go/types: use a TypeList type to hold type arguments

This resolves an asymmetry between the TParams and TArgs APIs, and
reduces the size of the Named struct at the cost of some additional nil
checks.

While at it, move TParamList and TypeList to a new file:typelists.go,
and change TParamList to access the tparams slice directly in At. There
is no reason to guard against a nil receiver, as accessing an index on
the empty slice will panic anyway.

Change-Id: I9b65247e06c697a57a4efe40c3390e0faff91441
Reviewed-on: https://go-review.googlesource.com/c/go/+/343933
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

11 files changed, 96 insertions, 69 deletions

diff --git a/src/go/types/infer.go b/src/go/types/infer.go
index 58456444ed..e6417545e9 100644
--- a/src/go/types/infer.go
+++ b/src/go/types/infer.go
@@ -330,7 +330,7 @@ func (w *tpWalker) isParameterized(typ Type) (res bool) {
 		return w.isParameterized(t.elem)
 
 	case *Named:
-		return w.isParameterizedTypeList(t.targs)
+		return w.isParameterizedTypeList(t.targs.list())
 
 	case *TypeParam:
 		// t must be one of w.tparams
diff --git a/src/go/types/instantiate.go b/src/go/types/instantiate.go
index fb2fddfc0f..3ee09b7e84 100644
--- a/src/go/types/instantiate.go
+++ b/src/go/types/instantiate.go
@@ -131,7 +131,7 @@ func (check *Checker) instance(pos token.Pos, typ Type, targs []Type) (res Type)
 
 		tname := NewTypeName(pos, t.obj.pkg, t.obj.name, nil)
 		named := check.newNamed(tname, t, nil, nil, nil) // methods and tparams are set when named is loaded
-		named.targs = targs
+		named.targs = &TypeList{targs}
 		named.instance = &instance{pos}
 		if check != nil {
 			check.typMap[h] = named
@@ -139,7 +139,7 @@ func (check *Checker) instance(pos token.Pos, typ Type, targs []Type) (res Type)
 		res = named
 	case *Signature:
 		tparams := t.TParams()
-		if !check.validateTArgLen(pos, tparams, targs) {
+		if !check.validateTArgLen(pos, tparams.Len(), len(targs)) {
 			return Typ[Invalid]
 		}
 		if tparams.Len() == 0 {
@@ -174,14 +174,14 @@ func (check *Checker) instance(pos token.Pos, typ Type, targs []Type) (res Type)
 // validateTArgLen verifies that the length of targs and tparams matches,
 // reporting an error if not. If validation fails and check is nil,
 // validateTArgLen panics.
-func (check *Checker) validateTArgLen(pos token.Pos, tparams *TParamList, targs []Type) bool {
-	if len(targs) != tparams.Len() {
+func (check *Checker) validateTArgLen(pos token.Pos, ntparams, ntargs int) bool {
+	if ntargs != ntparams {
 		// TODO(gri) provide better error message
 		if check != nil {
-			check.errorf(atPos(pos), _Todo, "got %d arguments but %d type parameters", len(targs), tparams.Len())
+			check.errorf(atPos(pos), _Todo, "got %d arguments but %d type parameters", ntargs, ntparams)
 			return false
 		}
-		panic(fmt.Sprintf("%v: got %d arguments but %d type parameters", pos, len(targs), tparams.Len()))
+		panic(fmt.Sprintf("%v: got %d arguments but %d type parameters", pos, ntargs, ntparams))
 	}
 	return true
 }
diff --git a/src/go/types/lookup.go b/src/go/types/lookup.go
index 20a24cab73..862e59401d 100644
--- a/src/go/types/lookup.go
+++ b/src/go/types/lookup.go
@@ -392,10 +392,10 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 			// here. Exit early in this case to prevent an assertion
 			// failure in makeSubstMap.
 			// TODO(gri) Can we avoid this check by fixing the lengths?
-			if len(ftyp.RParams().list()) != len(Vn.targs) {
+			if len(ftyp.RParams().list()) != Vn.targs.Len() {
 				return
 			}
-			ftyp = check.subst(token.NoPos, ftyp, makeSubstMap(ftyp.RParams().list(), Vn.targs), nil).(*Signature)
+			ftyp = check.subst(token.NoPos, ftyp, makeSubstMap(ftyp.RParams().list(), Vn.targs.list()), nil).(*Signature)
 		}
 
 		// If the methods have type parameters we don't care whether they
diff --git a/src/go/types/named.go b/src/go/types/named.go
index 105f34e1fb..6bc33b9538 100644
--- a/src/go/types/named.go
+++ b/src/go/types/named.go
@@ -21,7 +21,7 @@ type Named struct {
 	underlying Type        // possibly a *Named during setup; never a *Named once set up completely
 	instance   *instance   // syntactic information for lazy instantiation
 	tparams    *TParamList // type parameters, or nil
-	targs      []Type      // type arguments (after instantiation), or nil
+	targs      *TypeList   // 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) ([]*TypeParam, Type, []*Func)
@@ -46,7 +46,7 @@ func (t *Named) load() *Named {
 	// underlying is set when t is expanded.
 	//
 	// By convention, a type instance is loaded iff its tparams are set.
-	if len(t.targs) > 0 && t.tparams == nil {
+	if t.targs.Len() > 0 && t.tparams == nil {
 		t.orig.load()
 		t.tparams = t.orig.tparams
 		t.methods = t.orig.methods
@@ -128,12 +128,8 @@ func (t *Named) TParams() *TParamList { return t.load().tparams }
 // SetTParams sets the type parameters of the named type t.
 func (t *Named) SetTParams(tparams []*TypeParam) { t.load().tparams = bindTParams(tparams) }
 
-// NumTArgs returns the number of type arguments used to instantiate the named
-// type t, or 0 if t is not an instantiated type.
-func (t *Named) NumTArgs() int { return len(t.targs) }
-
-// TArgs returns the i'th type argument of the named type t for 0 <= i < t.NumTArgs().
-func (t *Named) TArg(i int) Type { return t.targs[i] }
+// TArgs returns the type arguments used to instantiate the named type t.
+func (t *Named) TArgs() *TypeList { return t.targs }
 
 // NumMethods returns the number of explicit methods whose receiver is named type t.
 func (t *Named) NumMethods() int { return len(t.load().methods) }
@@ -263,7 +259,7 @@ func (n *Named) expand(typMap map[string]*Named) *Named {
 		// explicit is harmless: load is idempotent.
 		n.load()
 		var u Type
-		if n.check.validateTArgLen(n.instance.pos, n.tparams, n.targs) {
+		if n.check.validateTArgLen(n.instance.pos, n.tparams.Len(), n.targs.Len()) {
 			if typMap == nil {
 				if n.check != nil {
 					typMap = n.check.typMap
@@ -272,11 +268,11 @@ func (n *Named) expand(typMap map[string]*Named) *Named {
 					// type-checking pass. In that case we won't have a pre-existing
 					// typMap, but don't want to create a duplicate of the current instance
 					// in the process of expansion.
-					h := instantiatedHash(n.orig, n.targs)
+					h := instantiatedHash(n.orig, n.targs.list())
 					typMap = map[string]*Named{h: n}
 				}
 			}
-			u = n.check.subst(n.instance.pos, n.orig.underlying, makeSubstMap(n.TParams().list(), n.targs), typMap)
+			u = n.check.subst(n.instance.pos, n.orig.underlying, makeSubstMap(n.TParams().list(), n.targs.list()), typMap)
 		} else {
 			u = Typ[Invalid]
 		}
diff --git a/src/go/types/predicates.go b/src/go/types/predicates.go
index 027e30c1a1..2f4ef9dace 100644
--- a/src/go/types/predicates.go
+++ b/src/go/types/predicates.go
@@ -305,22 +305,22 @@ func identical(x, y Type, cmpTags bool, p *ifacePair) bool {
 			x.expand(nil)
 			y.expand(nil)
 
-			// xargs := x.TArgs()
-			// yargs := y.TArgs()
+			xargs := x.TArgs().list()
+			yargs := y.TArgs().list()
 
-			if x.NumTArgs() != y.NumTArgs() {
+			if len(xargs) != len(yargs) {
 				return false
 			}
 
-			if nargs := x.NumTArgs(); nargs > 0 {
+			if nargs := len(xargs); nargs > 0 {
 				// Instances are identical if their original type and type arguments
 				// are identical.
 				if !Identical(x.orig, y.orig) {
 					return false
 				}
 				for i := 0; i < nargs; i++ {
-					xa := x.TArg(i)
-					ya := y.TArg(i)
+					xa := xargs[i]
+					ya := yargs[i]
 					if !Identical(xa, ya) {
 						return false
 					}
diff --git a/src/go/types/sizeof_test.go b/src/go/types/sizeof_test.go
index 403f2bbece..c2f5b3c333 100644
--- a/src/go/types/sizeof_test.go
+++ b/src/go/types/sizeof_test.go
@@ -30,7 +30,7 @@ func TestSizeof(t *testing.T) {
 		{Interface{}, 40, 80},
 		{Map{}, 16, 32},
 		{Chan{}, 12, 24},
-		{Named{}, 80, 152},
+		{Named{}, 72, 136},
 		{TypeParam{}, 28, 48},
 		{term{}, 12, 24},
 		{top{}, 0, 0},
diff --git a/src/go/types/subst.go b/src/go/types/subst.go
index 75f1ca5fab..8b8d6fb82a 100644
--- a/src/go/types/subst.go
+++ b/src/go/types/subst.go
@@ -191,21 +191,21 @@ func (subst *subster) typ(typ Type) Type {
 		}
 
 		var newTArgs []Type
-		assert(len(t.targs) == t.TParams().Len())
+		assert(t.targs.Len() == t.TParams().Len())
 
 		// already instantiated
 		dump(">>> %s already instantiated", t)
 		// For each (existing) type argument targ, determine if it needs
 		// to be substituted; i.e., if it is or contains a type parameter
 		// that has a type argument for it.
-		for i, targ := range t.targs {
+		for i, targ := range t.targs.list() {
 			dump(">>> %d targ = %s", i, targ)
 			new_targ := subst.typ(targ)
 			if new_targ != targ {
 				dump(">>> substituted %d targ %s => %s", i, targ, new_targ)
 				if newTArgs == nil {
 					newTArgs = make([]Type, t.TParams().Len())
-					copy(newTArgs, t.targs)
+					copy(newTArgs, t.targs.list())
 				}
 				newTArgs[i] = new_targ
 			}
@@ -233,7 +233,7 @@ func (subst *subster) typ(typ Type) Type {
 		// It's ok to provide a nil *Checker because the newly created type
 		// doesn't need to be (lazily) expanded; it's expanded below.
 		named := (*Checker)(nil).newNamed(tname, t.orig, nil, t.tparams, t.methods) // t is loaded, so tparams and methods are available
-		named.targs = newTArgs
+		named.targs = &TypeList{newTArgs}
 		subst.typMap[h] = named
 		t.expand(subst.typMap) // must happen after typMap update to avoid infinite recursion
 
diff --git a/src/go/types/typelists.go b/src/go/types/typelists.go
new file mode 100644
index 0000000000..a8181404bf
--- /dev/null
+++ b/src/go/types/typelists.go
@@ -0,0 +1,61 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+// TParamList holds a list of type parameters.
+type TParamList struct{ tparams []*TypeParam }
+
+// Len returns the number of type parameters in the list.
+// It is safe to call on a nil receiver.
+func (l *TParamList) Len() int { return len(l.list()) }
+
+// At returns the i'th type parameter in the list.
+func (l *TParamList) At(i int) *TypeParam { return l.tparams[i] }
+
+// list is for internal use where we expect a []*TypeParam.
+// TODO(rfindley): list should probably be eliminated: we can pass around a
+// TParamList instead.
+func (l *TParamList) list() []*TypeParam {
+	if l == nil {
+		return nil
+	}
+	return l.tparams
+}
+
+// TypeList holds a list of types.
+type TypeList struct{ types []Type }
+
+// Len returns the number of types in the list.
+// It is safe to call on a nil receiver.
+func (l *TypeList) Len() int { return len(l.list()) }
+
+// At returns the i'th type in the list.
+func (l *TypeList) At(i int) Type { return l.types[i] }
+
+// list is for internal use where we expect a []Type.
+// TODO(rfindley): list should probably be eliminated: we can pass around a
+// TypeList instead.
+func (l *TypeList) list() []Type {
+	if l == nil {
+		return nil
+	}
+	return l.types
+}
+
+// ----------------------------------------------------------------------------
+// Implementation
+
+func bindTParams(list []*TypeParam) *TParamList {
+	if len(list) == 0 {
+		return nil
+	}
+	for i, typ := range list {
+		if typ.index >= 0 {
+			panic("type parameter bound more than once")
+		}
+		typ.index = i
+	}
+	return &TParamList{tparams: list}
+}
diff --git a/src/go/types/typeparam.go b/src/go/types/typeparam.go
index 49eda1b43a..b6952489ca 100644
--- a/src/go/types/typeparam.go
+++ b/src/go/types/typeparam.go
@@ -88,40 +88,6 @@ func (t *TypeParam) SetConstraint(bound Type) {
 func (t *TypeParam) Underlying() Type { return t }
 func (t *TypeParam) String() string   { return TypeString(t, nil) }
 
-// TParamList holds a list of type parameters bound to a type.
-type TParamList struct{ tparams []*TypeParam }
-
-// Len returns the number of type parameters in the list.
-// It is safe to call on a nil receiver.
-func (tps *TParamList) Len() int {
-	return len(tps.list())
-}
-
-// At returns the i'th type parameter in the list.
-func (tps *TParamList) At(i int) *TypeParam {
-	return tps.list()[i]
-}
-
-func (tps *TParamList) list() []*TypeParam {
-	if tps == nil {
-		return nil
-	}
-	return tps.tparams
-}
-
-func bindTParams(list []*TypeParam) *TParamList {
-	if len(list) == 0 {
-		return nil
-	}
-	for i, typ := range list {
-		if typ.index >= 0 {
-			panic("type parameter bound more than once")
-		}
-		typ.index = i
-	}
-	return &TParamList{tparams: list}
-}
-
 // ----------------------------------------------------------------------------
 // Implementation
 
diff --git a/src/go/types/typestring.go b/src/go/types/typestring.go
index 41dde0e469..cdc7ea9f51 100644
--- a/src/go/types/typestring.go
+++ b/src/go/types/typestring.go
@@ -204,7 +204,7 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 		if t.targs != nil {
 			// instantiated type
 			buf.WriteByte('[')
-			writeTypeList(buf, t.targs, qf, visited)
+			writeTypeList(buf, t.targs.list(), qf, visited)
 			buf.WriteByte(']')
 		} else if t.TParams().Len() != 0 {
 			// parameterized type
diff --git a/src/go/types/unify.go b/src/go/types/unify.go
index 8f5b23ce39..1720646db9 100644
--- a/src/go/types/unify.go
+++ b/src/go/types/unify.go
@@ -426,13 +426,17 @@ func (u *unifier) nify(x, y Type, p *ifacePair) bool {
 		if y, ok := y.(*Named); ok {
 			x.expand(nil)
 			y.expand(nil)
+
+			xargs := x.targs.list()
+			yargs := y.targs.list()
+
 			// TODO(gri) This is not always correct: two types may have the same names
 			//           in the same package if one of them is nested in a function.
 			//           Extremely unlikely but we need an always correct solution.
 			if x.obj.pkg == y.obj.pkg && x.obj.name == y.obj.name {
-				assert(len(x.targs) == len(y.targs))
-				for i, x := range x.targs {
-					if !u.nify(x, y.targs[i], p) {
+				assert(len(xargs) == len(yargs))
+				for i, x := range xargs {
+					if !u.nify(x, yargs[i], p) {
 						return false
 					}
 				}