", typ)
+				continue
+			}
+			types = t
+		default:
+			if t == Typ[Invalid] {
+				continue
+			}
+			if check != nil && !check.allowVersion(check.pkg, 1, 18) {
+				check.errorf(atPos(pos), _InvalidIfaceEmbed, "%s is not an interface", typ)
+				continue
+			}
+			types = t
 		}
-		if etyp.allMethods == nil {
-			completeInterface(check, pos, etyp)
-		}
-		for _, m := range etyp.allMethods {
-			addMethod(pos, m, false) // use embedding position pos rather than m.pos
-		}
-		allTypes = intersect(allTypes, etyp.allTypes)
+		allTypes = intersect(allTypes, types)
 	}
 
 	// process todo's (this only happens if check == nil)
@@ -281,7 +277,7 @@ func completeInterface(check *Checker, pos token.Pos, ityp *Interface) {
 }
 
 // intersect computes the intersection of the types x and y.
-// Note: A incomming nil type stands for the top type. A top
+// Note: An incomming nil type stands for the top type. A top
 // type result is returned as nil.
 func intersect(x, y Type) (r Type) {
 	defer func() {
diff --git a/src/go/types/predicates.go b/src/go/types/predicates.go
index 7bb026414f..a72c0dc1fd 100644
--- a/src/go/types/predicates.go
+++ b/src/go/types/predicates.go
@@ -288,6 +288,9 @@ func (check *Checker) identical0(x, y Type, cmpTags bool, p *ifacePair) bool {
 			return true
 		}
 
+	case *Union:
+		panic("identical0 not implemented for union types")
+
 	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
diff --git a/src/go/types/sanitize.go b/src/go/types/sanitize.go
index 88fc3f8377..b9fd56001d 100644
--- a/src/go/types/sanitize.go
+++ b/src/go/types/sanitize.go
@@ -110,11 +110,11 @@ func (s sanitizer) typ(typ Type) Type {
 	case *_Sum:
 		s.typeList(t.types)
 
+	case *Union:
+		s.typeList(t.terms)
+
 	case *Interface:
 		s.funcList(t.methods)
-		if types := s.typ(t.types); types != t.types {
-			t.types = types
-		}
 		s.typeList(t.embeddeds)
 		s.funcList(t.allMethods)
 		if allTypes := s.typ(t.allTypes); allTypes != t.allTypes {
diff --git a/src/go/types/sizeof_test.go b/src/go/types/sizeof_test.go
index 3af9079a85..7454831843 100644
--- a/src/go/types/sizeof_test.go
+++ b/src/go/types/sizeof_test.go
@@ -27,7 +27,8 @@ func TestSizeof(t *testing.T) {
 		{Tuple{}, 12, 24},
 		{Signature{}, 44, 88},
 		{_Sum{}, 12, 24},
-		{Interface{}, 60, 120},
+		{Union{}, 24, 48},
+		{Interface{}, 52, 104},
 		{Map{}, 16, 32},
 		{Chan{}, 12, 24},
 		{Named{}, 68, 136},
diff --git a/src/go/types/sizes.go b/src/go/types/sizes.go
index 67052bb816..ae5d765a89 100644
--- a/src/go/types/sizes.go
+++ b/src/go/types/sizes.go
@@ -150,6 +150,8 @@ func (s *StdSizes) Sizeof(T Type) int64 {
 		return offsets[n-1] + s.Sizeof(t.fields[n-1].typ)
 	case *_Sum:
 		panic("Sizeof unimplemented for type sum")
+	case *Union:
+		panic("Sizeof unimplemented for type union")
 	case *Interface:
 		return s.WordSize * 2
 	}
diff --git a/src/go/types/subst.go b/src/go/types/subst.go
index 47b0c279db..d79c07a2fc 100644
--- a/src/go/types/subst.go
+++ b/src/go/types/subst.go
@@ -311,15 +311,19 @@ func (subst *subster) typ(typ Type) Type {
 			return _NewSum(types)
 		}
 
+	case *Union:
+		terms, copied := subst.typeList(t.terms)
+		if copied {
+			// TODO(gri) Do we need to remove duplicates that may have
+			//           crept in after substitution? It may not matter.
+			return newUnion(terms, t.tilde)
+		}
+
 	case *Interface:
 		methods, mcopied := subst.funcList(t.methods)
-		types := t.types
-		if t.types != nil {
-			types = subst.typ(t.types)
-		}
 		embeddeds, ecopied := subst.typeList(t.embeddeds)
-		if mcopied || types != t.types || ecopied {
-			iface := &Interface{methods: methods, types: types, embeddeds: embeddeds}
+		if mcopied || ecopied {
+			iface := &Interface{methods: methods, embeddeds: embeddeds}
 			if subst.check == nil {
 				panic("internal error: cannot instantiate interfaces yet")
 			}
diff --git a/src/go/types/testdata/check/decls0.src b/src/go/types/testdata/check/decls0.src
index 09904bb303..1224e46377 100644
--- a/src/go/types/testdata/check/decls0.src
+++ b/src/go/types/testdata/check/decls0.src
@@ -4,7 +4,7 @@
 
 // type declarations
 
-package decls0
+package go1_17 // don't permit non-interface elements in interfaces
 
 import "unsafe"
 
diff --git a/src/go/types/testdata/check/issues.src b/src/go/types/testdata/check/issues.src
index e2ac06759b..9d9fc7862f 100644
--- a/src/go/types/testdata/check/issues.src
+++ b/src/go/types/testdata/check/issues.src
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package issues
+package go1_17 // don't permit non-interface elements in interfaces
 
 import (
 	"fmt"
diff --git a/src/go/types/testdata/check/typeinst2.go2 b/src/go/types/testdata/check/typeinst2.go2
index 6e2104a515..1096bb42eb 100644
--- a/src/go/types/testdata/check/typeinst2.go2
+++ b/src/go/types/testdata/check/typeinst2.go2
@@ -164,12 +164,12 @@ type _ interface {
 // for them to be all in a single list, and we report the error
 // as well.)
 type _ interface {
-	type int, int /* ERROR duplicate type int */
-	type /* ERROR multiple type lists */ int /* ERROR duplicate type int */
+	type int, int /* ERROR duplicate term int */
+	type /* ERROR multiple type lists */ int /* ERROR duplicate term int */
 }
 
 type _ interface {
-	type struct{f int}, struct{g int}, struct /* ERROR duplicate type */ {f int}
+	type struct{f int}, struct{g int}, struct /* ERROR duplicate term */ {f int}
 }
 
 // Interface type lists can contain any type, incl. *Named types.
diff --git a/src/go/types/testdata/examples/constraints.go2 b/src/go/types/testdata/examples/constraints.go2
new file mode 100644
index 0000000000..e8b3912884
--- /dev/null
+++ b/src/go/types/testdata/examples/constraints.go2
@@ -0,0 +1,25 @@
+// 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 shows some examples of generic constraint interfaces.
+
+package p
+
+type (
+	// Arbitrary types may be embedded like interfaces.
+	_ interface{int}
+	_ interface{~int}
+
+	// Types may be combined into a union.
+	_ interface{int|~string}
+
+	// Union terms must be unique independent of whether they are ~ or not.
+	_ interface{int|int /* ERROR duplicate term int */ }
+	_ interface{int|~ /* ERROR duplicate term int */ int }
+	_ interface{~int|~ /* ERROR duplicate term int */ int }
+
+	// For now we do not permit interfaces with ~ or in unions.
+	_ interface{~ /* ERROR cannot use interface */ interface{}}
+	_ interface{int|interface /* ERROR cannot use interface */ {}}
+)
diff --git a/src/go/types/testdata/fixedbugs/issue39634.go2 b/src/go/types/testdata/fixedbugs/issue39634.go2
index a13ed13ce5..c759be0d93 100644
--- a/src/go/types/testdata/fixedbugs/issue39634.go2
+++ b/src/go/types/testdata/fixedbugs/issue39634.go2
@@ -36,7 +36,7 @@ func bar8[A foo8[A]](a A) {}
 func main8() {}
 
 // crash 9
-type foo9[A any] interface { type foo9 /* ERROR interface contains type constraints */ [A] }
+type foo9[A any] interface { type foo9 /* ERROR cannot use interface */ [A] }
 func _() { var _ = new(foo9 /* ERROR interface contains type constraints */ [int]) }
 
 // crash 12
diff --git a/src/go/types/testdata/fixedbugs/issue39693.go2 b/src/go/types/testdata/fixedbugs/issue39693.go2
index 316ab1982e..ec7641902a 100644
--- a/src/go/types/testdata/fixedbugs/issue39693.go2
+++ b/src/go/types/testdata/fixedbugs/issue39693.go2
@@ -4,11 +4,20 @@
 
 package p
 
-type Number interface {
-	int     /* ERROR int is not an interface */
-	float64 /* ERROR float64 is not an interface */
+type Number1 interface {
+	// embedding non-interface types is permitted
+	int
+	float64
 }
 
-func Add[T Number](a, b T) T {
+func Add[T Number1](a, b T) T {
 	return a /* ERROR not defined */ + b
 }
+
+type Number2 interface {
+	int|float64
+}
+
+func Add2[T Number2](a, b T) T {
+	return a + b
+}
diff --git a/src/go/types/testdata/fixedbugs/issue39711.go2 b/src/go/types/testdata/fixedbugs/issue39711.go2
index df621a4c17..cf1f90545f 100644
--- a/src/go/types/testdata/fixedbugs/issue39711.go2
+++ b/src/go/types/testdata/fixedbugs/issue39711.go2
@@ -7,5 +7,7 @@ package p
 // Do not report a duplicate type error for this type list.
 // (Check types after interfaces have been completed.)
 type _ interface {
-	type interface{ Error() string }, interface{ String() string }
+	// TODO(rfindley) Once we have full type sets we can enable this again.
+	// Fow now we don't permit interfaces in type lists.
+	// type interface{ Error() string }, interface{ String() string }
 }
diff --git a/src/go/types/testdata/fixedbugs/issue39723.go2 b/src/go/types/testdata/fixedbugs/issue39723.go2
index 55464e6b77..61bc606789 100644
--- a/src/go/types/testdata/fixedbugs/issue39723.go2
+++ b/src/go/types/testdata/fixedbugs/issue39723.go2
@@ -6,4 +6,4 @@ package p
 
 // A constraint must be an interface; it cannot
 // be a type parameter, for instance.
-func _[A interface{ type interface{} }, B A /* ERROR not an interface */ ]()
+func _[A interface{ type int }, B A /* ERROR not an interface */ ]()
diff --git a/src/go/types/testdata/fixedbugs/issue39948.go2 b/src/go/types/testdata/fixedbugs/issue39948.go2
index c2b460902c..d83084b52a 100644
--- a/src/go/types/testdata/fixedbugs/issue39948.go2
+++ b/src/go/types/testdata/fixedbugs/issue39948.go2
@@ -2,7 +2,13 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package p
+// TODO(rfindley) Eventually, once we disallow type lists, we need to
+//                adjust this code: for 1.17 we don't accept type parameters,
+//                and for 1.18 this code is valid.
+//                Leaving for now so we can see that existing errors
+//                are being reported.
+
+package go1_17 // don't permit non-interface elements in interfaces
 
 type T[P any] interface{
 	P // ERROR P is a type parameter, not an interface
diff --git a/src/go/types/type.go b/src/go/types/type.go
index 4a39499905..d487bf66f9 100644
--- a/src/go/types/type.go
+++ b/src/go/types/type.go
@@ -305,7 +305,6 @@ func (s *_Sum) is(pred func(Type) bool) bool {
 // An Interface represents an interface type.
 type Interface struct {
 	methods   []*Func // ordered list of explicitly declared methods
-	types     Type    // (possibly a Sum) type declared with a type list (TODO(gri) need better field name)
 	embeddeds []Type  // ordered list of explicitly embedded types
 
 	allMethods []*Func // ordered list of methods declared with or embedded in this interface (TODO(gri): replace with mset)
diff --git a/src/go/types/typestring.go b/src/go/types/typestring.go
index ff93f3b3c3..9e860dda22 100644
--- a/src/go/types/typestring.go
+++ b/src/go/types/typestring.go
@@ -159,11 +159,17 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 		writeSignature(buf, t, qf, visited)
 
 	case *_Sum:
-		for i, t := range t.types {
+		writeTypeList(buf, t.types, qf, visited)
+
+	case *Union:
+		for i, e := range t.terms {
 			if i > 0 {
-				buf.WriteString(", ")
+				buf.WriteString("|")
 			}
-			writeType(buf, t, qf, visited)
+			if t.tilde[i] {
+				buf.WriteByte('~')
+			}
+			writeType(buf, e, qf, visited)
 		}
 
 	case *Interface:
@@ -208,14 +214,6 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 				writeSignature(buf, m.typ.(*Signature), qf, visited)
 				empty = false
 			}
-			if !empty && t.types != nil {
-				buf.WriteString("; ")
-			}
-			if t.types != nil {
-				buf.WriteString("type ")
-				writeType(buf, t.types, qf, visited)
-				empty = false
-			}
 			if !empty && len(t.embeddeds) > 0 {
 				buf.WriteString("; ")
 			}
@@ -301,6 +299,7 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 
 	default:
 		// For externally defined implementations of Type.
+		// Note: In this case cycles won't be caught.
 		buf.WriteString(t.String())
 	}
 }
diff --git a/src/go/types/typestring_test.go b/src/go/types/typestring_test.go
index 55ee4b987f..0e35a3dbf1 100644
--- a/src/go/types/typestring_test.go
+++ b/src/go/types/typestring_test.go
@@ -95,6 +95,9 @@ var independentTestTypes = []testEntry{
 	dup("interface{}"),
 	dup("interface{m()}"),
 	dup(`interface{String() string; m(int) float32}`),
+	{"interface{type int, float32, complex128}", "interface{~int|~float32|~complex128}"},
+	dup("interface{int|float32|complex128}"),
+	dup("interface{int|~float32|~complex128}"),
 
 	// TODO(rFindley) uncomment this once this AST is accepted, and add more test
 	// cases.
diff --git a/src/go/types/unify.go b/src/go/types/unify.go
index db06e21cf7..4b541df4cd 100644
--- a/src/go/types/unify.go
+++ b/src/go/types/unify.go
@@ -356,6 +356,10 @@ func (u *unifier) nify(x, y Type, p *ifacePair) bool {
 		// This should not happen with the current internal use of sum types.
 		panic("type inference across sum types not implemented")
 
+	case *Union:
+		// This should not happen with the current internal use of union types.
+		panic("type inference across union 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
diff --git a/src/go/types/union.go b/src/go/types/union.go
new file mode 100644
index 0000000000..0df200c67b
--- /dev/null
+++ b/src/go/types/union.go
@@ -0,0 +1,108 @@
+// 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 types
+
+import (
+	"go/ast"
+	"go/token"
+)
+
+// ----------------------------------------------------------------------------
+// API
+
+// A Union represents a union of terms.
+// A term is a type, possibly with a ~ (tilde) indication.
+type Union struct {
+	terms []Type // terms are unique
+	tilde []bool // if tilde[i] is set, terms[i] is of the form ~T
+}
+
+func NewUnion(terms []Type, tilde []bool) Type { return newUnion(terms, tilde) }
+
+func (u *Union) NumTerms() int           { return len(u.terms) }
+func (u *Union) Term(i int) (Type, bool) { return u.terms[i], u.tilde[i] }
+
+func (u *Union) Underlying() Type { return u }
+func (u *Union) String() string   { return TypeString(u, nil) }
+
+// ----------------------------------------------------------------------------
+// Implementation
+
+func newUnion(terms []Type, tilde []bool) Type {
+	assert(len(terms) == len(tilde))
+	if terms == nil {
+		return nil
+	}
+	t := new(Union)
+	t.terms = terms
+	t.tilde = tilde
+	return t
+}
+
+func parseUnion(check *Checker, tlist []ast.Expr) Type {
+	var terms []Type
+	var tilde []bool
+	for _, x := range tlist {
+		t, d := parseTilde(check, x)
+		if len(tlist) == 1 && !d {
+			return t // single type
+		}
+		terms = append(terms, t)
+		tilde = append(tilde, d)
+	}
+
+	// Ensure that each type is only present once in the type list.
+	// It's ok to do this check at the end because it's not a requirement
+	// for correctness of the code.
+	// Note: This is a quadratic algorithm, but unions tend to be short.
+	check.later(func() {
+		for i, t := range terms {
+			t := expand(t)
+			if t == Typ[Invalid] {
+				continue
+			}
+
+			x := tlist[i]
+			pos := x.Pos()
+			// We may not know the position of x if it was a typechecker-
+			// introduced ~T type of a type list entry T. Use the position
+			// of T instead.
+			// TODO(rfindley) remove this test once we don't support type lists anymore
+			if !pos.IsValid() {
+				if op, _ := x.(*ast.UnaryExpr); op != nil {
+					pos = op.X.Pos()
+				}
+			}
+
+			u := under(t)
+			if tilde[i] {
+				// TODO(rfindley) enable this check once we have converted tests
+				// if !Identical(u, t) {
+				// 	check.errorf(x, "invalid use of ~ (underlying type of %s is %s)", t, u)
+				// }
+			}
+			if _, ok := u.(*Interface); ok {
+				check.errorf(atPos(pos), _Todo, "cannot use interface %s with ~ or inside a union (implementation restriction)", t)
+			}
+
+			// Complain about duplicate entries a|a, but also a|~a, and ~a|~a.
+			if includes(terms[:i], t) {
+				// TODO(rfindley) this currently doesn't print the ~ if present
+				check.softErrorf(atPos(pos), _Todo, "duplicate term %s in union element", t)
+			}
+		}
+	})
+
+	return newUnion(terms, tilde)
+}
+
+func parseTilde(check *Checker, x ast.Expr) (Type, bool) {
+	tilde := false
+	if op, _ := x.(*ast.UnaryExpr); op != nil && op.Op == token.TILDE {
+		x = op.X
+		tilde = true
+	}
+	return check.anyType(x), tilde
+}