diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 545247e3e537..54fe95f1ac96 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,9 @@
+2019-09-23  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/91788 (partial)
+	* include/std/variant (variant::index()): Improve codegen for cases
+	where conversion to size_t already works correctly.
+
 2019-09-23  Andreas Schwab  <schwab@suse.de>
 
 	* config/abi/post/riscv64-linux-gnu/baseline_symbols.txt: Update.
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index d93ea86ea296..c0043243ec29 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1518,7 +1518,17 @@ namespace __variant
       { return !this->_M_valid(); }
 
       constexpr size_t index() const noexcept
-      { return size_t(typename _Base::__index_type(this->_M_index + 1)) - 1; }
+      {
+	using __index_type = typename _Base::__index_type;
+	if constexpr (is_same_v<__index_type, size_t>)
+	  return this->_M_index;
+	else if constexpr (__detail::__variant::__never_valueless<_Types...>())
+	  return this->_M_index;
+	else if constexpr (sizeof...(_Types) <= __index_type(-1) / 2)
+	  return make_signed_t<__index_type>(this->_M_index);
+	else
+	  return size_t(__index_type(this->_M_index + 1)) - 1;
+      }
 
       void
       swap(variant& __rhs)