[dev.typeparams] cmd/compile: simple shape cleanups

- Changed some early returns to asserts (instantiateMethods and Shapify
   should never take a shape arg)

 - Added suggested change (by Ingo) to use copy() in getInstantiation()

 - Clarified that shape types never have methods in Shapify(), removed
   some TODO comments.

Change-Id: Ia2164ffe670a777f7797bbb45c7ef5e6e9e15357
Reviewed-on: https://go-review.googlesource.com/c/go/+/338971
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>

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

@@ -474,10 +474,7 @@ func (g *irgen) buildClosure(outer *ir.Func, x ir.Node) ir.Node {
 func (g *irgen) instantiateMethods() {
 	for i := 0; i < len(g.instTypeList); i++ {
 		typ := g.instTypeList[i]
-		if typ.HasShape() {
+		assert(!typ.HasShape())
-			// Shape types should not have any methods.
-			continue
-		}
 		// Mark runtime type as needed, since this ensures that the
 		// compiler puts out the needed DWARF symbols, when this
 		// instantiated type has a different package from the local
@@ -782,9 +779,7 @@ func (g *irgen) getInstantiation(nameNode *ir.Name, shapes []*types.Type, isMeth
 		if !t.HasShape() {
 			if s1 == nil {
 				s1 = make([]*types.Type, len(shapes))
-				for j := 0; j < i; j++ {
+				copy(s1[0:i], shapes[0:i])
-					s1[j] = shapes[j]
-				}
 			}
 			s1[i] = typecheck.Shapify(t)
 		} else if s1 != nil {

src/cmd/compile/internal/typecheck/subr.go

@@ -1338,6 +1338,9 @@ func genericTypeName(sym *types.Sym) string {
 
 // Shapify takes a concrete type and returns a GCshape type that can
 // be used in place of the input type and still generate identical code.
+// No methods are added - all methods calls directly on a shape should
+// be done by converting to an interface using the dictionary.
+//
 // TODO: this could take the generic function and base its decisions
 // on how that generic function uses this type argument. For instance,
 // if it doesn't use it as a function argument/return value, then
@@ -1345,9 +1348,7 @@ func genericTypeName(sym *types.Sym) string {
 // differ in how they get passed as arguments). For now, we only
 // unify two different types if they are identical in every possible way.
 func Shapify(t *types.Type) *types.Type {
-	if t.IsShape() {
+	assert(!t.HasShape())
-		return t // TODO: is this right?
-	}
 	// Map all types with the same underlying type to the same shape.
 	u := t.Underlying()
 
@@ -1358,7 +1359,7 @@ func Shapify(t *types.Type) *types.Type {
 	}
 
 	if s := shaped[u]; s != nil {
-		return s //TODO: keep?
+		return s
 	}
 
 	sym := Lookup(fmt.Sprintf(".shape%d", snum))
@@ -1370,7 +1371,6 @@ func Shapify(t *types.Type) *types.Type {
 	s.SetHasShape(true)
 	name.SetType(s)
 	name.SetTypecheck(1)
-	// TODO: add methods to s that the bound has?
 	shaped[u] = s
 	return s
 }