[PATCH 1/2] libstdc++: Move std::optional assertions out of _M_get()

[Jonathan Wakely]

Tested x86_64-linux.

Any reason not to do this? I don't think the assertions are useful to
catch implementation bugs where we access the contained value without
checking it - we should use tests for that.

-- >8 --

Currently we implement the precondition for accessing the contained
value of a std::optional in the _M_get() accessor in the base class.
This means that we always check the assertions even in internal
functions that have an explicit check for a contained value being
present, such as value() and value_or(U&&). Although those redundant
assertions should get optimized out in most cases, they might hurt
inliner heuristics and generally give the compiler more work to do.
And they won't be optimized out at all for non-optimized builds.

The current assertions also result in repeated invalid bug reports, such
as PR 91281, PR 101659, PR 102712, and PR 107894.

We can move the assertions from the internal accessors to the public
member functions where the preconditions are specified.

libstdc++-v3/ChangeLog:

        * include/std/optional (_Optional_base_impl::_M_get()): Move
        assertions to ...
        (optional::operator->, optional::operator*): ... here.
---
 libstdc++-v3/include/std/optional | 40 ++++++++++++++++++++-----------
 1 file changed, 26 insertions(+), 14 deletions(-)

diff --git a/libstdc++-v3/include/std/optional 
b/libstdc++-v3/include/std/optional
index 48e0f3d36f2..af72004645e 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -472,17 +472,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // The _M_get operations have _M_engaged as a precondition.
       constexpr _Tp&
       _M_get() noexcept
-      {
-       __glibcxx_assert(this->_M_is_engaged());
-       return static_cast<_Dp*>(this)->_M_payload._M_get();
-      }
+      { return static_cast<_Dp*>(this)->_M_payload._M_get(); }
 
       constexpr const _Tp&
       _M_get() const noexcept
-      {
-       __glibcxx_assert(this->_M_is_engaged());
-       return static_cast<const _Dp*>(this)->_M_payload._M_get();
-      }
+      { return static_cast<const _Dp*>(this)->_M_payload._M_get(); }
     };
 
   /**
@@ -958,27 +952,45 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // Observers.
       constexpr const _Tp*
       operator->() const noexcept
-      { return std::__addressof(this->_M_get()); }
+      {
+       __glibcxx_assert(this->_M_is_engaged());
+       return std::__addressof(this->_M_get());
+      }
 
       constexpr _Tp*
       operator->() noexcept
-      { return std::__addressof(this->_M_get()); }
+      {
+       __glibcxx_assert(this->_M_is_engaged());
+       return std::__addressof(this->_M_get());
+      }
 
       constexpr const _Tp&
       operator*() const& noexcept
-      { return this->_M_get(); }
+      {
+       __glibcxx_assert(this->_M_is_engaged());
+       return this->_M_get();
+      }
 
       constexpr _Tp&
       operator*()& noexcept
-      { return this->_M_get(); }
+      {
+       __glibcxx_assert(this->_M_is_engaged());
+       return this->_M_get();
+      }
 
       constexpr _Tp&&
       operator*()&& noexcept
-      { return std::move(this->_M_get()); }
+      {
+       __glibcxx_assert(this->_M_is_engaged());
+       return std::move(this->_M_get());
+      }
 
       constexpr const _Tp&&
       operator*() const&& noexcept
-      { return std::move(this->_M_get()); }
+      {
+       __glibcxx_assert(this->_M_is_engaged());
+       return std::move(this->_M_get());
+      }
 
       constexpr explicit operator bool() const noexcept
       { return this->_M_is_engaged(); }
-- 
2.45.2