[dev.typeparams] cmd/compile: set type parameter indices when they are bound

This is a port of CL 336249 with adjustments due to slightly
different handling of type parameter declaration in types2.

The CL also contains adjustments to the compiler front-end.

With this change it is not necessary to export type parameter
indices. Filed issue #47451 so we don't forget.

Change-Id: I2834f7be313fcb4763dff2a9058f1983ee6a81b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/338192
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>

src/cmd/compile/internal/importer/iimport.go

@@ -368,10 +368,13 @@ func (r *importReader) obj(name string) {
 		if r.p.exportVersion < iexportVersionGenerics {
 			errorf("unexpected type param type")
 		}
-		index := int(r.int64())
+		// Type parameter indices are lazily "allocated".
+		// There's no need to export them anymore.
+		// TODO change the export format accordingly
+		_ = int(r.int64())
 		name0, sub := parseSubscript(name)
 		tn := types2.NewTypeName(pos, r.currPkg, name0, nil)
-		t := (*types2.Checker)(nil).NewTypeParam(tn, index, nil)
+		t := (*types2.Checker)(nil).NewTypeParam(tn, nil)
 		if sub == 0 {
 			errorf("missing subscript")
 		}

src/cmd/compile/internal/noder/decl.go

@@ -167,10 +167,10 @@ func (g *irgen) typeDecl(out *ir.Nodes, decl *syntax.TypeDecl) {
 	ntyp.SetUnderlying(g.typeExpr(decl.Type))
 
 	tparams := otyp.(*types2.Named).TParams()
-	if len(tparams) > 0 {
+	if n := tparams.Len(); n > 0 {
-		rparams := make([]*types.Type, len(tparams))
+		rparams := make([]*types.Type, n)
 		for i := range rparams {
-			rparams[i] = g.typ(tparams[i].Type())
+			rparams[i] = g.typ(tparams.At(i).Type())
 		}
 		// This will set hasTParam flag if any rparams are not concrete types.
 		ntyp.SetRParams(rparams)

src/cmd/compile/internal/noder/expr.go

@@ -337,7 +337,7 @@ func (g *irgen) selectorExpr(pos src.XPos, typ types2.Type, expr *syntax.Selecto
 			if wantPtr {
 				recvType2Base = types2.AsPointer(recvType2).Elem()
 			}
-			if len(types2.AsNamed(recvType2Base).TParams()) > 0 {
+			if types2.AsNamed(recvType2Base).TParams().Len() > 0 {
 				// recvType2 is the original generic type that is
 				// instantiated for this method call.
 				// selinfo.Recv() is the instantiated type

src/cmd/compile/internal/noder/reader2.go

@@ -481,7 +481,7 @@ func (r *reader2) typeParamNames() []*types2.TypeName {
 		pkg, name := r.localIdent()
 
 		names[i] = types2.NewTypeName(pos, pkg, name, nil)
-		r.dict.tparams[i] = r.p.check.NewTypeParam(names[i], i, nil)
+		r.dict.tparams[i] = r.p.check.NewTypeParam(names[i], nil)
 	}
 
 	for i, bound := range r.dict.bounds {

src/cmd/compile/internal/noder/types.go

@@ -304,9 +304,9 @@ func (g *irgen) fillinMethods(typ *types2.Named, ntyp *types.Type) {
 				} else {
 					meth2 = ir.NewNameAt(meth.Pos(), newsym)
 					rparams := types2.AsSignature(m.Type()).RParams()
-					tparams := make([]*types.Type, len(rparams))
+					tparams := make([]*types.Type, rparams.Len())
-					for i, rparam := range rparams {
+					for i := range tparams {
-						tparams[i] = g.typ1(rparam.Type())
+						tparams[i] = g.typ1(rparams.At(i).Type())
 					}
 					assert(len(tparams) == len(targs))
 					ts := typecheck.Tsubster{
@@ -336,9 +336,9 @@ func (g *irgen) fillinMethods(typ *types2.Named, ntyp *types.Type) {
 
 func (g *irgen) signature(recv *types.Field, sig *types2.Signature) *types.Type {
 	tparams2 := sig.TParams()
-	tparams := make([]*types.Field, len(tparams2))
+	tparams := make([]*types.Field, tparams2.Len())
 	for i := range tparams {
-		tp := tparams2[i]
+		tp := tparams2.At(i)
 		tparams[i] = types.NewField(g.pos(tp), g.sym(tp), g.typ1(tp.Type()))
 	}
 

src/cmd/compile/internal/noder/writer.go

@@ -299,7 +299,7 @@ func (pw *pkgWriter) typIdx(typ types2.Type, dict *writerDict) typeInfo {
 		// Type aliases can refer to uninstantiated generic types, so we
 		// might see len(TParams) != 0 && len(TArgs) == 0 here.
 		// TODO(mdempsky): Revisit after #46477 is resolved.
-		assert(len(typ.TParams()) == len(typ.TArgs()) || len(typ.TArgs()) == 0)
+		assert(typ.TParams().Len() == len(typ.TArgs()) || len(typ.TArgs()) == 0)
 
 		// TODO(mdempsky): Why do we need to loop here?
 		orig := typ
@@ -615,9 +615,10 @@ func (w *writer) objDict(obj types2.Object, dict *writerDict) {
 	w.len(len(dict.implicits))
 
 	tparams := objTypeParams(obj)
-	w.len(len(tparams))
+	ntparams := tparams.Len()
-	for _, tparam := range tparams {
+	w.len(ntparams)
-		w.typ(tparam.Type().(*types2.TypeParam).Bound())
+	for i := 0; i < ntparams; i++ {
+		w.typ(tparams.At(i).Type().(*types2.TypeParam).Bound())
 	}
 
 	nderived := len(dict.derived)
@@ -641,10 +642,12 @@ func (w *writer) objDict(obj types2.Object, dict *writerDict) {
 	assert(len(dict.funcs) == nfuncs)
 }
 
-func (w *writer) typeParamNames(tparams []*types2.TypeName) {
+func (w *writer) typeParamNames(tparams *types2.TypeParams) {
 	w.sync(syncTypeParamNames)
 
-	for _, tparam := range tparams {
+	ntparams := tparams.Len()
+	for i := 0; i < ntparams; i++ {
+		tparam := tparams.At(i)
 		w.pos(tparam)
 		w.localIdent(tparam)
 	}
@@ -1468,13 +1471,16 @@ type declCollector struct {
 
 func (c *declCollector) withTParams(obj types2.Object) *declCollector {
 	tparams := objTypeParams(obj)
-	if len(tparams) == 0 {
+	n := tparams.Len()
+	if n == 0 {
 		return c
 	}
 
 	copy := *c
 	copy.implicits = copy.implicits[:len(copy.implicits):len(copy.implicits)]
-	copy.implicits = append(copy.implicits, objTypeParams(obj)...)
+	for i := 0; i < n; i++ {
+		copy.implicits = append(copy.implicits, tparams.At(i))
+	}
 	return &copy
 }
 
@@ -1705,7 +1711,7 @@ func (w *writer) pkgDecl(decl syntax.Decl) {
 		// TODO(mdempsky): Revisit after #46477 is resolved.
 		if name.IsAlias() {
 			named, ok := name.Type().(*types2.Named)
-			if ok && len(named.TParams()) != 0 && len(named.TArgs()) == 0 {
+			if ok && named.TParams().Len() != 0 && len(named.TArgs()) == 0 {
 				break
 			}
 		}
@@ -1851,7 +1857,7 @@ func fieldIndex(info *types2.Info, str *types2.Struct, key *syntax.Name) int {
 }
 
 // objTypeParams returns the type parameters on the given object.
-func objTypeParams(obj types2.Object) []*types2.TypeName {
+func objTypeParams(obj types2.Object) *types2.TypeParams {
 	switch obj := obj.(type) {
 	case *types2.Func:
 		sig := obj.Type().(*types2.Signature)

src/cmd/compile/internal/types2/api_test.go

@@ -1857,7 +1857,7 @@ func TestInstantiate(t *testing.T) {
 
 	// type T should have one type parameter
 	T := pkg.Scope().Lookup("T").Type().(*Named)
-	if n := len(T.TParams()); n != 1 {
+	if n := T.TParams().Len(); n != 1 {
 		t.Fatalf("expected 1 type parameter; found %d", n)
 	}
 

src/cmd/compile/internal/types2/assignments.go

@@ -68,7 +68,7 @@ func (check *Checker) assignment(x *operand, T Type, context string) {
 	// x.typ is typed
 
 	// A generic (non-instantiated) function value cannot be assigned to a variable.
-	if sig := asSignature(x.typ); sig != nil && len(sig.tparams) > 0 {
+	if sig := asSignature(x.typ); sig != nil && sig.TParams().Len() > 0 {
 		check.errorf(x, "cannot use generic function %s without instantiation in %s", x, context)
 	}
 

src/cmd/compile/internal/types2/builtins.go

@@ -837,7 +837,8 @@ func (check *Checker) applyTypeFunc(f func(Type) Type, x Type) Type {
 		// type param is placed in the current package so export/import
 		// works as expected.
 		tpar := NewTypeName(nopos, check.pkg, "<type parameter>", nil)
-		ptyp := check.NewTypeParam(tpar, tp.index, &emptyInterface) // assigns type to tpar as a side-effect
+		ptyp := check.NewTypeParam(tpar, &emptyInterface) // assigns type to tpar as a side-effect
+		ptyp.index = tp.index
 		tsum := newUnion(rtypes, tildes)
 		ptyp.bound = &Interface{complete: true, tset: &TypeSet{types: tsum}}
 

src/cmd/compile/internal/types2/call.go

@@ -26,7 +26,7 @@ func (check *Checker) funcInst(x *operand, inst *syntax.IndexExpr) {
 
 	// check number of type arguments (got) vs number of type parameters (want)
 	sig := x.typ.(*Signature)
-	got, want := len(targs), len(sig.tparams)
+	got, want := len(targs), sig.TParams().Len()
 	if !useConstraintTypeInference && got != want || got > want {
 		check.errorf(xlist[got-1], "got %d type arguments but want %d", got, want)
 		x.mode = invalid
@@ -37,7 +37,7 @@ func (check *Checker) funcInst(x *operand, inst *syntax.IndexExpr) {
 	// if we don't have enough type arguments, try type inference
 	inferred := false
 	if got < want {
-		targs = check.infer(inst.Pos(), sig.tparams, targs, nil, nil, true)
+		targs = check.infer(inst.Pos(), sig.TParams().list(), targs, nil, nil, true)
 		if targs == nil {
 			// error was already reported
 			x.mode = invalid
@@ -155,7 +155,7 @@ func (check *Checker) callExpr(x *operand, call *syntax.CallExpr) exprKind {
 		assert(len(targs) == len(xlist))
 
 		// check number of type arguments (got) vs number of type parameters (want)
-		got, want := len(targs), len(sig.tparams)
+		got, want := len(targs), sig.TParams().Len()
 		if got > want {
 			check.errorf(xlist[want], "got %d type arguments but want %d", got, want)
 			check.use(call.ArgList...)
@@ -189,7 +189,7 @@ func (check *Checker) callExpr(x *operand, call *syntax.CallExpr) exprKind {
 
 	// if type inference failed, a parametrized result must be invalidated
 	// (operands cannot have a parametrized type)
-	if x.mode == value && len(sig.tparams) > 0 && isParameterized(sig.tparams, x.typ) {
+	if x.mode == value && sig.TParams().Len() > 0 && isParameterized(sig.TParams().list(), x.typ) {
 		x.mode = invalid
 	}
 
@@ -317,10 +317,10 @@ func (check *Checker) arguments(call *syntax.CallExpr, sig *Signature, targs []T
 	}
 
 	// infer type arguments and instantiate signature if necessary
-	if len(sig.tparams) > 0 {
+	if sig.TParams().Len() > 0 {
 		// TODO(gri) provide position information for targs so we can feed
 		//           it to the instantiate call for better error reporting
-		targs = check.infer(call.Pos(), sig.tparams, targs, sigParams, args, true)
+		targs := check.infer(call.Pos(), sig.TParams().list(), targs, sigParams, args, true)
 		if targs == nil {
 			return // error already reported
 		}
@@ -334,7 +334,7 @@ func (check *Checker) arguments(call *syntax.CallExpr, sig *Signature, targs []T
 		// need to compute it from the adjusted list; otherwise we can
 		// simply use the result signature's parameter list.
 		if adjusted {
-			sigParams = check.subst(call.Pos(), sigParams, makeSubstMap(sig.tparams, targs)).(*Tuple)
+			sigParams = check.subst(call.Pos(), sigParams, makeSubstMap(sig.TParams().list(), targs)).(*Tuple)
 		} else {
 			sigParams = rsig.params
 		}
@@ -516,7 +516,7 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr) {
 		// the signature accordingly.
 		// TODO(gri) factor this code out
 		sig := m.typ.(*Signature)
-		if len(sig.rparams) > 0 {
+		if sig.RParams().Len() > 0 {
 			// For inference to work, we must use the receiver type
 			// matching the receiver in the actual method declaration.
 			// If the method is embedded, the matching receiver is the
@@ -545,7 +545,7 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr) {
 			// the receiver type arguments here, the receiver must be be otherwise invalid
 			// and an error has been reported elsewhere.
 			arg := operand{mode: variable, expr: x.expr, typ: recv}
-			targs := check.infer(m.pos, sig.rparams, nil, NewTuple(sig.recv), []*operand{&arg}, false /* no error reporting */)
+			targs := check.infer(m.pos, sig.RParams().list(), nil, NewTuple(sig.recv), []*operand{&arg}, false /* no error reporting */)
 			//check.dump("### inferred targs = %s", targs)
 			if targs == nil {
 				// We may reach here if there were other errors (see issue #40056).
@@ -555,7 +555,7 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr) {
 			// (If we modify m, some tests will fail; possibly because the m is in use.)
 			// TODO(gri) investigate and provide a correct explanation here
 			copy := *m
-			copy.typ = check.subst(e.Pos(), m.typ, makeSubstMap(sig.rparams, targs))
+			copy.typ = check.subst(e.Pos(), m.typ, makeSubstMap(sig.RParams().list(), targs))
 			obj = &copy
 		}
 		// TODO(gri) we also need to do substitution for parameterized interface methods

src/cmd/compile/internal/types2/decl.go

@@ -575,20 +575,20 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named
 	named.underlying = under(named)
 
 	// If the RHS is a type parameter, it must be from this type declaration.
-	if tpar, _ := named.underlying.(*TypeParam); tpar != nil && tparamIndex(named.tparams, tpar) < 0 {
+	if tpar, _ := named.underlying.(*TypeParam); tpar != nil && tparamIndex(named.tparams.list(), tpar) < 0 {
 		check.errorf(tdecl.Type, "cannot use function type parameter %s as RHS in type declaration", tpar)
 		named.underlying = Typ[Invalid]
 	}
 }
 
-func (check *Checker) collectTypeParams(list []*syntax.Field) []*TypeName {
+func (check *Checker) collectTypeParams(list []*syntax.Field) *TypeParams {
 	tparams := make([]*TypeName, len(list))
 
 	// Declare type parameters up-front.
 	// The scope of type parameters starts at the beginning of the type parameter
 	// list (so we can have mutually recursive parameterized type bounds).
 	for i, f := range list {
-		tparams[i] = check.declareTypeParam(i, f.Name)
+		tparams[i] = check.declareTypeParam(f.Name)
 	}
 
 	var bound Type
@@ -602,12 +602,12 @@ func (check *Checker) collectTypeParams(list []*syntax.Field) []*TypeName {
 		tparams[i].typ.(*TypeParam).bound = bound
 	}
 
-	return tparams
+	return bindTParams(tparams)
 }
 
-func (check *Checker) declareTypeParam(index int, name *syntax.Name) *TypeName {
+func (check *Checker) declareTypeParam(name *syntax.Name) *TypeName {
 	tpar := NewTypeName(name.Pos(), check.pkg, name.Value, nil)
-	check.NewTypeParam(tpar, index, nil)                    // assigns type to tpar as a side-effect
+	check.NewTypeParam(tpar, nil)                           // assigns type to tpar as a side-effect
 	check.declare(check.scope, name, tpar, check.scope.pos) // TODO(gri) check scope position
 	return tpar
 }

src/cmd/compile/internal/types2/index.go

@@ -32,7 +32,7 @@ func (check *Checker) indexExpr(x *operand, e *syntax.IndexExpr) (isFuncInst boo
 		return false
 
 	case value:
-		if sig := asSignature(x.typ); sig != nil && len(sig.tparams) > 0 {
+		if sig := asSignature(x.typ); sig != nil && sig.TParams().Len() > 0 {
 			// function instantiation
 			return true
 		}

src/cmd/compile/internal/types2/instance.go

@@ -25,7 +25,7 @@ func (n *Named) expand() {
 		// tparams. This is done implicitly by the call to n.TParams, but making it
 		// explicit is harmless: load is idempotent.
 		n.load()
-		inst := n.check.instantiate(n.instance.pos, n.orig.underlying, n.TParams(), n.targs, n.instance.posList)
+		inst := n.check.instantiate(n.instance.pos, n.orig.underlying, n.TParams().list(), n.targs, n.instance.posList)
 		n.underlying = inst
 		n.fromRHS = inst
 		n.instance = nil

src/cmd/compile/internal/types2/instantiate.go

@@ -29,9 +29,9 @@ func (check *Checker) Instantiate(pos syntax.Pos, typ Type, targs []Type, posLis
 	var tparams []*TypeName
 	switch t := typ.(type) {
 	case *Named:
-		tparams = t.TParams()
+		tparams = t.TParams().list()
 	case *Signature:
-		tparams = t.tparams
+		tparams = t.TParams().list()
 		defer func() {
 			// If we had an unexpected failure somewhere don't panic below when
 			// asserting res.(*Signature). Check for *Signature in case Typ[Invalid]
@@ -109,9 +109,9 @@ func (check *Checker) InstantiateLazy(pos syntax.Pos, typ Type, targs []Type, po
 		panic(fmt.Sprintf("%v: cannot instantiate %v", pos, typ))
 	}
 
-	if verify && len(base.tparams) == len(targs) {
+	if verify && base.TParams().Len() == len(targs) {
 		check.later(func() {
-			check.verify(pos, base.tparams, targs, posList)
+			check.verify(pos, base.tparams.list(), targs, posList)
 		})
 	}
 
@@ -125,7 +125,7 @@ func (check *Checker) InstantiateLazy(pos syntax.Pos, typ Type, targs []Type, po
 	}
 
 	tname := NewTypeName(pos, base.obj.pkg, base.obj.name, nil)
-	named := check.newNamed(tname, base, nil, nil, nil) // methods and tparams are set when named is loaded.
+	named := check.newNamed(tname, base, nil, nil, nil) // methods and tparams are set when named is loaded
 	named.targs = targs
 	named.instance = &instance{pos, posList}
 	if check != nil {

src/cmd/compile/internal/types2/lookup.go

@@ -315,10 +315,10 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 			// both methods must have the same number of type parameters
 			ftyp := f.typ.(*Signature)
 			mtyp := m.typ.(*Signature)
-			if len(ftyp.tparams) != len(mtyp.tparams) {
+			if ftyp.TParams().Len() != mtyp.TParams().Len() {
 				return m, f
 			}
-			if !acceptMethodTypeParams && len(ftyp.tparams) > 0 {
+			if !acceptMethodTypeParams && ftyp.TParams().Len() > 0 {
 				panic("internal error: method with type parameters")
 			}
 
@@ -328,7 +328,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 			// TODO(gri) is this always correct? what about type bounds?
 			// (Alternative is to rename/subst type parameters and compare.)
 			u := newUnifier(true)
-			u.x.init(ftyp.tparams)
+			u.x.init(ftyp.TParams().list())
 			if !u.unify(ftyp, mtyp) {
 				return m, f
 			}
@@ -367,10 +367,10 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 		// both methods must have the same number of type parameters
 		ftyp := f.typ.(*Signature)
 		mtyp := m.typ.(*Signature)
-		if len(ftyp.tparams) != len(mtyp.tparams) {
+		if ftyp.TParams().Len() != mtyp.TParams().Len() {
 			return m, f
 		}
-		if !acceptMethodTypeParams && len(ftyp.tparams) > 0 {
+		if !acceptMethodTypeParams && ftyp.TParams().Len() > 0 {
 			panic("internal error: method with type parameters")
 		}
 
@@ -381,17 +381,17 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 		// In order to compare the signatures, substitute the receiver
 		// type parameters of ftyp with V's instantiation type arguments.
 		// This lazily instantiates the signature of method f.
-		if Vn != nil && len(Vn.TParams()) > 0 {
+		if Vn != nil && Vn.TParams().Len() > 0 {
 			// Be careful: The number of type arguments may not match
 			// the number of receiver parameters. If so, an error was
 			// reported earlier but the length discrepancy is still
 			// 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) != len(Vn.targs) {
+			if len(ftyp.RParams().list()) != len(Vn.targs) {
 				return
 			}
-			ftyp = check.subst(nopos, ftyp, makeSubstMap(ftyp.rparams, Vn.targs)).(*Signature)
+			ftyp = check.subst(nopos, ftyp, makeSubstMap(ftyp.RParams().list(), Vn.targs)).(*Signature)
 		}
 
 		// If the methods have type parameters we don't care whether they
@@ -400,7 +400,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 		// TODO(gri) is this always correct? what about type bounds?
 		// (Alternative is to rename/subst type parameters and compare.)
 		u := newUnifier(true)
-		if len(ftyp.tparams) > 0 {
+		if ftyp.TParams().Len() > 0 {
 			// We reach here only if we accept method type parameters.
 			// In this case, unification must consider any receiver
 			// and method type parameters as "free" type parameters.
@@ -410,9 +410,9 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 			// unimplemented call so that we test this code if we
 			// enable method type parameters.
 			unimplemented()
-			u.x.init(append(ftyp.rparams, ftyp.tparams...))
+			u.x.init(append(ftyp.RParams().list(), ftyp.TParams().list()...))
 		} else {
-			u.x.init(ftyp.rparams)
+			u.x.init(ftyp.RParams().list())
 		}
 		if !u.unify(ftyp, mtyp) {
 			return m, f

src/cmd/compile/internal/types2/named.go

@@ -17,7 +17,7 @@ type Named struct {
 	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
 	instance   *instance   // position information for lazy instantiation, or nil
-	tparams    []*TypeName // type parameters, or nil
+	tparams    *TypeParams // 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
 
@@ -69,7 +69,7 @@ func (t *Named) load() *Named {
 			panic("invalid underlying type")
 		}
 
-		t.tparams = tparams
+		t.tparams = bindTParams(tparams)
 		t.underlying = underlying
 		t.methods = methods
 	})
@@ -77,7 +77,7 @@ func (t *Named) load() *Named {
 }
 
 // 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 {
+func (check *Checker) newNamed(obj *TypeName, orig *Named, underlying Type, tparams *TypeParams, 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
@@ -117,12 +117,10 @@ func (t *Named) Orig() *Named { return t.orig }
 
 // 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 {
+func (t *Named) TParams() *TypeParams { return t.load().tparams }
-	return t.load().tparams
-}
 
 // SetTParams sets the type parameters of the named type t.
-func (t *Named) SetTParams(tparams []*TypeName) { t.load().tparams = tparams }
+func (t *Named) SetTParams(tparams []*TypeName) { t.load().tparams = bindTParams(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 }

src/cmd/compile/internal/types2/object.go

@@ -475,8 +475,8 @@ func writeObject(buf *bytes.Buffer, obj Object, qf Qualifier) {
 		if _, ok := typ.(*Basic); ok {
 			return
 		}
-		if named, _ := typ.(*Named); named != nil && len(named.tparams) > 0 {
+		if named, _ := typ.(*Named); named != nil && named.TParams().Len() > 0 {
-			writeTParamList(buf, named.tparams, qf, nil)
+			writeTParamList(buf, named.TParams().list(), qf, nil)
 		}
 		if tname.IsAlias() {
 			buf.WriteString(" =")

src/cmd/compile/internal/types2/predicates.go

@@ -227,7 +227,7 @@ func identical(x, y Type, cmpTags bool, p *ifacePair) bool {
 		// parameter names.
 		if y, ok := y.(*Signature); ok {
 			return x.variadic == y.variadic &&
-				identicalTParams(x.tparams, y.tparams, cmpTags, p) &&
+				identicalTParams(x.TParams().list(), y.TParams().list(), cmpTags, p) &&
 				identical(x.params, y.params, cmpTags, p) &&
 				identical(x.results, y.results, cmpTags, p)
 		}

src/cmd/compile/internal/types2/signature.go

@@ -19,8 +19,8 @@ type Signature struct {
 	// and store it in the Func Object) because when type-checking a function
 	// literal we call the general type checker which returns a general Type.
 	// We then unpack the *Signature and use the scope for the literal body.
-	rparams  []*TypeName // receiver type parameters from left to right; or nil
+	rparams  *TypeParams // receiver type parameters from left to right, or nil
-	tparams  []*TypeName // type parameters from left to right; or nil
+	tparams  *TypeParams // type parameters from left to right, or nil
 	scope    *Scope      // function scope, present for package-local signatures
 	recv     *Var        // nil if not a method
 	params   *Tuple      // (incoming) parameters from left to right; or nil
@@ -54,16 +54,16 @@ func NewSignature(recv *Var, params, results *Tuple, variadic bool) *Signature {
 func (s *Signature) Recv() *Var { return s.recv }
 
 // TParams returns the type parameters of signature s, or nil.
-func (s *Signature) TParams() []*TypeName { return s.tparams }
+func (s *Signature) TParams() *TypeParams { return s.tparams }
-
-// RParams returns the receiver type params of signature s, or nil.
-func (s *Signature) RParams() []*TypeName { return s.rparams }
 
 // SetTParams sets the type parameters of signature s.
-func (s *Signature) SetTParams(tparams []*TypeName) { s.tparams = tparams }
+func (s *Signature) SetTParams(tparams []*TypeName) { s.tparams = bindTParams(tparams) }
+
+// RParams returns the receiver type parameters of signature s, or nil.
+func (s *Signature) RParams() *TypeParams { return s.rparams }
 
 // SetRParams sets the receiver type params of signature s.
-func (s *Signature) SetRParams(rparams []*TypeName) { s.rparams = rparams }
+func (s *Signature) SetRParams(rparams []*TypeName) { s.rparams = bindTParams(rparams) }
 
 // Params returns the parameters of signature s, or nil.
 func (s *Signature) Params() *Tuple { return s.params }
@@ -119,10 +119,11 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 				// blank identifiers were found => use rewritten receiver type
 				recvTyp = isubst(recvPar.Type, smap)
 			}
-			sig.rparams = make([]*TypeName, len(rparams))
+			rlist := make([]*TypeName, len(rparams))
 			for i, rparam := range rparams {
-				sig.rparams[i] = check.declareTypeParam(i, rparam)
+				rlist[i] = check.declareTypeParam(rparam)
 			}
+			sig.rparams = bindTParams(rlist)
 			// determine receiver type to get its type parameters
 			// and the respective type parameter bounds
 			var recvTParams []*TypeName
@@ -132,19 +133,19 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 				//       again when we type-check the signature.
 				// TODO(gri) maybe the receiver should be marked as invalid instead?
 				if recv := asNamed(check.genericType(rname, false)); recv != nil {
-					recvTParams = recv.TParams()
+					recvTParams = recv.TParams().list()
 				}
 			}
 			// provide type parameter bounds
 			// - only do this if we have the right number (otherwise an error is reported elsewhere)
-			if len(sig.rparams) == len(recvTParams) {
+			if sig.RParams().Len() == len(recvTParams) {
 				// We have a list of *TypeNames but we need a list of Types.
-				list := make([]Type, len(sig.rparams))
+				list := make([]Type, sig.RParams().Len())
-				for i, t := range sig.rparams {
+				for i, t := range sig.RParams().list() {
 					list[i] = t.typ
 				}
 				smap := makeSubstMap(recvTParams, list)
-				for i, tname := range sig.rparams {
+				for i, tname := range sig.RParams().list() {
 					bound := recvTParams[i].typ.(*TypeParam).bound
 					// bound is (possibly) parameterized in the context of the
 					// receiver type declaration. Substitute parameters for the

src/cmd/compile/internal/types2/sizeof_test.go

@@ -26,12 +26,12 @@ func TestSizeof(t *testing.T) {
 		{Struct{}, 24, 48},
 		{Pointer{}, 8, 16},
 		{Tuple{}, 12, 24},
-		{Signature{}, 44, 88},
+		{Signature{}, 28, 56},
 		{Union{}, 12, 24},
 		{Interface{}, 40, 80},
 		{Map{}, 16, 32},
 		{Chan{}, 12, 24},
-		{Named{}, 88, 168},
+		{Named{}, 80, 152},
 		{TypeParam{}, 28, 48},
 		{term{}, 12, 24},
 		{top{}, 0, 0},

src/cmd/compile/internal/types2/subst.go

@@ -197,7 +197,7 @@ func (subst *subster) typ(typ Type) Type {
 		if len(t.targs) > 0 {
 			// already instantiated
 			dump(">>> %s already instantiated", t)
-			assert(len(t.targs) == len(t.TParams()))
+			assert(len(t.targs) == t.TParams().Len())
 			// 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.
@@ -207,7 +207,7 @@ func (subst *subster) typ(typ Type) Type {
 				if new_targ != targ {
 					dump(">>> substituted %d targ %s => %s", i, targ, new_targ)
 					if new_targs == nil {
-						new_targs = make([]Type, len(t.TParams()))
+						new_targs = make([]Type, t.TParams().Len())
 						copy(new_targs, t.targs)
 					}
 					new_targs[i] = new_targ

src/cmd/compile/internal/types2/typeparam.go

@@ -28,15 +28,19 @@ type TypeParam struct {
 // 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).
+// NewTypeParam returns a new TypeParam. Type parameters may be set on a Named
-func (check *Checker) NewTypeParam(obj *TypeName, index int, bound Type) *TypeParam {
+// or Signature type by calling SetTParams. Setting a type parameter on more
+// than one type will result in a panic.
+//
+// The bound argument can be nil, and set later via SetBound.
+func (check *Checker) NewTypeParam(obj *TypeName, 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}
+	typ := &TypeParam{check: check, id: id, obj: obj, index: -1, bound: bound}
 	if obj.typ == nil {
 		obj.typ = typ
 	}
@@ -88,6 +92,42 @@ func (t *TypeParam) SetBound(bound Type) {
 func (t *TypeParam) Underlying() Type { return t }
 func (t *TypeParam) String() string   { return TypeString(t, nil) }
 
+// TypeParams holds a list of type parameters bound to a type.
+type TypeParams struct{ tparams []*TypeName }
+
+// Len returns the number of type parameters in the list.
+// It is safe to call on a nil receiver.
+func (tps *TypeParams) Len() int {
+	return len(tps.list())
+}
+
+// At returns the i'th type parameter in the list.
+// It is safe to call on a nil receiver.
+func (tps *TypeParams) At(i int) *TypeName {
+	return tps.list()[i]
+}
+
+func (tps *TypeParams) list() []*TypeName {
+	if tps == nil {
+		return nil
+	}
+	return tps.tparams
+}
+
+func bindTParams(list []*TypeName) *TypeParams {
+	if len(list) == 0 {
+		return nil
+	}
+	for i, tp := range list {
+		typ := tp.Type().(*TypeParam)
+		if typ.index >= 0 {
+			panic("internal error: type parameter bound more than once")
+		}
+		typ.index = i
+	}
+	return &TypeParams{tparams: list}
+}
+
 // ----------------------------------------------------------------------------
 // Implementation
 

src/cmd/compile/internal/types2/typestring.go

@@ -280,7 +280,7 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 			buf.WriteByte(']')
 		} else if t.TParams() != nil {
 			// parameterized type
-			writeTParamList(buf, t.TParams(), qf, visited)
+			writeTParamList(buf, t.TParams().list(), qf, visited)
 		}
 
 	case *TypeParam:
@@ -426,7 +426,7 @@ func WriteSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier) {
 
 func writeSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier, visited []Type) {
 	if sig.tparams != nil {
-		writeTParamList(buf, sig.tparams, qf, visited)
+		writeTParamList(buf, sig.TParams().list(), qf, visited)
 	}
 
 	writeTuple(buf, sig.params, sig.variadic, qf, visited)