cmd/compile: changed representation of typeparam bound in types1

Especially with typesets, we should be able to fully represent a
typeparam bound as just another type (actually an interface type).
Change the representation of a typeparam in types1 to include a bound,
which is just a type. Changed the signature for NewTypeParam() to take a
sym, and not a package, since we always set the sym (name) of the
typeparam when creating it. No need for an extra pkg field in Typeparam.

Also added index field in the types1 representation of typeparam. This
is especially needed to correctly export the typeparam, and re-import it
as a types2 type (which requires the index to be set correctly).

Change-Id: I50200e2489a97898c37d292b2bd025df790b0277
Reviewed-on: https://go-review.googlesource.com/c/go/+/319929
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

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

@@ -204,18 +204,15 @@ func (g *irgen) typ0(typ types2.Type) *types.Type {
 		return types.NewInterface(g.tpkg(typ), append(embeddeds, methods...))
 
 	case *types2.TypeParam:
-		tp := types.NewTypeParam(g.tpkg(typ))
 		// Save the name of the type parameter in the sym of the type.
 		// Include the types2 subscript in the sym name
 		sym := g.pkg(typ.Obj().Pkg()).Lookup(types2.TypeString(typ, func(*types2.Package) string { return "" }))
-		tp.SetSym(sym)
+		tp := types.NewTypeParam(sym, typ.Index())
 		// Set g.typs[typ] in case the bound methods reference typ.
 		g.typs[typ] = tp
 
-		// TODO(danscales): we don't currently need to use the bounds
-		// anywhere, so eventually we can probably remove.
 		bound := g.typ1(typ.Bound())
-		*tp.Methods() = *bound.Methods()
+		tp.SetBound(bound)
 		return tp
 
 	case *types2.Tuple:

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

@@ -151,7 +151,7 @@ type Type struct {
 	// TARRAY: *Array
 	// TSLICE: Slice
 	// TSSA: string
-	// TTYPEPARAM:  *Interface (though we may not need to store/use the Interface info)
+	// TTYPEPARAM:  *Typeparam
 	Extra interface{}
 
 	// Width is the width of this Type in bytes.
@@ -377,6 +377,12 @@ type Interface struct {
 	pkg *Pkg
 }
 
+// Typeparam contains Type fields specific to typeparam types.
+type Typeparam struct {
+	index int // type parameter index in source order, starting at 0
+	bound *Type
+}
+
 // Ptr contains Type fields specific to pointer types.
 type Ptr struct {
 	Elem *Type // element type
@@ -558,7 +564,7 @@ func New(et Kind) *Type {
 	case TRESULTS:
 		t.Extra = new(Results)
 	case TTYPEPARAM:
-		t.Extra = new(Interface)
+		t.Extra = new(Typeparam)
 	}
 	return t
 }
@@ -825,6 +831,8 @@ func (t *Type) copy() *Type {
 	case TARRAY:
 		x := *t.Extra.(*Array)
 		nt.Extra = &x
+	case TTYPEPARAM:
+		base.Fatalf("typeparam types cannot be copied")
 	case TTUPLE, TSSA, TRESULTS:
 		base.Fatalf("ssa types cannot be copied")
 	}
@@ -1766,14 +1774,34 @@ func NewInterface(pkg *Pkg, methods []*Field) *Type {
 	return t
 }
 
-// NewTypeParam returns a new type param.
+// NewTypeParam returns a new type param with the specified sym (package and name)
-func NewTypeParam(pkg *Pkg) *Type {
+// and specified index within the typeparam list.
+func NewTypeParam(sym *Sym, index int) *Type {
 	t := New(TTYPEPARAM)
-	t.Extra.(*Interface).pkg = pkg
+	t.sym = sym
+	t.Extra.(*Typeparam).index = index
 	t.SetHasTParam(true)
 	return t
 }
 
+// Index returns the index of the type param within its param list.
+func (t *Type) Index() int {
+	t.wantEtype(TTYPEPARAM)
+	return t.Extra.(*Typeparam).index
+}
+
+// SetBound sets the bound of a typeparam.
+func (t *Type) SetBound(bound *Type) {
+	t.wantEtype(TTYPEPARAM)
+	t.Extra.(*Typeparam).bound = bound
+}
+
+// Bound returns the bound of a typeparam.
+func (t *Type) Bound() *Type {
+	t.wantEtype(TTYPEPARAM)
+	return t.Extra.(*Typeparam).bound
+}
+
 const BOGUS_FUNARG_OFFSET = -1000000000
 
 func unzeroFieldOffsets(f []*Field) {

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

@@ -760,6 +760,11 @@ func (check *Checker) NewTypeParam(obj *TypeName, index int, bound Type) *TypePa
 	return typ
 }
 
+// Index returns the index of the type param within its param list.
+func (t *TypeParam) Index() int {
+	return t.index
+}
+
 func (t *TypeParam) Bound() *Interface {
 	iface := asInterface(t.bound)
 	// use the type bound position if we have one