[dev.typeparams] cmd/compile/internal/types2: move (remaining) type decls into their own files (cleanup)

This change moves the type declarations and associated methods for
each of the remaining Type types into their respective files. Except
for import and comment adjustments, and receiver name adjustments for
the Underlying and String methods, no functional changes are made.

Change-Id: I3b9ccab3c85abea4852bacd28c2e47cec05c0bac
Reviewed-on: https://go-review.googlesource.com/c/go/+/332093
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

11 files changed, 540 insertions, 483 deletions

diff --git a/src/cmd/compile/internal/types2/array.go b/src/cmd/compile/internal/types2/array.go
new file mode 100644
index 0000000000..502d49bc25
--- /dev/null
+++ b/src/cmd/compile/internal/types2/array.go
@@ -0,0 +1,25 @@
+// Copyright 2011 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
+
+// An Array represents an array type.
+type Array struct {
+	len  int64
+	elem Type
+}
+
+// NewArray returns a new array type for the given element type and length.
+// A negative length indicates an unknown length.
+func NewArray(elem Type, len int64) *Array { return &Array{len: len, elem: elem} }
+
+// Len returns the length of array a.
+// A negative result indicates an unknown length.
+func (a *Array) Len() int64 { return a.len }
+
+// Elem returns element type of array a.
+func (a *Array) Elem() Type { return a.elem }
+
+func (a *Array) Underlying() Type { return a }
+func (a *Array) String() string   { return TypeString(a, nil) }
diff --git a/src/cmd/compile/internal/types2/basic.go b/src/cmd/compile/internal/types2/basic.go
new file mode 100644
index 0000000000..2fd973cafb
--- /dev/null
+++ b/src/cmd/compile/internal/types2/basic.go
@@ -0,0 +1,82 @@
+// Copyright 2011 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
+
+// BasicKind describes the kind of basic type.
+type BasicKind int
+
+const (
+	Invalid BasicKind = iota // type is invalid
+
+	// predeclared types
+	Bool
+	Int
+	Int8
+	Int16
+	Int32
+	Int64
+	Uint
+	Uint8
+	Uint16
+	Uint32
+	Uint64
+	Uintptr
+	Float32
+	Float64
+	Complex64
+	Complex128
+	String
+	UnsafePointer
+
+	// types for untyped values
+	UntypedBool
+	UntypedInt
+	UntypedRune
+	UntypedFloat
+	UntypedComplex
+	UntypedString
+	UntypedNil
+
+	// aliases
+	Byte = Uint8
+	Rune = Int32
+)
+
+// BasicInfo is a set of flags describing properties of a basic type.
+type BasicInfo int
+
+// Properties of basic types.
+const (
+	IsBoolean BasicInfo = 1 << iota
+	IsInteger
+	IsUnsigned
+	IsFloat
+	IsComplex
+	IsString
+	IsUntyped
+
+	IsOrdered   = IsInteger | IsFloat | IsString
+	IsNumeric   = IsInteger | IsFloat | IsComplex
+	IsConstType = IsBoolean | IsNumeric | IsString
+)
+
+// A Basic represents a basic type.
+type Basic struct {
+	kind BasicKind
+	info BasicInfo
+	name string
+}
+
+// Kind returns the kind of basic type b.
+func (b *Basic) Kind() BasicKind { return b.kind }
+
+// Info returns information about properties of basic type b.
+func (b *Basic) Info() BasicInfo { return b.info }
+
+// Name returns the name of basic type b.
+func (b *Basic) Name() string { return b.name }
+
+func (b *Basic) Underlying() Type { return b }
+func (b *Basic) String() string   { return TypeString(b, nil) }
diff --git a/src/cmd/compile/internal/types2/chan.go b/src/cmd/compile/internal/types2/chan.go
new file mode 100644
index 0000000000..77650dfb09
--- /dev/null
+++ b/src/cmd/compile/internal/types2/chan.go
@@ -0,0 +1,35 @@
+// Copyright 2011 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
+
+// A Chan represents a channel type.
+type Chan struct {
+	dir  ChanDir
+	elem Type
+}
+
+// A ChanDir value indicates a channel direction.
+type ChanDir int
+
+// The direction of a channel is indicated by one of these constants.
+const (
+	SendRecv ChanDir = iota
+	SendOnly
+	RecvOnly
+)
+
+// NewChan returns a new channel type for the given direction and element type.
+func NewChan(dir ChanDir, elem Type) *Chan {
+	return &Chan{dir: dir, elem: elem}
+}
+
+// Dir returns the direction of channel c.
+func (c *Chan) Dir() ChanDir { return c.dir }
+
+// Elem returns the element type of channel c.
+func (c *Chan) Elem() Type { return c.elem }
+
+func (c *Chan) Underlying() Type { return c }
+func (c *Chan) String() string   { return TypeString(c, nil) }
diff --git a/src/cmd/compile/internal/types2/instance.go b/src/cmd/compile/internal/types2/instance.go
new file mode 100644
index 0000000000..9d6097e874
--- /dev/null
+++ b/src/cmd/compile/internal/types2/instance.go
@@ -0,0 +1,58 @@
+// Copyright 2011 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 "cmd/compile/internal/syntax"
+
+// An instance represents an instantiated generic type syntactically
+// (without expanding the instantiation). Type instances appear only
+// during type-checking and are replaced by their fully instantiated
+// (expanded) types before the end of type-checking.
+type instance struct {
+	check   *Checker     // for lazy instantiation
+	pos     syntax.Pos   // position of type instantiation; for error reporting only
+	base    *Named       // parameterized type to be instantiated
+	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
+}
+
+// expand returns the instantiated (= expanded) type of t.
+// The result is either an instantiated *Named type, or
+// Typ[Invalid] if there was an error.
+func (t *instance) expand() Type {
+	v := t.value
+	if v == nil {
+		v = t.check.instantiate(t.pos, t.base, t.targs, t.poslist)
+		if v == nil {
+			v = Typ[Invalid]
+		}
+		t.value = v
+	}
+	// After instantiation we must have an invalid or a *Named type.
+	if debug && v != Typ[Invalid] {
+		_ = v.(*Named)
+	}
+	return v
+}
+
+// expand expands a type instance into its instantiated
+// type and leaves all other types alone. expand does
+// not recurse.
+func expand(typ Type) Type {
+	if t, _ := typ.(*instance); t != nil {
+		return t.expand()
+	}
+	return typ
+}
+
+// expandf is set to expand.
+// Call expandf when calling expand causes compile-time cycle error.
+var expandf func(Type) Type
+
+func init() { expandf = expand }
+
+func (t *instance) Underlying() Type { return t }
+func (t *instance) String() string   { return TypeString(t, nil) }
diff --git a/src/cmd/compile/internal/types2/map.go b/src/cmd/compile/internal/types2/map.go
new file mode 100644
index 0000000000..0d3464caae
--- /dev/null
+++ b/src/cmd/compile/internal/types2/map.go
@@ -0,0 +1,24 @@
+// Copyright 2011 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
+
+// A Map represents a map type.
+type Map struct {
+	key, elem Type
+}
+
+// NewMap returns a new map for the given key and element types.
+func NewMap(key, elem Type) *Map {
+	return &Map{key: key, elem: elem}
+}
+
+// Key returns the key type of map m.
+func (m *Map) Key() Type { return m.key }
+
+// Elem returns the element type of map m.
+func (m *Map) Elem() Type { return m.elem }
+
+func (t *Map) Underlying() Type { return t }
+func (t *Map) String() string   { return TypeString(t, nil) }
diff --git a/src/cmd/compile/internal/types2/named.go b/src/cmd/compile/internal/types2/named.go
new file mode 100644
index 0000000000..2005dfbd84
--- /dev/null
+++ b/src/cmd/compile/internal/types2/named.go
@@ -0,0 +1,144 @@
+// Copyright 2011 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 "sync"
+
+// TODO(gri) Clean up Named struct below; specifically the fromRHS field (can we use underlying?).
+
+// A Named represents a named (defined) type.
+type Named struct {
+	check      *Checker    // for Named.under implementation; nilled once under has been called
+	info       typeInfo    // for cycle detection
+	obj        *TypeName   // corresponding declared object
+	orig       *Named      // original, uninstantiated type
+	fromRHS    Type        // type (on RHS of declaration) this *Named type is derived from (for cycle reporting)
+	underlying Type        // possibly a *Named during setup; never a *Named once set up completely
+	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
+
+	resolve func(*Named) ([]*TypeName, Type, []*Func)
+	once    sync.Once
+}
+
+// NewNamed returns a new named type for the given type name, underlying type, and associated methods.
+// If the given type name obj doesn't have a type yet, its type is set to the returned named type.
+// The underlying type must not be a *Named.
+func NewNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
+	if _, ok := underlying.(*Named); ok {
+		panic("types2.NewNamed: underlying type must not be *Named")
+	}
+	return (*Checker)(nil).newNamed(obj, nil, underlying, nil, methods)
+}
+
+func (t *Named) expand() *Named {
+	if t.resolve == nil {
+		return t
+	}
+
+	t.once.Do(func() {
+		// TODO(mdempsky): Since we're passing t to resolve anyway
+		// (necessary because types2 expects the receiver type for methods
+		// on defined interface types to be the Named rather than the
+		// underlying Interface), maybe it should just handle calling
+		// SetTParams, SetUnderlying, and AddMethod instead?  Those
+		// methods would need to support reentrant calls though.  It would
+		// also make the API more future-proof towards further extensions
+		// (like SetTParams).
+
+		tparams, underlying, methods := t.resolve(t)
+
+		switch underlying.(type) {
+		case nil, *Named:
+			panic("invalid underlying type")
+		}
+
+		t.tparams = tparams
+		t.underlying = underlying
+		t.methods = methods
+	})
+	return t
+}
+
+// newNamed is like NewNamed but with a *Checker receiver and additional orig argument.
+func (check *Checker) newNamed(obj *TypeName, orig *Named, underlying Type, tparams []*TypeName, methods []*Func) *Named {
+	typ := &Named{check: check, obj: obj, orig: orig, fromRHS: underlying, underlying: underlying, tparams: tparams, methods: methods}
+	if typ.orig == nil {
+		typ.orig = typ
+	}
+	if obj.typ == nil {
+		obj.typ = typ
+	}
+	// Ensure that typ is always expanded, at which point the check field can be
+	// nilled out.
+	//
+	// Note that currently we cannot nil out check inside typ.under(), because
+	// it's possible that typ is expanded multiple times.
+	//
+	// TODO(gri): clean this up so that under is the only function mutating
+	//            named types.
+	if check != nil {
+		check.later(func() {
+			switch typ.under().(type) {
+			case *Named, *instance:
+				panic("internal error: unexpanded underlying type")
+			}
+			typ.check = nil
+		})
+	}
+	return typ
+}
+
+// Obj returns the type name for the named type t.
+func (t *Named) Obj() *TypeName { return t.obj }
+
+// Orig returns the original generic type an instantiated type is derived from.
+// If t is not an instantiated type, the result is t.
+func (t *Named) Orig() *Named { return t.orig }
+
+// TODO(gri) Come up with a better representation and API to distinguish
+//           between parameterized instantiated and non-instantiated types.
+
+// TParams returns the type parameters of the named type t, or nil.
+// The result is non-nil for an (originally) parameterized type even if it is instantiated.
+func (t *Named) TParams() []*TypeName { return t.expand().tparams }
+
+// SetTParams sets the type parameters of the named type t.
+func (t *Named) SetTParams(tparams []*TypeName) { t.expand().tparams = tparams }
+
+// TArgs returns the type arguments after instantiation of the named type t, or nil if not instantiated.
+func (t *Named) TArgs() []Type { return t.targs }
+
+// SetTArgs sets the type arguments of the named type t.
+func (t *Named) SetTArgs(args []Type) { t.targs = args }
+
+// NumMethods returns the number of explicit methods whose receiver is named type t.
+func (t *Named) NumMethods() int { return len(t.expand().methods) }
+
+// Method returns the i'th method of named type t for 0 <= i < t.NumMethods().
+func (t *Named) Method(i int) *Func { return t.expand().methods[i] }
+
+// SetUnderlying sets the underlying type and marks t as complete.
+func (t *Named) SetUnderlying(underlying Type) {
+	if underlying == nil {
+		panic("types2.Named.SetUnderlying: underlying type must not be nil")
+	}
+	if _, ok := underlying.(*Named); ok {
+		panic("types2.Named.SetUnderlying: underlying type must not be *Named")
+	}
+	t.expand().underlying = underlying
+}
+
+// AddMethod adds method m unless it is already in the method list.
+func (t *Named) AddMethod(m *Func) {
+	t.expand()
+	if i, _ := lookupMethod(t.methods, m.pkg, m.name); i < 0 {
+		t.methods = append(t.methods, m)
+	}
+}
+
+func (t *Named) Underlying() Type { return t.expand().underlying }
+func (t *Named) String() string   { return TypeString(t, nil) }
diff --git a/src/cmd/compile/internal/types2/pointer.go b/src/cmd/compile/internal/types2/pointer.go
new file mode 100644
index 0000000000..63055fc6b0
--- /dev/null
+++ b/src/cmd/compile/internal/types2/pointer.go
@@ -0,0 +1,19 @@
+// Copyright 2011 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
+
+// A Pointer represents a pointer type.
+type Pointer struct {
+	base Type // element type
+}
+
+// NewPointer returns a new pointer type for the given element (base) type.
+func NewPointer(elem Type) *Pointer { return &Pointer{base: elem} }
+
+// Elem returns the element type for the given pointer p.
+func (p *Pointer) Elem() Type { return p.base }
+
+func (p *Pointer) Underlying() Type { return p }
+func (p *Pointer) String() string   { return TypeString(p, nil) }
diff --git a/src/cmd/compile/internal/types2/slice.go b/src/cmd/compile/internal/types2/slice.go
new file mode 100644
index 0000000000..9c22a6fb1b
--- /dev/null
+++ b/src/cmd/compile/internal/types2/slice.go
@@ -0,0 +1,19 @@
+// Copyright 2011 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
+
+// A Slice represents a slice type.
+type Slice struct {
+	elem Type
+}
+
+// NewSlice returns a new slice type for the given element type.
+func NewSlice(elem Type) *Slice { return &Slice{elem: elem} }
+
+// Elem returns the element type of slice s.
+func (s *Slice) Elem() Type { return s.elem }
+
+func (s *Slice) Underlying() Type { return s }
+func (s *Slice) String() string   { return TypeString(s, nil) }
diff --git a/src/cmd/compile/internal/types2/tuple.go b/src/cmd/compile/internal/types2/tuple.go
new file mode 100644
index 0000000000..a3946beab5
--- /dev/null
+++ b/src/cmd/compile/internal/types2/tuple.go
@@ -0,0 +1,36 @@
+// Copyright 2011 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
+
+// A Tuple represents an ordered list of variables; a nil *Tuple is a valid (empty) tuple.
+// Tuples are used as components of signatures and to represent the type of multiple
+// assignments; they are not first class types of Go.
+type Tuple struct {
+	vars []*Var
+}
+
+// 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
+}
+
+// Len returns the number variables of tuple t.
+func (t *Tuple) Len() int {
+	if t != nil {
+		return len(t.vars)
+	}
+	return 0
+}
+
+// At returns the i'th variable of tuple t.
+func (t *Tuple) At(i int) *Var { return t.vars[i] }
+
+func (t *Tuple) Underlying() Type { return t }
+func (t *Tuple) String() string   { return TypeString(t, nil) }
diff --git a/src/cmd/compile/internal/types2/type.go b/src/cmd/compile/internal/types2/type.go
index e2e10d21ae..e3f63f63b5 100644
--- a/src/cmd/compile/internal/types2/type.go
+++ b/src/cmd/compile/internal/types2/type.go
@@ -4,12 +4,6 @@
 
 package types2
 
-import (
-	"cmd/compile/internal/syntax"
-	"sync"
-	"sync/atomic"
-)
-
 // A Type represents a type of Go.
 // All types implement the Type interface.
 type Type interface {
@@ -22,394 +16,31 @@ type Type interface {
 	String() string
 }
 
-// BasicKind describes the kind of basic type.
-type BasicKind int
-
-const (
-	Invalid BasicKind = iota // type is invalid
-
-	// predeclared types
-	Bool
-	Int
-	Int8
-	Int16
-	Int32
-	Int64
-	Uint
-	Uint8
-	Uint16
-	Uint32
-	Uint64
-	Uintptr
-	Float32
-	Float64
-	Complex64
-	Complex128
-	String
-	UnsafePointer
-
-	// types for untyped values
-	UntypedBool
-	UntypedInt
-	UntypedRune
-	UntypedFloat
-	UntypedComplex
-	UntypedString
-	UntypedNil
-
-	// aliases
-	Byte = Uint8
-	Rune = Int32
-)
-
-// BasicInfo is a set of flags describing properties of a basic type.
-type BasicInfo int
-
-// Properties of basic types.
-const (
-	IsBoolean BasicInfo = 1 << iota
-	IsInteger
-	IsUnsigned
-	IsFloat
-	IsComplex
-	IsString
-	IsUntyped
-
-	IsOrdered   = IsInteger | IsFloat | IsString
-	IsNumeric   = IsInteger | IsFloat | IsComplex
-	IsConstType = IsBoolean | IsNumeric | IsString
-)
-
-// A Basic represents a basic type.
-type Basic struct {
-	kind BasicKind
-	info BasicInfo
-	name string
-}
-
-// Kind returns the kind of basic type b.
-func (b *Basic) Kind() BasicKind { return b.kind }
-
-// Info returns information about properties of basic type b.
-func (b *Basic) Info() BasicInfo { return b.info }
-
-// Name returns the name of basic type b.
-func (b *Basic) Name() string { return b.name }
-
-// An Array represents an array type.
-type Array struct {
-	len  int64
-	elem Type
-}
-
-// NewArray returns a new array type for the given element type and length.
-// A negative length indicates an unknown length.
-func NewArray(elem Type, len int64) *Array { return &Array{len: len, elem: elem} }
-
-// Len returns the length of array a.
-// A negative result indicates an unknown length.
-func (a *Array) Len() int64 { return a.len }
-
-// Elem returns element type of array a.
-func (a *Array) Elem() Type { return a.elem }
-
-// A Slice represents a slice type.
-type Slice struct {
-	elem Type
-}
-
-// NewSlice returns a new slice type for the given element type.
-func NewSlice(elem Type) *Slice { return &Slice{elem: elem} }
-
-// Elem returns the element type of slice s.
-func (s *Slice) Elem() Type { return s.elem }
-
-// A Pointer represents a pointer type.
-type Pointer struct {
-	base Type // element type
-}
-
-// NewPointer returns a new pointer type for the given element (base) type.
-func NewPointer(elem Type) *Pointer { return &Pointer{base: elem} }
-
-// Elem returns the element type for the given pointer p.
-func (p *Pointer) Elem() Type { return p.base }
-
-// A Tuple represents an ordered list of variables; a nil *Tuple is a valid (empty) tuple.
-// Tuples are used as components of signatures and to represent the type of multiple
-// assignments; they are not first class types of Go.
-type Tuple struct {
-	vars []*Var
-}
-
-// 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
-}
-
-// Len returns the number variables of tuple t.
-func (t *Tuple) Len() int {
-	if t != nil {
-		return len(t.vars)
-	}
-	return 0
-}
-
-// At returns the i'th variable of tuple t.
-func (t *Tuple) At(i int) *Var { return t.vars[i] }
-
-// A Map represents a map type.
-type Map struct {
-	key, elem Type
-}
-
-// NewMap returns a new map for the given key and element types.
-func NewMap(key, elem Type) *Map {
-	return &Map{key: key, elem: elem}
-}
-
-// Key returns the key type of map m.
-func (m *Map) Key() Type { return m.key }
-
-// Elem returns the element type of map m.
-func (m *Map) Elem() Type { return m.elem }
-
-// A Chan represents a channel type.
-type Chan struct {
-	dir  ChanDir
-	elem Type
-}
-
-// A ChanDir value indicates a channel direction.
-type ChanDir int
-
-// The direction of a channel is indicated by one of these constants.
-const (
-	SendRecv ChanDir = iota
-	SendOnly
-	RecvOnly
-)
-
-// NewChan returns a new channel type for the given direction and element type.
-func NewChan(dir ChanDir, elem Type) *Chan {
-	return &Chan{dir: dir, elem: elem}
-}
-
-// Dir returns the direction of channel c.
-func (c *Chan) Dir() ChanDir { return c.dir }
-
-// Elem returns the element type of channel c.
-func (c *Chan) Elem() Type { return c.elem }
-
-// TODO(gri) Clean up Named struct below; specifically the fromRHS field (can we use underlying?).
-
-// A Named represents a named (defined) type.
-type Named struct {
-	check      *Checker    // for Named.under implementation; nilled once under has been called
-	info       typeInfo    // for cycle detection
-	obj        *TypeName   // corresponding declared object
-	orig       *Named      // original, uninstantiated type
-	fromRHS    Type        // type (on RHS of declaration) this *Named type is derived from (for cycle reporting)
-	underlying Type        // possibly a *Named during setup; never a *Named once set up completely
-	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
+// top represents the top of the type lattice.
+// It is the underlying type of a type parameter that
+// can be satisfied by any type (ignoring methods),
+// because its type constraint contains no restrictions
+// besides methods.
+type top struct{}
 
-	resolve func(*Named) ([]*TypeName, Type, []*Func)
-	once    sync.Once
-}
+// theTop is the singleton top type.
+var theTop = &top{}
 
-// NewNamed returns a new named type for the given type name, underlying type, and associated methods.
-// If the given type name obj doesn't have a type yet, its type is set to the returned named type.
-// The underlying type must not be a *Named.
-func NewNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
-	if _, ok := underlying.(*Named); ok {
-		panic("types2.NewNamed: underlying type must not be *Named")
-	}
-	return (*Checker)(nil).newNamed(obj, nil, underlying, nil, methods)
-}
+func (t *top) Underlying() Type { return t }
+func (t *top) String() string   { return TypeString(t, nil) }
 
-func (t *Named) expand() *Named {
-	if t.resolve == nil {
-		return t
+// 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.
+func under(t Type) Type {
+	// TODO(gri) is this correct for *Union?
+	if n := asNamed(t); n != nil {
+		return n.under()
 	}
-
-	t.once.Do(func() {
-		// TODO(mdempsky): Since we're passing t to resolve anyway
-		// (necessary because types2 expects the receiver type for methods
-		// on defined interface types to be the Named rather than the
-		// underlying Interface), maybe it should just handle calling
-		// SetTParams, SetUnderlying, and AddMethod instead?  Those
-		// methods would need to support reentrant calls though.  It would
-		// also make the API more future-proof towards further extensions
-		// (like SetTParams).
-
-		tparams, underlying, methods := t.resolve(t)
-
-		switch underlying.(type) {
-		case nil, *Named:
-			panic("invalid underlying type")
-		}
-
-		t.tparams = tparams
-		t.underlying = underlying
-		t.methods = methods
-	})
 	return t
 }
 
-// newNamed is like NewNamed but with a *Checker receiver and additional orig argument.
-func (check *Checker) newNamed(obj *TypeName, orig *Named, underlying Type, tparams []*TypeName, methods []*Func) *Named {
-	typ := &Named{check: check, obj: obj, orig: orig, fromRHS: underlying, underlying: underlying, tparams: tparams, methods: methods}
-	if typ.orig == nil {
-		typ.orig = typ
-	}
-	if obj.typ == nil {
-		obj.typ = typ
-	}
-	// Ensure that typ is always expanded, at which point the check field can be
-	// nilled out.
-	//
-	// Note that currently we cannot nil out check inside typ.under(), because
-	// it's possible that typ is expanded multiple times.
-	//
-	// TODO(gri): clean this up so that under is the only function mutating
-	//            named types.
-	if check != nil {
-		check.later(func() {
-			switch typ.under().(type) {
-			case *Named, *instance:
-				panic("internal error: unexpanded underlying type")
-			}
-			typ.check = nil
-		})
-	}
-	return typ
-}
-
-// Obj returns the type name for the named type t.
-func (t *Named) Obj() *TypeName { return t.obj }
-
-// Orig returns the original generic type an instantiated type is derived from.
-// If t is not an instantiated type, the result is t.
-func (t *Named) Orig() *Named { return t.orig }
-
-// TODO(gri) Come up with a better representation and API to distinguish
-//           between parameterized instantiated and non-instantiated types.
-
-// TParams returns the type parameters of the named type t, or nil.
-// The result is non-nil for an (originally) parameterized type even if it is instantiated.
-func (t *Named) TParams() []*TypeName { return t.expand().tparams }
-
-// SetTParams sets the type parameters of the named type t.
-func (t *Named) SetTParams(tparams []*TypeName) { t.expand().tparams = tparams }
-
-// TArgs returns the type arguments after instantiation of the named type t, or nil if not instantiated.
-func (t *Named) TArgs() []Type { return t.targs }
-
-// SetTArgs sets the type arguments of the named type t.
-func (t *Named) SetTArgs(args []Type) { t.targs = args }
-
-// NumMethods returns the number of explicit methods whose receiver is named type t.
-func (t *Named) NumMethods() int { return len(t.expand().methods) }
-
-// Method returns the i'th method of named type t for 0 <= i < t.NumMethods().
-func (t *Named) Method(i int) *Func { return t.expand().methods[i] }
-
-// SetUnderlying sets the underlying type and marks t as complete.
-func (t *Named) SetUnderlying(underlying Type) {
-	if underlying == nil {
-		panic("types2.Named.SetUnderlying: underlying type must not be nil")
-	}
-	if _, ok := underlying.(*Named); ok {
-		panic("types2.Named.SetUnderlying: underlying type must not be *Named")
-	}
-	t.expand().underlying = underlying
-}
-
-// AddMethod adds method m unless it is already in the method list.
-func (t *Named) AddMethod(m *Func) {
-	t.expand()
-	if i, _ := lookupMethod(t.methods, m.pkg, m.name); i < 0 {
-		t.methods = append(t.methods, m)
-	}
-}
-
-// Note: This is a uint32 rather than a uint64 because the
-// respective 64 bit atomic instructions are not available
-// on all platforms.
-var lastID uint32
-
-// nextID returns a value increasing monotonically by 1 with
-// each call, starting with 1. It may be called concurrently.
-func nextID() uint64 { return uint64(atomic.AddUint32(&lastID, 1)) }
-
-// A TypeParam represents a type parameter type.
-type TypeParam struct {
-	check *Checker  // for lazy type bound completion
-	id    uint64    // unique id, for debugging only
-	obj   *TypeName // corresponding type name
-	index int       // type parameter index in source order, starting at 0
-	bound Type      // *Named or *Interface; underlying type is always *Interface
-}
-
-// Obj returns the type name for the type parameter t.
-func (t *TypeParam) Obj() *TypeName { return t.obj }
-
-// NewTypeParam returns a new TypeParam.  bound can be nil (and set later).
-func (check *Checker) NewTypeParam(obj *TypeName, index int, bound Type) *TypeParam {
-	// Always increment lastID, even if it is not used.
-	id := nextID()
-	if check != nil {
-		check.nextID++
-		id = check.nextID
-	}
-	typ := &TypeParam{check: check, id: id, obj: obj, index: index, bound: bound}
-	if obj.typ == nil {
-		obj.typ = typ
-	}
-	return typ
-}
-
-// Index returns the index of the type param within its param list.
-func (t *TypeParam) Index() int {
-	return t.index
-}
-
-// SetId sets the unique id of a type param. Should only be used for type params
-// in imported generic types.
-func (t *TypeParam) SetId(id uint64) {
-	t.id = id
-}
-
-func (t *TypeParam) Bound() *Interface {
-	// we may not have an interface (error reported elsewhere)
-	iface, _ := under(t.bound).(*Interface)
-	if iface == nil {
-		return &emptyInterface
-	}
-	// use the type bound position if we have one
-	pos := nopos
-	if n, _ := t.bound.(*Named); n != nil {
-		pos = n.obj.pos
-	}
-	// TODO(gri) switch this to an unexported method on Checker.
-	computeTypeSet(t.check, pos, iface)
-	return iface
-}
-
-func (t *TypeParam) SetBound(bound Type) {
-	if bound == nil {
-		panic("types2.TypeParam.SetBound: bound must not be nil")
-	}
-	t.bound = bound
-}
-
 // 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
@@ -441,102 +72,6 @@ func optype(typ Type) Type {
 	return under(typ)
 }
 
-// An instance represents an instantiated generic type syntactically
-// (without expanding the instantiation). Type instances appear only
-// during type-checking and are replaced by their fully instantiated
-// (expanded) types before the end of type-checking.
-type instance struct {
-	check   *Checker     // for lazy instantiation
-	pos     syntax.Pos   // position of type instantiation; for error reporting only
-	base    *Named       // parameterized type to be instantiated
-	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
-}
-
-// expand returns the instantiated (= expanded) type of t.
-// The result is either an instantiated *Named type, or
-// Typ[Invalid] if there was an error.
-func (t *instance) expand() Type {
-	v := t.value
-	if v == nil {
-		v = t.check.instantiate(t.pos, t.base, t.targs, t.poslist)
-		if v == nil {
-			v = Typ[Invalid]
-		}
-		t.value = v
-	}
-	// After instantiation we must have an invalid or a *Named type.
-	if debug && v != Typ[Invalid] {
-		_ = v.(*Named)
-	}
-	return v
-}
-
-// expand expands a type instance into its instantiated
-// type and leaves all other types alone. expand does
-// not recurse.
-func expand(typ Type) Type {
-	if t, _ := typ.(*instance); t != nil {
-		return t.expand()
-	}
-	return typ
-}
-
-// expandf is set to expand.
-// Call expandf when calling expand causes compile-time cycle error.
-var expandf func(Type) Type
-
-func init() { expandf = expand }
-
-// top represents the top of the type lattice.
-// It is the underlying type of a type parameter that
-// can be satisfied by any type (ignoring methods),
-// because its type constraint contains no restrictions
-// besides methods.
-type top struct{}
-
-// theTop is the singleton top type.
-var theTop = &top{}
-
-// Type-specific implementations of Underlying.
-func (t *Basic) Underlying() Type     { return t }
-func (t *Array) Underlying() Type     { return t }
-func (t *Slice) Underlying() Type     { return t }
-func (t *Pointer) Underlying() Type   { return t }
-func (t *Tuple) Underlying() Type     { return t }
-func (t *Map) Underlying() Type       { return t }
-func (t *Chan) Underlying() Type      { return t }
-func (t *Named) Underlying() Type     { return t.expand().underlying }
-func (t *TypeParam) Underlying() Type { return t }
-func (t *instance) Underlying() Type  { return t }
-func (t *top) Underlying() 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) }
-func (t *Slice) String() string     { return TypeString(t, nil) }
-func (t *Pointer) String() string   { return TypeString(t, nil) }
-func (t *Tuple) String() string     { return TypeString(t, nil) }
-func (t *Map) String() string       { return TypeString(t, nil) }
-func (t *Chan) String() string      { return TypeString(t, nil) }
-func (t *Named) String() string     { return TypeString(t, nil) }
-func (t *TypeParam) String() string { return TypeString(t, nil) }
-func (t *instance) 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.
-func under(t Type) Type {
-	// TODO(gri) is this correct for *Union?
-	if n := asNamed(t); n != nil {
-		return n.under()
-	}
-	return t
-}
-
 // Converters
 //
 // A converter must only be called when a type is
diff --git a/src/cmd/compile/internal/types2/typeparam.go b/src/cmd/compile/internal/types2/typeparam.go
new file mode 100644
index 0000000000..5c74405912
--- /dev/null
+++ b/src/cmd/compile/internal/types2/typeparam.go
@@ -0,0 +1,80 @@
+// Copyright 2011 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 "sync/atomic"
+
+// Note: This is a uint32 rather than a uint64 because the
+// respective 64 bit atomic instructions are not available
+// on all platforms.
+var lastID uint32
+
+// nextID returns a value increasing monotonically by 1 with
+// each call, starting with 1. It may be called concurrently.
+func nextID() uint64 { return uint64(atomic.AddUint32(&lastID, 1)) }
+
+// A TypeParam represents a type parameter type.
+type TypeParam struct {
+	check *Checker  // for lazy type bound completion
+	id    uint64    // unique id, for debugging only
+	obj   *TypeName // corresponding type name
+	index int       // type parameter index in source order, starting at 0
+	bound Type      // *Named or *Interface; underlying type is always *Interface
+}
+
+// Obj returns the type name for the type parameter t.
+func (t *TypeParam) Obj() *TypeName { return t.obj }
+
+// NewTypeParam returns a new TypeParam.  bound can be nil (and set later).
+func (check *Checker) NewTypeParam(obj *TypeName, index int, bound Type) *TypeParam {
+	// Always increment lastID, even if it is not used.
+	id := nextID()
+	if check != nil {
+		check.nextID++
+		id = check.nextID
+	}
+	typ := &TypeParam{check: check, id: id, obj: obj, index: index, bound: bound}
+	if obj.typ == nil {
+		obj.typ = typ
+	}
+	return typ
+}
+
+// Index returns the index of the type param within its param list.
+func (t *TypeParam) Index() int {
+	return t.index
+}
+
+// SetId sets the unique id of a type param. Should only be used for type params
+// in imported generic types.
+func (t *TypeParam) SetId(id uint64) {
+	t.id = id
+}
+
+func (t *TypeParam) Bound() *Interface {
+	// we may not have an interface (error reported elsewhere)
+	iface, _ := under(t.bound).(*Interface)
+	if iface == nil {
+		return &emptyInterface
+	}
+	// use the type bound position if we have one
+	pos := nopos
+	if n, _ := t.bound.(*Named); n != nil {
+		pos = n.obj.pos
+	}
+	// TODO(gri) switch this to an unexported method on Checker.
+	computeTypeSet(t.check, pos, iface)
+	return iface
+}
+
+func (t *TypeParam) SetBound(bound Type) {
+	if bound == nil {
+		panic("types2.TypeParam.SetBound: bound must not be nil")
+	}
+	t.bound = bound
+}
+
+func (t *TypeParam) Underlying() Type { return t }
+func (t *TypeParam) String() string   { return TypeString(t, nil) }