[dev.typeparams] cmd/compile/internal/types2: remove Type.Under method in favor of function

This removes the need for the aType embedded type and brings the types2.Type
API in sync with the go/types.Type API.

For reasons not fully understood yet, introducing the new under function
causes a very long initialization cycle error, which doesn't exist in
go/types. For now, circumvent the problem through a helper function variable.

This CL also eliminates superflous (former) Under() method calls
inside optype calls (optype takes care of this).

Plus some minor misc. cleanups and comment adjustments.

Change-Id: I86e13ccf6f0b34d7496240ace61a1c84856b6033
Reviewed-on: https://go-review.googlesource.com/c/go/+/293470
Reviewed-by: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>

16 files changed, 113 insertions, 149 deletions

diff --git a/src/cmd/compile/internal/types2/builtins.go b/src/cmd/compile/internal/types2/builtins.go
index 16e294d226..a6a9b51dd1 100644
--- a/src/cmd/compile/internal/types2/builtins.go
+++ b/src/cmd/compile/internal/types2/builtins.go
@@ -142,7 +142,7 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 		mode := invalid
 		var typ Type
 		var val constant.Value
-		switch typ = implicitArrayDeref(optype(x.typ.Under())); t := typ.(type) {
+		switch typ = implicitArrayDeref(optype(x.typ)); t := typ.(type) {
 		case *Basic:
 			if isString(t) && id == _Len {
 				if x.mode == constant_ {
@@ -178,7 +178,7 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 
 		case *Sum:
 			if t.is(func(t Type) bool {
-				switch t := t.Under().(type) {
+				switch t := under(t).(type) {
 				case *Basic:
 					if isString(t) && id == _Len {
 						return true
@@ -330,7 +330,7 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 			return
 		}
 		var src Type
-		switch t := optype(y.typ.Under()).(type) {
+		switch t := optype(y.typ).(type) {
 		case *Basic:
 			if isString(y.typ) {
 				src = universeByte
@@ -453,7 +453,7 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 		var valid func(t Type) bool
 		valid = func(t Type) bool {
 			var m int
-			switch t := optype(t.Under()).(type) {
+			switch t := optype(t).(type) {
 			case *Slice:
 				m = 2
 			case *Map, *Chan:
diff --git a/src/cmd/compile/internal/types2/conversions.go b/src/cmd/compile/internal/types2/conversions.go
index c9603b263c..dc0621919e 100644
--- a/src/cmd/compile/internal/types2/conversions.go
+++ b/src/cmd/compile/internal/types2/conversions.go
@@ -90,8 +90,8 @@ func (x *operand) convertibleTo(check *Checker, T Type) bool {
 
 	// "x's type and T have identical underlying types if tags are ignored"
 	V := x.typ
-	Vu := V.Under()
-	Tu := T.Under()
+	Vu := under(V)
+	Tu := under(T)
 	if check.identicalIgnoreTags(Vu, Tu) {
 		return true
 	}
@@ -100,7 +100,7 @@ func (x *operand) convertibleTo(check *Checker, T Type) bool {
 	// have identical underlying types if tags are ignored"
 	if V, ok := V.(*Pointer); ok {
 		if T, ok := T.(*Pointer); ok {
-			if check.identicalIgnoreTags(V.base.Under(), T.base.Under()) {
+			if check.identicalIgnoreTags(under(V.base), under(T.base)) {
 				return true
 			}
 		}
@@ -146,7 +146,7 @@ func isUintptr(typ Type) bool {
 
 func isUnsafePointer(typ Type) bool {
 	// TODO(gri): Is this asBasic(typ) instead of typ.(*Basic) correct?
-	//            (The former calls typ.Under(), while the latter doesn't.)
+	//            (The former calls under(), while the latter doesn't.)
 	//            The spec does not say so, but gc claims it is. See also
 	//            issue 6326.
 	t := asBasic(typ)
diff --git a/src/cmd/compile/internal/types2/decl.go b/src/cmd/compile/internal/types2/decl.go
index e9fc08df37..677172d40f 100644
--- a/src/cmd/compile/internal/types2/decl.go
+++ b/src/cmd/compile/internal/types2/decl.go
@@ -445,7 +445,7 @@ func (check *Checker) constDecl(obj *Const, typ, init syntax.Expr, inherited boo
 		if !isConstType(t) {
 			// don't report an error if the type is an invalid C (defined) type
 			// (issue #22090)
-			if t.Under() != Typ[Invalid] {
+			if under(t) != Typ[Invalid] {
 				check.errorf(typ, "invalid constant type %s", t)
 			}
 			obj.typ = Typ[Invalid]
@@ -545,13 +545,13 @@ func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init syntax.Expr) {
 	check.initVars(lhs, []syntax.Expr{init}, nopos)
 }
 
-// Under returns the expanded underlying type of n0; possibly by following
+// under returns the expanded underlying type of n0; possibly by following
 // forward chains of named types. If an underlying type is found, resolve
 // the chain by setting the underlying type for each defined type in the
 // chain before returning it. If no underlying type is found or a cycle
 // is detected, the result is Typ[Invalid]. If a cycle is detected and
 // n0.check != nil, the cycle is reported.
-func (n0 *Named) Under() Type {
+func (n0 *Named) under() Type {
 	u := n0.underlying
 	if u == nil {
 		return Typ[Invalid]
@@ -584,6 +584,8 @@ func (n0 *Named) Under() Type {
 
 		if i, ok := seen[n]; ok {
 			// cycle
+			// TODO(gri) revert this to a method on Checker. Having a possibly
+			// nil Checker on Named and TypeParam is too subtle.
 			if n0.check != nil {
 				n0.check.cycleError(path[i:])
 			}
@@ -667,7 +669,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named
 		// any forward chain.
 		// TODO(gri) Investigate if we can just use named.origin here
 		//           and rely on lazy computation of the underlying type.
-		named.underlying = named.Under()
+		named.underlying = under(named)
 	}
 
 }
@@ -716,7 +718,7 @@ func (check *Checker) collectTypeParams(list []*syntax.Field) (tparams []*TypeNa
 		//           we may not have a complete interface yet:
 		//           type C(type T C) interface {}
 		//           (issue #39724).
-		if _, ok := bound.Under().(*Interface); ok {
+		if _, ok := under(bound).(*Interface); ok {
 			// set the type bounds
 			for i < j {
 				tparams[i].typ.(*TypeParam).bound = bound
diff --git a/src/cmd/compile/internal/types2/expr.go b/src/cmd/compile/internal/types2/expr.go
index 57c8896e0d..a284c8c8b6 100644
--- a/src/cmd/compile/internal/types2/expr.go
+++ b/src/cmd/compile/internal/types2/expr.go
@@ -656,7 +656,7 @@ func (check *Checker) convertUntypedInternal(x *operand, target Type) {
 	}
 
 	// typed target
-	switch t := optype(target.Under()).(type) {
+	switch t := optype(target).(type) {
 	case *Basic:
 		if x.mode == constant_ {
 			check.representable(x, t)
@@ -1258,7 +1258,7 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 		case hint != nil:
 			// no composite literal type present - use hint (element type of enclosing type)
 			typ = hint
-			base, _ = deref(typ.Under()) // *T implies &T{}
+			base, _ = deref(under(typ)) // *T implies &T{}
 
 		default:
 			// TODO(gri) provide better error messages depending on context
@@ -1266,7 +1266,7 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 			goto Error
 		}
 
-		switch utyp := optype(base.Under()).(type) {
+		switch utyp := optype(base).(type) {
 		case *Struct:
 			if len(e.ElemList) == 0 {
 				break
@@ -1475,7 +1475,7 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 		// ordinary index expression
 		valid := false
 		length := int64(-1) // valid if >= 0
-		switch typ := optype(x.typ.Under()).(type) {
+		switch typ := optype(x.typ).(type) {
 		case *Basic:
 			if isString(typ) {
 				valid = true
@@ -1528,7 +1528,7 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 			nmaps := 0           // number of map types in sum type
 			if typ.is(func(t Type) bool {
 				var e Type
-				switch t := t.Under().(type) {
+				switch t := under(t).(type) {
 				case *Basic:
 					if isString(t) {
 						e = universeByte
@@ -1637,7 +1637,7 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 
 		valid := false
 		length := int64(-1) // valid if >= 0
-		switch typ := optype(x.typ.Under()).(type) {
+		switch typ := optype(x.typ).(type) {
 		case *Basic:
 			if isString(typ) {
 				if e.Full {
@@ -1738,7 +1738,7 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 		if x.mode == invalid {
 			goto Error
 		}
-		xtyp, _ := x.typ.Under().(*Interface)
+		xtyp, _ := under(x.typ).(*Interface)
 		if xtyp == nil {
 			check.errorf(x, "%s is not an interface type", x)
 			goto Error
diff --git a/src/cmd/compile/internal/types2/infer.go b/src/cmd/compile/internal/types2/infer.go
index 09d099e625..061b919239 100644
--- a/src/cmd/compile/internal/types2/infer.go
+++ b/src/cmd/compile/internal/types2/infer.go
@@ -289,7 +289,7 @@ func (check *Checker) inferB(tparams []*TypeName, targs []Type) (types []Type, i
 	// Unify type parameters with their structural constraints, if any.
 	for _, tpar := range tparams {
 		typ := tpar.typ.(*TypeParam)
-		sbound := check.structuralType(typ.bound.Under())
+		sbound := check.structuralType(typ.bound)
 		if sbound != nil {
 			if !u.unify(typ, sbound) {
 				check.errorf(tpar.pos, "%s does not match %s", tpar, sbound)
@@ -344,7 +344,7 @@ func (check *Checker) inferB(tparams []*TypeName, targs []Type) (types []Type, i
 
 // structuralType returns the structural type of a constraint, if any.
 func (check *Checker) structuralType(constraint Type) Type {
-	if iface, _ := constraint.(*Interface); iface != nil {
+	if iface, _ := under(constraint).(*Interface); iface != nil {
 		check.completeInterface(nopos, iface)
 		types := unpack(iface.allTypes)
 		if len(types) == 1 {
diff --git a/src/cmd/compile/internal/types2/lookup.go b/src/cmd/compile/internal/types2/lookup.go
index e210850ba0..34d18acdfc 100644
--- a/src/cmd/compile/internal/types2/lookup.go
+++ b/src/cmd/compile/internal/types2/lookup.go
@@ -141,7 +141,7 @@ func (check *Checker) rawLookupFieldOrMethod(T Type, addressable bool, pkg *Pack
 
 				// continue with underlying type, but only if it's not a type parameter
 				// TODO(gri) is this what we want to do for type parameters? (spec question)
-				typ = named.Under()
+				typ = under(named)
 				if asTypeParam(typ) != nil {
 					continue
 				}
diff --git a/src/cmd/compile/internal/types2/object.go b/src/cmd/compile/internal/types2/object.go
index 956646499a..844bc34b6a 100644
--- a/src/cmd/compile/internal/types2/object.go
+++ b/src/cmd/compile/internal/types2/object.go
@@ -461,7 +461,7 @@ func writeObject(buf *bytes.Buffer, obj Object, qf Qualifier) {
 		if tname.IsAlias() {
 			buf.WriteString(" =")
 		} else {
-			typ = typ.Under()
+			typ = under(typ)
 		}
 	}
 
diff --git a/src/cmd/compile/internal/types2/operand.go b/src/cmd/compile/internal/types2/operand.go
index 238c9b8ee0..001e905a7b 100644
--- a/src/cmd/compile/internal/types2/operand.go
+++ b/src/cmd/compile/internal/types2/operand.go
@@ -257,8 +257,8 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) bool {
 		return true
 	}
 
-	Vu := optype(V.Under())
-	Tu := optype(T.Under())
+	Vu := optype(V)
+	Tu := optype(T)
 
 	// x is an untyped value representable by a value of type T
 	// TODO(gri) This is borrowing from checker.convertUntyped and
diff --git a/src/cmd/compile/internal/types2/predicates.go b/src/cmd/compile/internal/types2/predicates.go
index a48e72b9c4..ae186a0b5d 100644
--- a/src/cmd/compile/internal/types2/predicates.go
+++ b/src/cmd/compile/internal/types2/predicates.go
@@ -25,7 +25,7 @@ func isGeneric(typ Type) bool {
 }
 
 func is(typ Type, what BasicInfo) bool {
-	switch t := optype(typ.Under()).(type) {
+	switch t := optype(typ).(type) {
 	case *Basic:
 		return t.info&what != 0
 	case *Sum:
@@ -73,7 +73,7 @@ func isOrdered(typ Type) bool { return is(typ, IsOrdered) }
 
 func isConstType(typ Type) bool {
 	// Type parameters are never const types.
-	t, _ := typ.Under().(*Basic)
+	t, _ := under(typ).(*Basic)
 	return t != nil && t.info&IsConstType != 0
 }
 
@@ -108,7 +108,7 @@ func comparable(T Type, seen map[Type]bool) bool {
 		return t.Bound().IsComparable()
 	}
 
-	switch t := optype(T.Under()).(type) {
+	switch t := optype(T).(type) {
 	case *Basic:
 		// assume invalid types to be comparable
 		// to avoid follow-up errors
@@ -137,7 +137,7 @@ func comparable(T Type, seen map[Type]bool) bool {
 
 // hasNil reports whether a type includes the nil value.
 func hasNil(typ Type) bool {
-	switch t := optype(typ.Under()).(type) {
+	switch t := optype(typ).(type) {
 	case *Basic:
 		return t.kind == UnsafePointer
 	case *Slice, *Pointer, *Signature, *Interface, *Map, *Chan:
diff --git a/src/cmd/compile/internal/types2/sizes.go b/src/cmd/compile/internal/types2/sizes.go
index 9d8f3ae5ad..aa0fbf40fc 100644
--- a/src/cmd/compile/internal/types2/sizes.go
+++ b/src/cmd/compile/internal/types2/sizes.go
@@ -48,7 +48,7 @@ type StdSizes struct {
 func (s *StdSizes) Alignof(T Type) int64 {
 	// For arrays and structs, alignment is defined in terms
 	// of alignment of the elements and fields, respectively.
-	switch t := optype(T.Under()).(type) {
+	switch t := optype(T).(type) {
 	case *Array:
 		// spec: "For a variable x of array type: unsafe.Alignof(x)
 		// is the same as unsafe.Alignof(x[0]), but at least 1."
@@ -118,7 +118,7 @@ var basicSizes = [...]byte{
 }
 
 func (s *StdSizes) Sizeof(T Type) int64 {
-	switch t := optype(T.Under()).(type) {
+	switch t := optype(T).(type) {
 	case *Basic:
 		assert(isTyped(T))
 		k := t.kind
diff --git a/src/cmd/compile/internal/types2/stmt.go b/src/cmd/compile/internal/types2/stmt.go
index 9d74e0e588..490cd0fc19 100644
--- a/src/cmd/compile/internal/types2/stmt.go
+++ b/src/cmd/compile/internal/types2/stmt.go
@@ -680,7 +680,7 @@ func (check *Checker) typeSwitchStmt(inner stmtContext, s *syntax.SwitchStmt, gu
 	if x.mode == invalid {
 		return
 	}
-	xtyp, _ := x.typ.Under().(*Interface)
+	xtyp, _ := under(x.typ).(*Interface)
 	if xtyp == nil {
 		check.errorf(&x, "%s is not an interface type", &x)
 		return
@@ -769,7 +769,7 @@ func (check *Checker) rangeStmt(inner stmtContext, s *syntax.ForStmt, rclause *s
 	// determine key/value types
 	var key, val Type
 	if x.mode != invalid {
-		typ := optype(x.typ.Under())
+		typ := optype(x.typ)
 		if _, ok := typ.(*Chan); ok && sValue != nil {
 			// TODO(gri) this also needs to happen for channels in generic variables
 			check.softErrorf(sValue, "range over %s permits only one iteration variable", &x)
@@ -906,7 +906,7 @@ func rangeKeyVal(typ Type, wantKey, wantVal bool) (Type, Type, string) {
 		var key, val Type
 		var msg string
 		typ.is(func(t Type) bool {
-			k, v, m := rangeKeyVal(t.Under(), wantKey, wantVal)
+			k, v, m := rangeKeyVal(under(t), wantKey, wantVal)
 			if k == nil || m != "" {
 				key, val, msg = k, v, m
 				return false
diff --git a/src/cmd/compile/internal/types2/subst.go b/src/cmd/compile/internal/types2/subst.go
index fc4b228e33..d730642831 100644
--- a/src/cmd/compile/internal/types2/subst.go
+++ b/src/cmd/compile/internal/types2/subst.go
@@ -61,7 +61,7 @@ func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, poslis
 			check.indent--
 			var under Type
 			if res != nil {
-				// Calling Under() here may lead to endless instantiations.
+				// Calling under() here may lead to endless instantiations.
 				// Test case: type T[P any] T[P]
 				// TODO(gri) investigate if that's a bug or to be expected.
 				under = res.Underlying()
@@ -186,7 +186,7 @@ func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, poslis
 				break
 			}
 			for _, t := range unpack(targBound.allTypes) {
-				if !iface.isSatisfiedBy(t.Under()) {
+				if !iface.isSatisfiedBy(t) {
 					// 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, t, iface.allTypes)
 					break
@@ -197,7 +197,7 @@ func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, poslis
 
 		// 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.Under(), iface.allTypes)
+			check.softErrorf(pos, "%s does not satisfy %s (%s not found in %s)", targ, tpar.bound, under(targ), iface.allTypes)
 			break
 		}
 	}
diff --git a/src/cmd/compile/internal/types2/type.go b/src/cmd/compile/internal/types2/type.go
index 4b6f507393..c1c3a4629e 100644
--- a/src/cmd/compile/internal/types2/type.go
+++ b/src/cmd/compile/internal/types2/type.go
@@ -18,24 +18,10 @@ type Type interface {
 	// client packages (here for backward-compatibility).
 	Underlying() Type
 
-	// Under returns the true expanded underlying type.
-	// If it doesn't exist, the result is Typ[Invalid].
-	// Under must only be called when a type is known
-	// to be fully set up.
-	Under() Type
-
 	// String returns a string representation of a type.
 	String() string
 }
 
-// aType implements default type behavior
-type aType struct{}
-
-// These methods must be implemented by each type.
-func (aType) Underlying() Type { panic("unreachable") }
-func (aType) Under() Type      { panic("unreachable") }
-func (aType) String() string   { panic("unreachable") }
-
 // BasicKind describes the kind of basic type.
 type BasicKind int
 
@@ -99,7 +85,6 @@ type Basic struct {
 	kind BasicKind
 	info BasicInfo
 	name string
-	aType
 }
 
 // Kind returns the kind of basic type b.
@@ -115,7 +100,6 @@ func (b *Basic) Name() string { return b.name }
 type Array struct {
 	len  int64
 	elem Type
-	aType
 }
 
 // NewArray returns a new array type for the given element type and length.
@@ -132,7 +116,6 @@ func (a *Array) Elem() Type { return a.elem }
 // A Slice represents a slice type.
 type Slice struct {
 	elem Type
-	aType
 }
 
 // NewSlice returns a new slice type for the given element type.
@@ -145,7 +128,6 @@ func (s *Slice) Elem() Type { return s.elem }
 type Struct struct {
 	fields []*Var
 	tags   []string // field tags; nil if there are no tags
-	aType
 }
 
 // NewStruct returns a new struct with the given fields and corresponding field tags.
@@ -182,7 +164,6 @@ func (s *Struct) Tag(i int) string {
 // A Pointer represents a pointer type.
 type Pointer struct {
 	base Type // element type
-	aType
 }
 
 // NewPointer returns a new pointer type for the given element (base) type.
@@ -196,17 +177,15 @@ func (p *Pointer) Elem() Type { return p.base }
 // assignments; they are not first class types of Go.
 type Tuple struct {
 	vars []*Var
-	aType
 }
 
-// TODO(gri) Don't represent empty tuples with a (*Tuple)(nil) pointer;
-//           it's too subtle and causes problems. Use a singleton instead.
-
 // NewTuple returns a new tuple for the given variables.
 func NewTuple(x ...*Var) *Tuple {
 	if len(x) > 0 {
 		return &Tuple{vars: x}
 	}
+	// TODO(gri) Don't represent empty tuples with a (*Tuple)(nil) pointer;
+	//           it's too subtle and causes problems.
 	return nil
 }
 
@@ -235,7 +214,6 @@ type Signature struct {
 	params   *Tuple      // (incoming) parameters from left to right; or nil
 	results  *Tuple      // (outgoing) results from left to right; or nil
 	variadic bool        // true if the last parameter's type is of the form ...T (or string, for append built-in only)
-	aType
 }
 
 // NewSignature returns a new function type for the given receiver, parameters,
@@ -284,7 +262,6 @@ func (s *Signature) Variadic() bool { return s.variadic }
 // first class types of Go.
 type Sum struct {
 	types []Type // types are unique
-	aType
 }
 
 // NewSum returns a new Sum type consisting of the provided
@@ -336,8 +313,6 @@ type Interface struct {
 	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)
-
-	aType
 }
 
 // unpack unpacks a type into a list of types.
@@ -468,10 +443,7 @@ func (t *Interface) Empty() bool {
 		return len(t.allMethods) == 0 && t.allTypes == nil
 	}
 	return !t.iterate(func(t *Interface) bool {
-		if len(t.methods) > 0 || t.types != nil {
-			return true
-		}
-		return false
+		return len(t.methods) > 0 || t.types != nil
 	}, nil)
 }
 
@@ -483,10 +455,7 @@ func (t *Interface) HasTypeList() bool {
 	}
 
 	return t.iterate(func(t *Interface) bool {
-		if t.types != nil {
-			return true
-		}
-		return false
+		return t.types != nil
 	}, nil)
 }
 
@@ -560,7 +529,7 @@ func (t *Interface) isSatisfiedBy(typ Type) bool {
 		return true
 	}
 	types := unpack(t.allTypes)
-	return includes(types, typ) || includes(types, typ.Under())
+	return includes(types, typ) || includes(types, under(typ))
 }
 
 // Complete computes the interface's method set. It must be called by users of
@@ -598,7 +567,7 @@ func (t *Interface) Complete() *Interface {
 	allTypes := t.types
 
 	for _, typ := range t.embeddeds {
-		utyp := typ.Under()
+		utyp := under(typ)
 		etyp := asInterface(utyp)
 		if etyp == nil {
 			if utyp != Typ[Invalid] {
@@ -633,7 +602,6 @@ func (t *Interface) Complete() *Interface {
 // A Map represents a map type.
 type Map struct {
 	key, elem Type
-	aType
 }
 
 // NewMap returns a new map for the given key and element types.
@@ -651,7 +619,6 @@ func (m *Map) Elem() Type { return m.elem }
 type Chan struct {
 	dir  ChanDir
 	elem Type
-	aType
 }
 
 // A ChanDir value indicates a channel direction.
@@ -677,7 +644,7 @@ func (c *Chan) Elem() Type { return c.elem }
 
 // A Named represents a named (defined) type.
 type Named struct {
-	check      *Checker    // for Named.Under implementation
+	check      *Checker    // for Named.under implementation
 	info       typeInfo    // for cycle detection
 	obj        *TypeName   // corresponding declared object
 	orig       Type        // type (on RHS of declaration) this *Named type is derived of (for cycle reporting)
@@ -685,7 +652,6 @@ type Named struct {
 	tparams    []*TypeName // type parameters, or nil
 	targs      []Type      // type arguments (after instantiation), or nil
 	methods    []*Func     // methods declared for this type (not the method set of this type); signatures are type-checked lazily
-	aType
 }
 
 // NewNamed returns a new named type for the given type name, underlying type, and associated methods.
@@ -757,7 +723,6 @@ type TypeParam struct {
 	obj   *TypeName // corresponding type name
 	index int       // parameter index
 	bound Type      // *Named or *Interface; underlying type is always *Interface
-	aType
 }
 
 func (t *TypeParam) Obj() *TypeName {
@@ -788,7 +753,7 @@ func (t *TypeParam) Bound() *Interface {
 
 // optype returns a type's operational type. Except for
 // type parameters, the operational type is the same
-// as the underlying type (as returned by Under). For
+// as the underlying type (as returned by under). For
 // Type parameters, the operational type is determined
 // by the corresponding type bound's type list. The
 // result may be the bottom or top type, but it is never
@@ -802,12 +767,12 @@ func optype(typ Type) Type {
 		// (type T interface { type T }).
 		// See also issue #39680.
 		if u := t.Bound().allTypes; u != nil && u != typ {
-			// u != typ and u is a type parameter => u.Under() != typ, so this is ok
-			return u.Under()
+			// u != typ and u is a type parameter => under(u) != typ, so this is ok
+			return under(u)
 		}
 		return theTop
 	}
-	return typ.Under()
+	return under(typ)
 }
 
 // An instance represents an instantiated generic type syntactically
@@ -821,7 +786,6 @@ type instance struct {
 	targs   []Type       // type arguments
 	poslist []syntax.Pos // position of each targ; for error reporting only
 	value   Type         // base(targs...) after instantiation or Typ[Invalid]; nil if not yet set
-	aType
 }
 
 // expand returns the instantiated (= expanded) type of t.
@@ -863,9 +827,7 @@ func init() { expandf = expand }
 // It is the underlying type of a type parameter that
 // cannot be satisfied by any type, usually because
 // the intersection of type constraints left nothing).
-type bottom struct {
-	aType
-}
+type bottom struct{}
 
 // theBottom is the singleton bottom type.
 var theBottom = &bottom{}
@@ -875,9 +837,7 @@ var theBottom = &bottom{}
 // can be satisfied by any type (ignoring methods),
 // usually because the type constraint has no type
 // list.
-type top struct {
-	aType
-}
+type top struct{}
 
 // theTop is the singleton top type.
 var theTop = &top{}
@@ -900,25 +860,6 @@ func (t *instance) Underlying() Type  { return t }
 func (t *bottom) Underlying() Type    { return t }
 func (t *top) Underlying() Type       { return t }
 
-// Type-specific implementations of Under.
-func (t *Basic) Under() Type     { return t }
-func (t *Array) Under() Type     { return t }
-func (t *Slice) Under() Type     { return t }
-func (t *Struct) Under() Type    { return t }
-func (t *Pointer) Under() Type   { return t }
-func (t *Tuple) Under() Type     { return t }
-func (t *Signature) Under() Type { return t }
-func (t *Sum) Under() Type       { return t } // TODO(gri) is this correct?
-func (t *Interface) Under() Type { return t }
-func (t *Map) Under() Type       { return t }
-func (t *Chan) Under() Type      { return t }
-
-// see decl.go for implementation of Named.Under
-func (t *TypeParam) Under() Type { return t }
-func (t *instance) Under() Type  { return t.expand().Under() }
-func (t *bottom) Under() Type    { return t }
-func (t *top) Under() Type       { return t }
-
 // Type-specific implementations of String.
 func (t *Basic) String() string     { return TypeString(t, nil) }
 func (t *Array) String() string     { return TypeString(t, nil) }
@@ -937,6 +878,27 @@ func (t *instance) String() string  { return TypeString(t, nil) }
 func (t *bottom) String() string    { return TypeString(t, nil) }
 func (t *top) String() string       { return TypeString(t, nil) }
 
+// under returns the true expanded underlying type.
+// If it doesn't exist, the result is Typ[Invalid].
+// under must only be called when a type is known
+// to be fully set up.
+//
+// under is set to underf to avoid an initialization cycle.
+// TODO(gri) this doesn't happen in go/types - investigate
+var under func(Type) Type
+
+func init() {
+	under = underf
+}
+
+func underf(t Type) Type {
+	// TODO(gri) is this correct for *Sum?
+	if n := asNamed(t); n != nil {
+		return n.under()
+	}
+	return t
+}
+
 // Converters
 //
 // A converter must only be called when a type is
@@ -1007,6 +969,6 @@ func asNamed(t Type) *Named {
 }
 
 func asTypeParam(t Type) *TypeParam {
-	u, _ := t.Under().(*TypeParam)
+	u, _ := under(t).(*TypeParam)
 	return u
 }
diff --git a/src/cmd/compile/internal/types2/typexpr.go b/src/cmd/compile/internal/types2/typexpr.go
index cf9d7c0a40..87eabbe28d 100644
--- a/src/cmd/compile/internal/types2/typexpr.go
+++ b/src/cmd/compile/internal/types2/typexpr.go
@@ -138,7 +138,7 @@ func (check *Checker) varType(e syntax.Expr) Type {
 // ordinaryType reports an error if typ is an interface type containing
 // type lists or is (or embeds) the predeclared type comparable.
 func (check *Checker) ordinaryType(pos syntax.Pos, typ Type) {
-	// We don't want to call Under() (via Interface) or complete interfaces while we
+	// We don't want to call under() (via Interface) or complete interfaces while we
 	// are in the middle of type-checking parameter declarations that might belong to
 	// interface methods. Delay this check to the end of type-checking.
 	check.atEnd(func() {
@@ -393,7 +393,7 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 						err = ""
 					}
 				} else {
-					switch u := optype(T.Under()).(type) {
+					switch u := optype(T).(type) {
 					case *Basic:
 						// unsafe.Pointer is treated like a regular pointer
 						if u.kind == UnsafePointer {
@@ -442,7 +442,7 @@ func (check *Checker) typInternal(e0 syntax.Expr, def *Named) (T Type) {
 			check.indent--
 			var under Type
 			if T != nil {
-				// Calling Under() here may lead to endless instantiations.
+				// Calling under() here may lead to endless instantiations.
 				// Test case: type T[P any] *T[P]
 				// TODO(gri) investigate if that's a bug or to be expected
 				// (see also analogous comment in Checker.instantiate).
@@ -967,7 +967,7 @@ func (check *Checker) completeInterface(pos syntax.Pos, ityp *Interface) {
 	posList := check.posMap[ityp]
 	for i, typ := range ityp.embeddeds {
 		pos := posList[i] // embedding position
-		utyp := typ.Under()
+		utyp := under(typ)
 		etyp := asInterface(utyp)
 		if etyp == nil {
 			if utyp != Typ[Invalid] {
@@ -1159,12 +1159,12 @@ func (check *Checker) structType(styp *Struct, e *syntax.StructType) {
 			// Because we have a name, typ must be of the form T or *T, where T is the name
 			// of a (named or alias) type, and t (= deref(typ)) must be the type of T.
 			// We must delay this check to the end because we don't want to instantiate
-			// (via t.Under()) a possibly incomplete type.
+			// (via under(t)) a possibly incomplete type.
 			embeddedTyp := typ // for closure below
 			embeddedPos := pos
 			check.atEnd(func() {
 				t, isPtr := deref(embeddedTyp)
-				switch t := optype(t.Under()).(type) {
+				switch t := optype(t).(type) {
 				case *Basic:
 					if t == Typ[Invalid] {
 						// error was reported before
diff --git a/src/cmd/compile/internal/types2/unify.go b/src/cmd/compile/internal/types2/unify.go
index 153df9d622..d2ea2b952b 100644
--- a/src/cmd/compile/internal/types2/unify.go
+++ b/src/cmd/compile/internal/types2/unify.go
@@ -215,9 +215,9 @@ func (u *unifier) nify(x, y Type, p *ifacePair) bool {
 		// want *Named types.)
 		switch {
 		case !isNamed(x) && y != nil && asNamed(y) != nil:
-			return u.nify(x, y.Under(), p)
+			return u.nify(x, under(y), p)
 		case x != nil && asNamed(x) != nil && !isNamed(y):
-			return u.nify(x.Under(), y, p)
+			return u.nify(under(x), y, p)
 		}
 	}
 
diff --git a/src/cmd/compile/internal/types2/universe.go b/src/cmd/compile/internal/types2/universe.go
index 994e298a6c..3654ab4945 100644
--- a/src/cmd/compile/internal/types2/universe.go
+++ b/src/cmd/compile/internal/types2/universe.go
@@ -34,39 +34,39 @@ var (
 // Use Universe.Lookup("byte").Type() to obtain the specific
 // alias basic type named "byte" (and analogous for "rune").
 var Typ = [...]*Basic{
-	Invalid: {Invalid, 0, "invalid type", aType{}},
-
-	Bool:          {Bool, IsBoolean, "bool", aType{}},
-	Int:           {Int, IsInteger, "int", aType{}},
-	Int8:          {Int8, IsInteger, "int8", aType{}},
-	Int16:         {Int16, IsInteger, "int16", aType{}},
-	Int32:         {Int32, IsInteger, "int32", aType{}},
-	Int64:         {Int64, IsInteger, "int64", aType{}},
-	Uint:          {Uint, IsInteger | IsUnsigned, "uint", aType{}},
-	Uint8:         {Uint8, IsInteger | IsUnsigned, "uint8", aType{}},
-	Uint16:        {Uint16, IsInteger | IsUnsigned, "uint16", aType{}},
-	Uint32:        {Uint32, IsInteger | IsUnsigned, "uint32", aType{}},
-	Uint64:        {Uint64, IsInteger | IsUnsigned, "uint64", aType{}},
-	Uintptr:       {Uintptr, IsInteger | IsUnsigned, "uintptr", aType{}},
-	Float32:       {Float32, IsFloat, "float32", aType{}},
-	Float64:       {Float64, IsFloat, "float64", aType{}},
-	Complex64:     {Complex64, IsComplex, "complex64", aType{}},
-	Complex128:    {Complex128, IsComplex, "complex128", aType{}},
-	String:        {String, IsString, "string", aType{}},
-	UnsafePointer: {UnsafePointer, 0, "Pointer", aType{}},
-
-	UntypedBool:    {UntypedBool, IsBoolean | IsUntyped, "untyped bool", aType{}},
-	UntypedInt:     {UntypedInt, IsInteger | IsUntyped, "untyped int", aType{}},
-	UntypedRune:    {UntypedRune, IsInteger | IsUntyped, "untyped rune", aType{}},
-	UntypedFloat:   {UntypedFloat, IsFloat | IsUntyped, "untyped float", aType{}},
-	UntypedComplex: {UntypedComplex, IsComplex | IsUntyped, "untyped complex", aType{}},
-	UntypedString:  {UntypedString, IsString | IsUntyped, "untyped string", aType{}},
-	UntypedNil:     {UntypedNil, IsUntyped, "untyped nil", aType{}},
+	Invalid: {Invalid, 0, "invalid type"},
+
+	Bool:          {Bool, IsBoolean, "bool"},
+	Int:           {Int, IsInteger, "int"},
+	Int8:          {Int8, IsInteger, "int8"},
+	Int16:         {Int16, IsInteger, "int16"},
+	Int32:         {Int32, IsInteger, "int32"},
+	Int64:         {Int64, IsInteger, "int64"},
+	Uint:          {Uint, IsInteger | IsUnsigned, "uint"},
+	Uint8:         {Uint8, IsInteger | IsUnsigned, "uint8"},
+	Uint16:        {Uint16, IsInteger | IsUnsigned, "uint16"},
+	Uint32:        {Uint32, IsInteger | IsUnsigned, "uint32"},
+	Uint64:        {Uint64, IsInteger | IsUnsigned, "uint64"},
+	Uintptr:       {Uintptr, IsInteger | IsUnsigned, "uintptr"},
+	Float32:       {Float32, IsFloat, "float32"},
+	Float64:       {Float64, IsFloat, "float64"},
+	Complex64:     {Complex64, IsComplex, "complex64"},
+	Complex128:    {Complex128, IsComplex, "complex128"},
+	String:        {String, IsString, "string"},
+	UnsafePointer: {UnsafePointer, 0, "Pointer"},
+
+	UntypedBool:    {UntypedBool, IsBoolean | IsUntyped, "untyped bool"},
+	UntypedInt:     {UntypedInt, IsInteger | IsUntyped, "untyped int"},
+	UntypedRune:    {UntypedRune, IsInteger | IsUntyped, "untyped rune"},
+	UntypedFloat:   {UntypedFloat, IsFloat | IsUntyped, "untyped float"},
+	UntypedComplex: {UntypedComplex, IsComplex | IsUntyped, "untyped complex"},
+	UntypedString:  {UntypedString, IsString | IsUntyped, "untyped string"},
+	UntypedNil:     {UntypedNil, IsUntyped, "untyped nil"},
 }
 
 var aliases = [...]*Basic{
-	{Byte, IsInteger | IsUnsigned, "byte", aType{}},
-	{Rune, IsInteger, "rune", aType{}},
+	{Byte, IsInteger | IsUnsigned, "byte"},
+	{Rune, IsInteger, "rune"},
 }
 
 func defPredeclaredTypes() {