internal/typeparams: use alias rather than indirection for IndexListExpr

Using aliases make it easier to translate code using the new APIs.

Change-Id: I9887569a3ba1d5b1434ac6cc185485433aca7090
Reviewed-on: https://go-review.googlesource.com/c/tools/+/352898
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

go/ast/astutil/rewrite.go

@@ -253,6 +253,10 @@ func (a *application) apply(parent ast.Node, name string, iter *iterator, n ast.
 		a.apply(n, "X", nil, n.X)
 		a.apply(n, "Index", nil, n.Index)
 
+	case *typeparams.IndexListExpr:
+		a.apply(n, "X", nil, n.X)
+		a.applyList(n, "Indices")
+
 	case *ast.SliceExpr:
 		a.apply(n, "X", nil, n.X)
 		a.apply(n, "Low", nil, n.Low)
@@ -439,13 +443,7 @@ func (a *application) apply(parent ast.Node, name string, iter *iterator, n ast.
 		}
 
 	default:
-		if ix := typeparams.GetIndexExprData(n); ix != nil {
-			a.apply(n, "X", nil, ix.X)
-			// *ast.IndexExpr was handled above, so n must be an *ast.MultiIndexExpr.
-			a.applyList(n, "Indices")
-		} else {
-			panic(fmt.Sprintf("Apply: unexpected node type %T", n))
-		}
+		panic(fmt.Sprintf("Apply: unexpected node type %T", n))
 	}
 
 	if a.post != nil && !a.post(&a.cursor) {

internal/typeparams/typeparams_go117.go

@@ -53,6 +53,10 @@ func PackIndexExpr(x ast.Expr, lbrack token.Pos, indices []ast.Expr, rbrack toke
 // this Go version. Its methods panic on use.
 type IndexListExpr struct {
 	ast.Expr
+	X       ast.Expr   // expression
+	Lbrack  token.Pos  // position of "["
+	Indices []ast.Expr // index expressions
+	Rbrack  token.Pos  // position of "]"
 }
 
 // ForTypeSpec returns an empty field list, as type parameters on not supported