go/types: fix iota undefined after ConstDecl inside function in ConstSpec

When reaching const declaration, Checker override context iota to use
correct iota value, but does not restore the old value when exit, and
always set context iota to nil. It ends up with undefined iota after
const declaration.

To fix it, preserve the original iota value and restore it after const
declaration.

Fixes #34228

Change-Id: I42d5efb55a57e5ddc369bb72d31f1f039c92361c
Reviewed-on: https://go-review.googlesource.com/c/go/+/194737
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/decl.go

@@ -333,8 +333,8 @@ func (check *Checker) constDecl(obj *Const, typ, init ast.Expr) {
 	assert(obj.typ == nil)
 
 	// use the correct value of iota
+	defer func(iota constant.Value) { check.iota = iota }(check.iota)
 	check.iota = obj.val
-	defer func() { check.iota = nil }()
 
 	// provide valid constant value under all circumstances
 	obj.val = constant.MakeUnknown()

src/go/types/testdata/const0.src

@@ -308,6 +308,8 @@ const (
 			_ = unsafe.Sizeof([iota-1]int{} == x) // assert types are equal
 			_ = unsafe.Sizeof([Two]int{} == x)    // assert types are equal
 		)
+		var z [iota]int                           // [2]int
+		_ = unsafe.Sizeof([2]int{} == z)          // assert types are equal
 	})
 	three = iota // the sequence continues
 )
@@ -334,3 +336,15 @@ var _ = []int64{
 	1 * 1e9,
 	5 * 1e9,
 }
+
+const _ = unsafe.Sizeof(func() {
+	const _ = 0
+	_ = iota
+
+	const (
+	   zero = iota
+	   one
+	)
+	assert(one == 1)
+	assert(iota == 0)
+})