[dev.typeparams] cmd/compile/internal/types2: introduce type set abstraction for interfaces

With this change, interfaces are "completed" on-demand, when needed,
and the respective information (set of all methods, type constraints)
is recorded in a new typeSet data structure.

As a consequence, interfaces don't need to be explicitly completed
anymore and (internal) uses of interfaces have become much simpler.

This change also introduces a new field Interface.complete to indicate
that all methods and embedded elements have been set up. This prevent
the computation and recording (!) of a partial type set for erroneous
programs (if we compute the partial type set and store it, subsequent
type set accesses use the wrong type set which may lead to follow-on
errors).

Change-Id: I1ffc907f7d0fb93b3e987fe5ff9c6fa5cae00d7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/329309
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

17 files changed, 205 insertions, 186 deletions

diff --git a/src/cmd/compile/internal/types2/builtins.go b/src/cmd/compile/internal/types2/builtins.go
index 8f2d849ef5..ffe872e7ab 100644
--- a/src/cmd/compile/internal/types2/builtins.go
+++ b/src/cmd/compile/internal/types2/builtins.go
@@ -783,7 +783,7 @@ func (check *Checker) applyTypeFunc(f func(Type) Type, x Type) Type {
 		tpar := NewTypeName(nopos, check.pkg, "<type parameter>", nil)
 		ptyp := check.NewTypeParam(tpar, 0, &emptyInterface) // assigns type to tpar as a side-effect
 		tsum := newUnion(rtypes, tildes)
-		ptyp.bound = &Interface{allMethods: markComplete, allTypes: tsum}
+		ptyp.bound = &Interface{complete: true, tset: &TypeSet{types: tsum}}
 
 		return ptyp
 	}
diff --git a/src/cmd/compile/internal/types2/call.go b/src/cmd/compile/internal/types2/call.go
index 8c717cd1e5..8c17a2f808 100644
--- a/src/cmd/compile/internal/types2/call.go
+++ b/src/cmd/compile/internal/types2/call.go
@@ -99,7 +99,6 @@ func (check *Checker) callExpr(x *operand, call *syntax.CallExpr) exprKind {
 			check.expr(x, call.ArgList[0])
 			if x.mode != invalid {
 				if t := asInterface(T); t != nil {
-					check.completeInterface(nopos, t)
 					if t.IsConstraint() {
 						check.errorf(call, "cannot use interface %s in conversion (contains type list or is comparable)", T)
 						break
diff --git a/src/cmd/compile/internal/types2/expr.go b/src/cmd/compile/internal/types2/expr.go
index d1cb27de16..7fba179e44 100644
--- a/src/cmd/compile/internal/types2/expr.go
+++ b/src/cmd/compile/internal/types2/expr.go
@@ -735,7 +735,6 @@ func (check *Checker) implicitTypeAndValue(x *operand, target Type) (Type, const
 		// Update operand types to the default type rather than the target
 		// (interface) type: values must have concrete dynamic types.
 		// Untyped nil was handled upfront.
-		check.completeInterface(nopos, t)
 		if !t.Empty() {
 			return nil, nil, _InvalidUntypedConversion // cannot assign untyped values to non-empty interfaces
 		}
diff --git a/src/cmd/compile/internal/types2/infer.go b/src/cmd/compile/internal/types2/infer.go
index 63cd63aacc..791e25e9f0 100644
--- a/src/cmd/compile/internal/types2/infer.go
+++ b/src/cmd/compile/internal/types2/infer.go
@@ -321,24 +321,13 @@ func (w *tpWalker) isParameterized(typ Type) (res bool) {
 		return w.isParameterized(t.params) || w.isParameterized(t.results)
 
 	case *Interface:
-		if t.allMethods != nil {
-			// interface is complete - quick test
-			for _, m := range t.allMethods {
-				if w.isParameterized(m.typ) {
-					return true
-				}
+		tset := t.typeSet()
+		for _, m := range tset.methods {
+			if w.isParameterized(m.typ) {
+				return true
 			}
-			return w.isParameterized(t.allTypes)
 		}
-
-		return t.iterate(func(t *Interface) bool {
-			for _, m := range t.methods {
-				if w.isParameterized(m.typ) {
-					return true
-				}
-			}
-			return w.isParameterizedList(t.embeddeds)
-		}, nil)
+		return w.isParameterized(tset.types)
 
 	case *Map:
 		return w.isParameterized(t.key) || w.isParameterized(t.elem)
@@ -476,15 +465,15 @@ func (check *Checker) inferB(tparams []*TypeName, targs []Type, report bool) (ty
 // 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(nopos, iface)
-		if u, _ := iface.allTypes.(*Union); u != nil {
+		types := iface.typeSet().types
+		if u, _ := types.(*Union); u != nil {
 			if u.NumTerms() == 1 {
 				// TODO(gri) do we need to respect tilde?
 				return u.types[0]
 			}
 			return nil
 		}
-		return iface.allTypes
+		return types
 	}
 	return nil
 }
diff --git a/src/cmd/compile/internal/types2/interface.go b/src/cmd/compile/internal/types2/interface.go
index c79026f00d..4dee923422 100644
--- a/src/cmd/compile/internal/types2/interface.go
+++ b/src/cmd/compile/internal/types2/interface.go
@@ -95,9 +95,13 @@ func (check *Checker) interfaceType(ityp *Interface, iface *syntax.InterfaceType
 		check.posMap[ityp] = append(check.posMap[ityp], tlist[0].(*syntax.Operation).X.Pos())
 	}
 
+	// All methods and embedded elements for this interface are collected;
+	// i.e., this interface is may be used in a type set computation.
+	ityp.complete = true
+
 	if len(ityp.methods) == 0 && len(ityp.embeddeds) == 0 {
 		// empty interface
-		ityp.allMethods = markComplete
+		ityp.tset = &topTypeSet
 		return
 	}
 
@@ -105,7 +109,10 @@ func (check *Checker) interfaceType(ityp *Interface, iface *syntax.InterfaceType
 	sortMethods(ityp.methods)
 	sortTypes(ityp.embeddeds)
 
-	check.later(func() { check.completeInterface(iface.Pos(), ityp) })
+	// Compute type set with a non-nil *Checker as soon as possible
+	// to report any errors. Subsequent uses of type sets should be
+	// using this computed type set and won't need to pass in a *Checker.
+	check.later(func() { newTypeSet(check, iface.Pos(), ityp) })
 }
 
 func flattenUnion(list []syntax.Expr, x syntax.Expr) []syntax.Expr {
@@ -116,26 +123,27 @@ func flattenUnion(list []syntax.Expr, x syntax.Expr) []syntax.Expr {
 	return append(list, x)
 }
 
-func (check *Checker) completeInterface(pos syntax.Pos, ityp *Interface) {
-	if ityp.allMethods != nil {
-		return
+// newTypeSet may be called with check == nil.
+// TODO(gri) move this function into typeset.go eventually
+func newTypeSet(check *Checker, pos syntax.Pos, ityp *Interface) *TypeSet {
+	if ityp.tset != nil {
+		return ityp.tset
 	}
 
-	// completeInterface may be called via the LookupFieldOrMethod,
-	// MissingMethod, Identical, or IdenticalIgnoreTags external API
-	// in which case check will be nil. In this case, type-checking
-	// must be finished and all interfaces should have been completed.
-	if check == nil {
-		panic("internal error: incomplete interface")
+	// If the interface is not fully set up yet, the type set will
+	// not be complete, which may lead to errors when using the the
+	// type set (e.g. missing method). Don't compute a partial type
+	// set (and don't store it!), so that we still compute the full
+	// type set eventually. Instead, return the top type set and
+	// let any follow-on errors play out.
+	//
+	// TODO(gri) Consider recording when this happens and reporting
+	// it as an error (but only if there were no other errors so to
+	// to not have unnecessary follow-on errors).
+	if !ityp.complete {
+		return &topTypeSet
 	}
 
-	completeInterface(check, pos, ityp)
-}
-
-// completeInterface may be called with check == nil.
-func completeInterface(check *Checker, pos syntax.Pos, ityp *Interface) {
-	assert(ityp.allMethods == nil)
-
 	if check != nil && check.conf.Trace {
 		// Types don't generally have position information.
 		// If we don't have a valid pos provided, try to use
@@ -144,11 +152,11 @@ func completeInterface(check *Checker, pos syntax.Pos, ityp *Interface) {
 			pos = ityp.methods[0].pos
 		}
 
-		check.trace(pos, "complete %s", ityp)
+		check.trace(pos, "type set for %s", ityp)
 		check.indent++
 		defer func() {
 			check.indent--
-			check.trace(pos, "=> %s (methods = %v, types = %v)", ityp, ityp.allMethods, ityp.allTypes)
+			check.trace(pos, "=> %s ", ityp.typeSet())
 		}()
 	}
 
@@ -157,7 +165,7 @@ func completeInterface(check *Checker, pos syntax.Pos, ityp *Interface) {
 	// have valid interfaces. Mark the interface as complete to avoid
 	// infinite recursion if the validType check occurs later for some
 	// reason.
-	ityp.allMethods = markComplete
+	ityp.tset = new(TypeSet) // TODO(gri) is this sufficient?
 
 	// Methods of embedded interfaces are collected unchanged; i.e., the identity
 	// of a method I.m's Func Object of an interface I is the same as that of
@@ -231,13 +239,11 @@ func completeInterface(check *Checker, pos syntax.Pos, ityp *Interface) {
 		var types Type
 		switch t := under(typ).(type) {
 		case *Interface:
-			if t.allMethods == nil {
-				completeInterface(check, pos, t)
-			}
-			for _, m := range t.allMethods {
+			tset := newTypeSet(check, pos, t)
+			for _, m := range tset.methods {
 				addMethod(pos, m, false) // use embedding position pos rather than m.pos
 			}
-			types = t.allTypes
+			types = tset.types
 		case *Union:
 			// TODO(gri) combine with default case once we have
 			//           converted all tests to new notation and we
@@ -274,9 +280,11 @@ func completeInterface(check *Checker, pos syntax.Pos, ityp *Interface) {
 
 	if methods != nil {
 		sortMethods(methods)
-		ityp.allMethods = methods
+		ityp.tset.methods = methods
 	}
-	ityp.allTypes = allTypes
+	ityp.tset.types = allTypes
+
+	return ityp.tset
 }
 
 func sortTypes(list []Type) {
diff --git a/src/cmd/compile/internal/types2/lookup.go b/src/cmd/compile/internal/types2/lookup.go
index 93ed620449..9fcec44d53 100644
--- a/src/cmd/compile/internal/types2/lookup.go
+++ b/src/cmd/compile/internal/types2/lookup.go
@@ -182,9 +182,7 @@ func (check *Checker) rawLookupFieldOrMethod(T Type, addressable bool, pkg *Pack
 
 			case *Interface:
 				// look for a matching method
-				// TODO(gri) t.allMethods is sorted - use binary search
-				check.completeInterface(nopos, t)
-				if i, m := lookupMethod(t.allMethods, pkg, name); m != nil {
+				if i, m := t.typeSet().LookupMethod(pkg, name); m != nil {
 					assert(m.typ != nil)
 					index = concat(e.index, i)
 					if obj != nil || e.multiples {
@@ -195,7 +193,7 @@ func (check *Checker) rawLookupFieldOrMethod(T Type, addressable bool, pkg *Pack
 				}
 
 			case *TypeParam:
-				if i, m := lookupMethod(t.Bound().allMethods, pkg, name); m != nil {
+				if i, m := t.Bound().typeSet().LookupMethod(pkg, name); m != nil {
 					assert(m.typ != nil)
 					index = concat(e.index, i)
 					if obj != nil || e.multiples {
@@ -307,18 +305,15 @@ func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType b
 // To improve error messages, also report the wrong signature
 // when the method exists on *V instead of V.
 func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method, wrongType *Func) {
-	check.completeInterface(nopos, T)
-
 	// fast path for common case
 	if T.Empty() {
 		return
 	}
 
 	if ityp := asInterface(V); ityp != nil {
-		check.completeInterface(nopos, ityp)
-		// TODO(gri) allMethods is sorted - can do this more efficiently
-		for _, m := range T.allMethods {
-			_, f := lookupMethod(ityp.allMethods, m.pkg, m.name)
+		// TODO(gri) the methods are sorted - could do this more efficiently
+		for _, m := range T.typeSet().methods {
+			_, f := ityp.typeSet().LookupMethod(m.pkg, m.name)
 
 			if f == nil {
 				// if m is the magic method == we're ok (interfaces are comparable)
@@ -356,7 +351,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 	// A concrete type implements T if it implements all methods of T.
 	Vd, _ := deref(V)
 	Vn := asNamed(Vd)
-	for _, m := range T.allMethods {
+	for _, m := range T.typeSet().methods {
 		// TODO(gri) should this be calling lookupFieldOrMethod instead (and why not)?
 		obj, _, _ := check.rawLookupFieldOrMethod(V, false, m.pkg, m.name)
 
diff --git a/src/cmd/compile/internal/types2/predicates.go b/src/cmd/compile/internal/types2/predicates.go
index 66de249044..73af127188 100644
--- a/src/cmd/compile/internal/types2/predicates.go
+++ b/src/cmd/compile/internal/types2/predicates.go
@@ -287,16 +287,8 @@ func (check *Checker) identical0(x, y Type, cmpTags bool, p *ifacePair) bool {
 		// 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 check != nil {
-				check.completeInterface(nopos, x)
-				check.completeInterface(nopos, y)
-			}
-			a := x.allMethods
-			b := y.allMethods
+			a := x.typeSet().methods
+			b := y.typeSet().methods
 			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
diff --git a/src/cmd/compile/internal/types2/sanitize.go b/src/cmd/compile/internal/types2/sanitize.go
index 406b46e574..3d2323a0a2 100644
--- a/src/cmd/compile/internal/types2/sanitize.go
+++ b/src/cmd/compile/internal/types2/sanitize.go
@@ -112,9 +112,11 @@ func (s sanitizer) typ(typ Type) Type {
 	case *Interface:
 		s.funcList(t.methods)
 		s.typeList(t.embeddeds)
-		s.funcList(t.allMethods)
-		if allTypes := s.typ(t.allTypes); allTypes != t.allTypes {
-			t.allTypes = allTypes
+		// TODO(gri) do we need to sanitize type sets?
+		tset := t.typeSet()
+		s.funcList(tset.methods)
+		if types := s.typ(tset.types); types != tset.types {
+			tset.types = types
 		}
 
 	case *Map:
diff --git a/src/cmd/compile/internal/types2/sizeof_test.go b/src/cmd/compile/internal/types2/sizeof_test.go
index 3cb162764c..0b1f7dacad 100644
--- a/src/cmd/compile/internal/types2/sizeof_test.go
+++ b/src/cmd/compile/internal/types2/sizeof_test.go
@@ -28,7 +28,7 @@ func TestSizeof(t *testing.T) {
 		{Tuple{}, 12, 24},
 		{Signature{}, 44, 88},
 		{Union{}, 24, 48},
-		{Interface{}, 52, 104},
+		{Interface{}, 40, 80},
 		{Map{}, 16, 32},
 		{Chan{}, 12, 24},
 		{Named{}, 84, 160},
@@ -49,6 +49,7 @@ func TestSizeof(t *testing.T) {
 		// Misc
 		{Scope{}, 56, 96},
 		{Package{}, 40, 80},
+		{TypeSet{}, 20, 40},
 	}
 
 	for _, test := range tests {
diff --git a/src/cmd/compile/internal/types2/subst.go b/src/cmd/compile/internal/types2/subst.go
index 3ef65c2e92..38bd07b8a2 100644
--- a/src/cmd/compile/internal/types2/subst.go
+++ b/src/cmd/compile/internal/types2/subst.go
@@ -136,6 +136,7 @@ func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, poslis
 // 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) bool {
 	iface := tpar.Bound()
 	if iface.Empty() {
@@ -150,8 +151,7 @@ func (check *Checker) satisfies(pos syntax.Pos, targ Type, tpar *TypeParam, smap
 
 	// targ must implement iface (methods)
 	// - check only if we have methods
-	check.completeInterface(nopos, iface)
-	if len(iface.allMethods) > 0 {
+	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)
@@ -182,7 +182,7 @@ func (check *Checker) satisfies(pos syntax.Pos, targ Type, tpar *TypeParam, smap
 	}
 
 	// targ's underlying type must also be one of the interface types listed, if any
-	if iface.allTypes == nil {
+	if iface.typeSet().types == nil {
 		return true // nothing to do
 	}
 
@@ -190,7 +190,7 @@ func (check *Checker) satisfies(pos syntax.Pos, targ Type, tpar *TypeParam, smap
 	// 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.Bound()
-		if targBound.allTypes == nil {
+		if targBound.typeSet().types == nil {
 			check.softErrorf(pos, "%s does not satisfy %s (%s has no type constraints)", targ, tpar.bound, targ)
 			return false
 		}
@@ -198,7 +198,7 @@ func (check *Checker) satisfies(pos syntax.Pos, targ Type, tpar *TypeParam, smap
 			// TODO(gri) incorporate tilde information!
 			if !iface.isSatisfiedBy(typ) {
 				// TODO(gri) match this error message with the one below (or vice versa)
-				check.softErrorf(pos, "%s does not satisfy %s (%s type constraint %s not found in %s)", targ, tpar.bound, targ, typ, iface.allTypes)
+				check.softErrorf(pos, "%s does not satisfy %s (%s type constraint %s not found in %s)", targ, tpar.bound, targ, typ, iface.typeSet().types)
 				return false
 			}
 			return true
@@ -207,7 +207,7 @@ func (check *Checker) satisfies(pos syntax.Pos, targ Type, tpar *TypeParam, smap
 
 	// Otherwise, targ's type or underlying type must also be one of the interface types listed, if any.
 	if !iface.isSatisfiedBy(targ) {
-		check.softErrorf(pos, "%s does not satisfy %s (%s not found in %s)", targ, tpar.bound, targ, iface.allTypes)
+		check.softErrorf(pos, "%s does not satisfy %s (%s not found in %s)", targ, tpar.bound, targ, iface.typeSet().types)
 		return false
 	}
 
@@ -312,12 +312,11 @@ func (subst *subster) typ(typ Type) Type {
 		methods, mcopied := subst.funcList(t.methods)
 		embeddeds, ecopied := subst.typeList(t.embeddeds)
 		if mcopied || ecopied {
-			iface := &Interface{methods: methods, embeddeds: embeddeds}
+			iface := &Interface{methods: methods, embeddeds: embeddeds, complete: t.complete}
 			if subst.check == nil {
 				panic("internal error: cannot instantiate interfaces yet")
 			}
 			subst.check.posMap[iface] = subst.check.posMap[t] // satisfy completeInterface requirement
-			subst.check.completeInterface(nopos, iface)
 			return iface
 		}
 
diff --git a/src/cmd/compile/internal/types2/testdata/check/cycles4.src b/src/cmd/compile/internal/types2/testdata/check/cycles4.src
index 445babca68..924aabf475 100644
--- a/src/cmd/compile/internal/types2/testdata/check/cycles4.src
+++ b/src/cmd/compile/internal/types2/testdata/check/cycles4.src
@@ -4,6 +4,8 @@
 
 package p
 
+import "unsafe"
+
 // Check that all methods of T are collected before
 // determining the result type of m (which embeds
 // all methods of T).
@@ -13,7 +15,7 @@ type T interface {
 	E
 }
 
-var _ = T.m(nil).m().e()
+var _ int = T.m(nil).m().e()
 
 type E interface {
 	e() int
@@ -22,7 +24,7 @@ type E interface {
 // Check that unresolved forward chains are followed
 // (see also comment in resolver.go, checker.typeDecl).
 
-var _ = C.m(nil).m().e()
+var _ int = C.m(nil).m().e()
 
 type A B
 
@@ -108,3 +110,12 @@ type Element interface {
 type Event interface {
 	Target() Element
 }
+
+// Check that accessing an interface method too early doesn't lead
+// to follow-on errors due to an incorrectly computed type set.
+
+type T8 interface {
+	m() [unsafe.Sizeof(T8.m /* ERROR undefined */ )]int
+}
+
+var _ = T8.m // no error expected here
diff --git a/src/cmd/compile/internal/types2/type.go b/src/cmd/compile/internal/types2/type.go
index 10cb651d0c..122e408ead 100644
--- a/src/cmd/compile/internal/types2/type.go
+++ b/src/cmd/compile/internal/types2/type.go
@@ -264,18 +264,20 @@ func (s *Signature) Variadic() bool { return s.variadic }
 
 // An Interface represents an interface type.
 type Interface struct {
+	obj       Object  // type name object defining this interface; or nil (for better error messages)
 	methods   []*Func // ordered list of explicitly declared methods
-	embeddeds []Type  // ordered list of explicitly embedded types
+	embeddeds []Type  // ordered list of explicitly embedded elements
+	complete  bool    // indicates that obj, methods, and embeddeds are set and type set can be computed
 
-	allMethods []*Func // ordered list of methods declared with or embedded in this interface (TODO(gri): replace with mset)
-	allTypes   Type    // intersection of all embedded and locally declared types  (TODO(gri) need better field name)
-
-	obj Object // type declaration defining this interface; or nil (for better error messages)
+	tset *TypeSet // type set described by this interface, computed lazily
 }
 
+// typeSet returns the type set for interface t.
+func (t *Interface) typeSet() *TypeSet { return newTypeSet(nil, nopos, t) }
+
 // is reports whether interface t represents types that all satisfy f.
 func (t *Interface) is(f func(Type, bool) bool) bool {
-	switch t := t.allTypes.(type) {
+	switch t := t.typeSet().types.(type) {
 	case nil, *top:
 		// TODO(gri) should settle on top or nil to represent this case
 		return false // we must have at least one type! (was bug)
@@ -286,21 +288,14 @@ func (t *Interface) is(f func(Type, bool) bool) bool {
 	}
 }
 
-// emptyInterface represents the empty (completed) interface
-var emptyInterface = Interface{allMethods: markComplete}
-
-// markComplete is used to mark an empty interface as completely
-// set up by setting the allMethods field to a non-nil empty slice.
-var markComplete = make([]*Func, 0)
+// emptyInterface represents the empty interface
+var emptyInterface = Interface{complete: true, tset: &topTypeSet}
 
-// NewInterface returns a new (incomplete) interface for the given methods and embedded types.
-// Each embedded type must have an underlying type of interface type.
-// NewInterface takes ownership of the provided methods and may modify their types by setting
-// missing receivers. To compute the method set of the interface, Complete must be called.
+// NewInterface returns a new interface for the given methods and embedded types.
+// NewInterface takes ownership of the provided methods and may modify their types
+// by setting missing receivers.
 //
-// Deprecated: Use NewInterfaceType instead which allows any (even non-defined) interface types
-// to be embedded. This is necessary for interfaces that embed alias type names referring to
-// non-defined (literal) interface types.
+// Deprecated: Use NewInterfaceType instead which allows arbitrary embedded types.
 func NewInterface(methods []*Func, embeddeds []*Named) *Interface {
 	tnames := make([]Type, len(embeddeds))
 	for i, t := range embeddeds {
@@ -309,9 +304,9 @@ func NewInterface(methods []*Func, embeddeds []*Named) *Interface {
 	return NewInterfaceType(methods, tnames)
 }
 
-// NewInterfaceType returns a new (incomplete) interface for the given methods and embedded types.
-// NewInterfaceType takes ownership of the provided methods and may modify their types by setting
-// missing receivers. To compute the method set of the interface, Complete must be called.
+// NewInterfaceType returns a new interface for the given methods and embedded types.
+// NewInterfaceType takes ownership of the provided methods and may modify their types
+// by setting missing receivers.
 func NewInterfaceType(methods []*Func, embeddeds []Type) *Interface {
 	if len(methods) == 0 && len(embeddeds) == 0 {
 		return &emptyInterface
@@ -331,6 +326,8 @@ func NewInterfaceType(methods []*Func, embeddeds []Type) *Interface {
 
 	typ.methods = methods
 	typ.embeddeds = embeddeds
+	typ.complete = true
+
 	return typ
 }
 
@@ -354,72 +351,27 @@ func (t *Interface) Embedded(i int) *Named { tname, _ := t.embeddeds[i].(*Named)
 func (t *Interface) EmbeddedType(i int) Type { return t.embeddeds[i] }
 
 // NumMethods returns the total number of methods of interface t.
-// The interface must have been completed.
-func (t *Interface) NumMethods() int { t.Complete(); return len(t.allMethods) }
+func (t *Interface) NumMethods() int { return t.typeSet().NumMethods() }
 
 // Method returns the i'th method of interface t for 0 <= i < t.NumMethods().
 // The methods are ordered by their unique Id.
-// The interface must have been completed.
-func (t *Interface) Method(i int) *Func { t.Complete(); return t.allMethods[i] }
+func (t *Interface) Method(i int) *Func { return t.typeSet().Method(i) }
 
 // Empty reports whether t is the empty interface.
-func (t *Interface) Empty() bool {
-	t.Complete()
-	return len(t.allMethods) == 0 && t.allTypes == nil
-}
-
-// HasTypeList reports whether interface t has a type list, possibly from an embedded type.
-func (t *Interface) HasTypeList() bool {
-	t.Complete()
-	return t.allTypes != nil
-}
+func (t *Interface) Empty() bool { return t.typeSet().IsTop() }
 
 // IsComparable reports whether interface t is or embeds the predeclared interface "comparable".
-func (t *Interface) IsComparable() bool {
-	t.Complete()
-	_, m := lookupMethod(t.allMethods, nil, "==")
-	return m != nil
-}
+func (t *Interface) IsComparable() bool { return t.typeSet().IsComparable() }
 
-// IsConstraint reports t.HasTypeList() || t.IsComparable().
-func (t *Interface) IsConstraint() bool {
-	return t.HasTypeList() || t.IsComparable()
-}
-
-// iterate calls f with t and then with any embedded interface of t, recursively, until f returns true.
-// iterate reports whether any call to f returned true.
-// TODO(gri) This is now only used by infer.go - see if we can eliminate it.
-func (t *Interface) iterate(f func(*Interface) bool, seen map[*Interface]bool) bool {
-	if f(t) {
-		return true
-	}
-	for _, e := range t.embeddeds {
-		// e should be an interface but be careful (it may be invalid)
-		if e := asInterface(e); e != nil {
-			// Cyclic interfaces such as "type E interface { E }" are not permitted
-			// but they are still constructed and we need to detect such cycles.
-			if seen[e] {
-				continue
-			}
-			if seen == nil {
-				seen = make(map[*Interface]bool)
-			}
-			seen[e] = true
-			if e.iterate(f, seen) {
-				return true
-			}
-		}
-	}
-	return false
-}
+// IsConstraint reports whether interface t is not just a method set.
+func (t *Interface) IsConstraint() bool { return !t.typeSet().IsMethodSet() }
 
 // isSatisfiedBy reports whether interface t's type list is satisfied by the type typ.
 // If the type list is empty (absent), typ trivially satisfies the interface.
 // TODO(gri) This is not a great name. Eventually, we should have a more comprehensive
 //           "implements" predicate.
 func (t *Interface) isSatisfiedBy(typ Type) bool {
-	t.Complete()
-	switch t := t.allTypes.(type) {
+	switch t := t.typeSet().types.(type) {
 	case nil:
 		return true // no type restrictions
 	case *Union:
@@ -430,15 +382,22 @@ func (t *Interface) isSatisfiedBy(typ Type) bool {
 	}
 }
 
-// Complete computes the interface's method set. It must be called by users of
+// Complete computes the interface's type set. It must be called by users of
 // NewInterfaceType and NewInterface after the interface's embedded types are
 // fully defined and before using the interface type in any way other than to
 // form other types. The interface must not contain duplicate methods or a
 // panic occurs. Complete returns the receiver.
+//
+// Deprecated: Type sets are now computed lazily, on demand; this function
+//             is only here for backward-compatibility. It does not have to
+//             be called explicitly anymore.
 func (t *Interface) Complete() *Interface {
-	if t.allMethods == nil {
-		completeInterface(nil, nopos, t)
-	}
+	// Some tests are still depending on the state change
+	// (string representation of an Interface not containing an
+	// /* incomplete */ marker) caused by the explicit Complete
+	// call, so we compute the type set eagerly here.
+	t.complete = true
+	t.typeSet()
 	return t
 }
 
@@ -674,7 +633,7 @@ func (t *TypeParam) Bound() *Interface {
 		pos = n.obj.pos
 	}
 	// TODO(gri) switch this to an unexported method on Checker.
-	t.check.completeInterface(pos, iface)
+	newTypeSet(t.check, pos, iface)
 	return iface
 }
 
@@ -700,7 +659,7 @@ func optype(typ Type) Type {
 		// for a type parameter list of the form:
 		// (type T interface { type T }).
 		// See also issue #39680.
-		if a := t.Bound().allTypes; a != nil {
+		if a := t.Bound().typeSet().types; a != nil {
 			// If we have a union with a single entry, ignore
 			// any tilde because under(~t) == under(t).
 			if u, _ := a.(*Union); u != nil && u.NumTerms() == 1 {
diff --git a/src/cmd/compile/internal/types2/typeset.go b/src/cmd/compile/internal/types2/typeset.go
new file mode 100644
index 0000000000..44e5929413
--- /dev/null
+++ b/src/cmd/compile/internal/types2/typeset.go
@@ -0,0 +1,70 @@
+// 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 types2
+
+import (
+	"bytes"
+)
+
+// topTypeSet may be used as type set for the empty interface.
+var topTypeSet TypeSet
+
+// A TypeSet represents the type set of an interface.
+type TypeSet struct {
+	// TODO(gri) consider using a set for the methods for faster lookup
+	methods []*Func // all methods of the interface; sorted by unique ID
+	types   Type    // typically a *Union; nil means no type restrictions
+}
+
+func (s *TypeSet) String() string {
+	if s.IsTop() {
+		return "⊤"
+	}
+
+	var buf bytes.Buffer
+	buf.WriteByte('{')
+	for i, m := range s.methods {
+		if i > 0 {
+			buf.WriteByte(';')
+		}
+		buf.WriteByte(' ')
+		buf.WriteString(m.String())
+	}
+	if len(s.methods) > 0 && s.types != nil {
+		buf.WriteByte(';')
+	}
+	if s.types != nil {
+		buf.WriteByte(' ')
+		writeType(&buf, s.types, nil, nil)
+	}
+
+	buf.WriteString(" }") // there was a least one method or type
+	return buf.String()
+}
+
+// IsTop reports whether type set s is the top type set (corresponding to the empty interface).
+func (s *TypeSet) IsTop() bool { return len(s.methods) == 0 && s.types == nil }
+
+// IsMethodSet reports whether the type set s is described by a single set of methods.
+func (s *TypeSet) IsMethodSet() bool { return s.types == nil && !s.IsComparable() }
+
+// IsComparable reports whether each type in the set is comparable.
+func (s *TypeSet) IsComparable() bool {
+	_, m := s.LookupMethod(nil, "==")
+	return m != nil
+}
+
+// NumMethods returns the number of methods available.
+func (s *TypeSet) NumMethods() int { return len(s.methods) }
+
+// Method returns the i'th method of type set s for 0 <= i < s.NumMethods().
+// The methods are ordered by their unique ID.
+func (s *TypeSet) Method(i int) *Func { return s.methods[i] }
+
+// LookupMethod returns the index of and method with matching package and name, or (-1, nil).
+func (s *TypeSet) LookupMethod(pkg *Package, name string) (int, *Func) {
+	// TODO(gri) s.methods is sorted - consider binary search
+	return lookupMethod(s.methods, pkg, name)
+}
diff --git a/src/cmd/compile/internal/types2/typestring.go b/src/cmd/compile/internal/types2/typestring.go
index f08c41c2a3..4925252b39 100644
--- a/src/cmd/compile/internal/types2/typestring.go
+++ b/src/cmd/compile/internal/types2/typestring.go
@@ -189,7 +189,8 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 		if gcCompatibilityMode {
 			// print flattened interface
 			// (useful to compare against gc-generated interfaces)
-			for i, m := range t.allMethods {
+			tset := t.typeSet()
+			for i, m := range tset.methods {
 				if i > 0 {
 					buf.WriteString("; ")
 				}
@@ -197,12 +198,12 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 				writeSignature(buf, m.typ.(*Signature), qf, visited)
 				empty = false
 			}
-			if !empty && t.allTypes != nil {
+			if !empty && tset.types != nil {
 				buf.WriteString("; ")
 			}
-			if t.allTypes != nil {
+			if tset.types != nil {
 				buf.WriteString("type ")
-				writeType(buf, t.allTypes, qf, visited)
+				writeType(buf, tset.types, qf, visited)
 			}
 		} else {
 			// print explicit interface methods and embedded types
@@ -225,7 +226,9 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 				empty = false
 			}
 		}
-		if debug && (t.allMethods == nil || len(t.methods) > len(t.allMethods)) {
+		// print /* incomplete */ if needed to satisfy existing tests
+		// TODO(gri) get rid of this eventually
+		if debug && t.tset == nil {
 			if !empty {
 				buf.WriteByte(' ')
 			}
diff --git a/src/cmd/compile/internal/types2/typexpr.go b/src/cmd/compile/internal/types2/typexpr.go
index 583bb464b2..fe676be2ef 100644
--- a/src/cmd/compile/internal/types2/typexpr.go
+++ b/src/cmd/compile/internal/types2/typexpr.go
@@ -141,12 +141,12 @@ func (check *Checker) ordinaryType(pos syntax.Pos, typ Type) {
 	// interface methods. Delay this check to the end of type-checking.
 	check.later(func() {
 		if t := asInterface(typ); t != nil {
-			check.completeInterface(pos, t) // TODO(gri) is this the correct position?
-			if t.allTypes != nil {
-				check.softErrorf(pos, "interface contains type constraints (%s)", t.allTypes)
+			tset := newTypeSet(check, pos, t) // TODO(gri) is this the correct position?
+			if tset.types != nil {
+				check.softErrorf(pos, "interface contains type constraints (%s)", tset.types)
 				return
 			}
-			if t.IsComparable() {
+			if tset.IsComparable() {
 				check.softErrorf(pos, "interface is (or embeds) comparable")
 			}
 		}
diff --git a/src/cmd/compile/internal/types2/unify.go b/src/cmd/compile/internal/types2/unify.go
index e5983dd40c..9a51dcb6d4 100644
--- a/src/cmd/compile/internal/types2/unify.go
+++ b/src/cmd/compile/internal/types2/unify.go
@@ -361,16 +361,8 @@ func (u *unifier) nify(x, y Type, p *ifacePair) bool {
 		// 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(nopos, x)
-				u.check.completeInterface(nopos, y)
-			}
-			a := x.allMethods
-			b := y.allMethods
+			a := x.typeSet().methods
+			b := y.typeSet().methods
 			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
diff --git a/src/cmd/compile/internal/types2/universe.go b/src/cmd/compile/internal/types2/universe.go
index 76d4e55e84..2bcc49778e 100644
--- a/src/cmd/compile/internal/types2/universe.go
+++ b/src/cmd/compile/internal/types2/universe.go
@@ -88,7 +88,7 @@ func defPredeclaredTypes() {
 		res := NewVar(nopos, nil, "", Typ[String])
 		sig := &Signature{results: NewTuple(res)}
 		err := NewFunc(nopos, nil, "Error", sig)
-		typ := &Named{underlying: NewInterfaceType([]*Func{err}, nil).Complete()}
+		typ := &Named{underlying: NewInterfaceType([]*Func{err}, nil)}
 		sig.recv = NewVar(nopos, nil, "", typ)
 		def(NewTypeName(nopos, nil, "error", typ))
 	}
@@ -216,7 +216,7 @@ func defPredeclaredComparable() {
 	// set up later to match the usual interface method assumptions.
 	sig := new(Signature)
 	eql := NewFunc(nopos, nil, "==", sig)
-	iface := NewInterfaceType([]*Func{eql}, nil).Complete()
+	iface := NewInterfaceType([]*Func{eql}, nil)
 
 	// set up the defined type for the interface
 	obj := NewTypeName(nopos, nil, "comparable", nil)