c++: Make sure fold_sizeof_expr returns the correct type [PR117775]

We currently ICE upon the following code, that is valid under
-Wno-pointer-arith:

=== cut here ===
int main() {
  decltype( [](auto) { return sizeof(void); } ) x;
  return x.operator()(0);
}
=== cut here ===

The problem is that "fold_sizeof_expr (sizeof(void))" returns
size_one_node, that has a different TREE_TYPE from that of the sizeof
expression, which later triggers an assert in cxx_eval_store_expression.

This patch makes sure that fold_sizeof_expr always returns a tree with
the size_type_node type.

	PR c++/117775

gcc/cp/ChangeLog:

	* decl.cc (fold_sizeof_expr): Make sure the folded result has
	type size_type_node.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-117775.C: New test.

2 files changed, 14 insertions, 0 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index e5f8cd7..ef88791 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -11921,6 +11921,7 @@ fold_sizeof_expr (tree t)
 				    false, false);
   if (r == error_mark_node)
     r = size_one_node;
+  r = cp_fold_convert (size_type_node, r);
   return r;
 }
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-117775.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-117775.C
new file mode 100644
index 0000000..59fc0d3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-117775.C
@@ -0,0 +1,13 @@
+// PR c++/117775
+// Check that we don't ICE and have sizeof(void)==1 under -Wno-pointer-arith
+// { dg-do run { target c++20 } }
+// { dg-additional-options "-Wno-pointer-arith" }
+
+int main() {
+  struct why :
+    decltype( [](auto) {
+		return sizeof(void);
+	      })
+  {} x;
+  return 1 - x.operator()(0);
+}