clang, c++17, std::optional, and libc++.dylib

[Ken Cunningham]

As software using c++17 is upon us, we have a slight problem with std::optional.

clang has supported std::optional for some time, and clang-9.0, at least, appears to support std::optional right back to the ancient systems.

however, there is one small part of std::optional, std::optional::value, that needs a callback function in case of an exception being thrown, called “bad_optional_access”. That symbol is compiled into libc++.dylib as of 10.13.

So clang-9.0 supports 99% of std::optional all the way back, but that bad_optional_access function limits that one part of it, std::optional::value, to 10.13. 

There are availability tests build into the libc++ headers based on system version to turn that off.

However, on 10.6, where I build and install libc++ 5.0, that symbol exists in libc++.dylib, so I disable clang’s availability tests.

It is quite simple (I think) to inline a function implementing bad_optional_access into the <optional> header. That could be used for 10.7 to 10.12, and then all those systems could comfortably implement c++17, and no problem. Otherwise we have to install a newer libc++.dylib on older systems, which is a simple but big problem, as it were.

Here’s the <optional> patch. I only speak basic c++, so improve this if you can.

So I am proposing to use this patch, and disable Apple’s availability test in <__config> for bad_optional_access.

Then all systems from 10.5 to current can use c++17 std::optional, and disaster averted until the next crisis.

Warts?

Ken


<optional> patch:


$ diff -u optional.orig optional
--- optional.orig	2020-07-04 18:51:05.000000000 -0700
+++ optional	2020-07-04 18:51:13.000000000 -0700
@@ -168,13 +168,10 @@
 namespace std  // purposefully not using versioning namespace
 {
 
-class _LIBCPP_EXCEPTION_ABI _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS bad_optional_access
-    : public exception
-{
+// 20.5.8, class bad_optional_access
+class bad_optional_access : public logic_error {
 public:
-    // Get the key function ~bad_optional_access() into the dylib
-    virtual ~bad_optional_access() _NOEXCEPT;
-    virtual const char* what() const _NOEXCEPT;
+  explicit bad_optional_access() : logic_error{"bad optional access"} {}
 };
 
 }  // std

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