[committed v2] libstdc++: Simplify std::__throw_bad_variant_access

[Jonathan Wakely]

This removes the overload of __throw_bad_variant_access that must be
called with a string literal. This avoids a potential source of
undefined behaviour if that function got misused. The other overload
that takes a bool parameter can be adjusted to take an integer index
selecting one of the four possible string literals to use, ensuring
that the std::bad_variant_access constructor is only called with those
literals.

Passing an index outside the range [0,3] is bogus, but will still select
a valid string literal and avoid undefined behaviour.

libstdc++-v3/ChangeLog:

	* include/std/variant (__throw_bad_variant_access(unsigned)):
	Define new function as inline friend, with namespace-scope
	declaration using noreturn attribute.
	(__throw_bad_variant_access(const char*)): Remove.
	(__throw_bad_variant_access(bool)): Remove.
	(visit, visit<R>): Adjust calls to __throw_bad_variant_access.

Reviewed-by: Patrick Palka <ppalka@redhat.com>
---
Patch v2 based on Patrick's review comments at:
https://forge.sourceware.org/gcc/gcc-TEST/pulls/3

Changes:

- Changed parameter type to unsigned.
- Added [[maybe_unused]] to the parameter and array.
- Added [[noreturn]] (which required declaring it before the
  class and then defining it as an inline friend).
- Made the array static constexpr.
- Added a trailing comma after the last array element.

Tested x86_64-linux.

Pushed to trunk.

 libstdc++-v3/include/std/variant | 32 +++++++++++++++-----------------
 1 file changed, 15 insertions(+), 17 deletions(-)

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index cf532126d79..bd0f9c3252a 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1402,6 +1402,8 @@ namespace __detail::__variant
 		   && (is_swappable_v<_Types> && ...))>
     swap(variant<_Types...>&, variant<_Types...>&) = delete;
 
+  [[noreturn]] void __throw_bad_variant_access(unsigned);
+
   class bad_variant_access : public exception
   {
   public:
@@ -1411,28 +1413,24 @@ namespace __detail::__variant
     { return _M_reason; }
 
   private:
+    // Must only be called with a string literal
     bad_variant_access(const char* __reason) noexcept : _M_reason(__reason) { }
 
     // Must point to a string with static storage duration:
     const char* _M_reason = "bad variant access";
 
-    friend void __throw_bad_variant_access(const char* __what);
+    friend void __throw_bad_variant_access([[maybe_unused]] unsigned __n)
+    {
+      [[maybe_unused]] static constexpr const char* __reasons[] = {
+	"std::get: wrong index for variant",
+	"std::get: variant is valueless",
+	"std::visit: variant is valueless",
+	"std::visit<R>: variant is valueless",
+      };
+      _GLIBCXX_THROW_OR_ABORT(bad_variant_access(__reasons[__n % 4u]));
+    }
   };
 
-  // Must only be called with a string literal
-  inline void
-  __throw_bad_variant_access(const char* __what)
-  { _GLIBCXX_THROW_OR_ABORT(bad_variant_access(__what)); }
-
-  inline void
-  __throw_bad_variant_access(bool __valueless)
-  {
-    if (__valueless) [[__unlikely__]]
-      __throw_bad_variant_access("std::get: variant is valueless");
-    else
-      __throw_bad_variant_access("std::get: wrong index for variant");
-  }
-
   template<typename... _Types>
     class variant
     : private __detail::__variant::_Variant_base<_Types...>,
@@ -1941,7 +1939,7 @@ namespace __detail::__variant
       namespace __variant = std::__detail::__variant;
 
       if ((__variant::__as(__variants).valueless_by_exception() || ...))
-	__throw_bad_variant_access("std::visit: variant is valueless");
+	__throw_bad_variant_access(2);
 
       using _Result_type
 	= __detail::__variant::__visit_result_t<_Visitor, _Variants...>;
@@ -1981,7 +1979,7 @@ namespace __detail::__variant
       namespace __variant = std::__detail::__variant;
 
       if ((__variant::__as(__variants).valueless_by_exception() || ...))
-	__throw_bad_variant_access("std::visit<R>: variant is valueless");
+	__throw_bad_variant_access(3);
 
       return std::__do_visit<_Res>(std::forward<_Visitor>(__visitor),
 	  __variant::__as(std::forward<_Variants>(__variants))...);
-- 
2.47.0