Clarify docs around manylinux and c++

docs/cpp_standards.md

@@ -5,10 +5,39 @@ title: Modern C++ standards
 Building Python wheels with modern C++ standards (C++11 and later) requires a few tricks.
 
 
-## manylinux1 and C++14
-The default `manylinux1` image (based on CentOS 5) contains a version of GCC and libstdc++ that only supports C++11 and earlier standards. There are however ways to compile wheels with the C++14 standard (and later): https://github.com/pypa/manylinux/issues/118
+## manylinux and C++
+The `manylinux*` standards imply a limit on which C++ standard you can use. There are workarounds to this, and `cibuildwheel` uses some workarounds by default. You should be aware of them!
 
-`manylinux2010` and `manylinux2014` are newer and support all C++ standards (up to C++17).
+Consider that the `manylinux*` standards constrain _symbol versions_ in `libstdc++.so`. So when *dynamically linking* to `libstdc++.so`, the desired `manylinux*` standard constrains the gcc version, and thus constrains the C++ standard.
+
+Cross-referencing [current manylinux standards](https://github.com/mayeut/pep600_compliance/blob/f86a7d7c153cc45aa3f2add6ffcf610c80501657/pep600_compliance/tools/policy.json) with [gcc's symbol versions](https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html) and [libstdc++'s language support](https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html), we have:
+ * `manylinux1`: gcc 4.1.x
+ * `manylinux2010`: gcc 4.4.x
+ * `manylinux2014`: gcc 4.8.x
+ * `manylinux_2_24`: gcc 6.x (stable C++14 ABI)
+ * `manylinux_2_27`: gcc 7.x (stable C++14 ABI)
+
+([The first release of a complete and stable C++17 ABI is gcc 9.1](https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2017), but as of writing there is no official `manylinux*` standard that supports this version. Prior standards like C++11 *are* supported, but gcc doesn't document the per-version support as clearly as C++14 and later)
+
+We *can* use newer C++ standards and support older `manylinux*`, if we use *static linking*.
+
+The default `manylinux1`, `manylinux2010` and `manylinux2014` docker images include *more recent* gcc than the one matched to their standard. These CentOS-based images use automagic *selective static linking* for newer `libstdc++.so` symbols, and dynamic linking for older ones (this is done by having `libstdc++.so` from the `devtools` package be a linker script, and splitting newer symbols into a static library).
+
+The debian-based `manylinux_2_24` image *does not* do selective static linking as of writing. See https://github.com/pypa/manylinux/issues/1012
+
+As of 2021-06-24, the gcc versions on each (x86-64) image are:
+ * `manylinux1`: gcc 4.8.2 (C++11 stable)
+ * `manylinux2010`: gcc 8.3.1 (C++14 stable)
+ * `manylinux2014`: gcc 9.3.1 (C++17 stable)
+ * `manylinux_2_24`: gcc 6.3.0 (C++14 stable)
+
+Each gcc version may support later C++ standards incompletely or experimentally. In some cases gcc may support a standard with an unstable ABI, but this won't matter with static linking (so using C++17 on `manylinux2010` will probably just work). You will have to experiment for yourself to see what works.
+
+If you hit an edge case with selective static linking, or want to try to support an older `manylinux*` standard, you can use unconditional static linking with `LDFLAGS=-static-libstdc++`. Note that this will bloat your wheels, and pypi *does* have a limit on total project size!
+
+If you really want dynamic linking with a newer C++ standard, you could just declare the non-specific `linux` platform tag, instead of a `manylinux*` tag. Use `AUDITWHEEL_PLAT=linux`.
+
+For more information, see https://github.com/pypa/manylinux/issues/118
 
 ## macOS and deployment target versions