message: Bump gcc version to 9.2.1.
  This lets us use designated initializers with fields which have
  nontrivial constructors (e.g. std::optional).
  This is still not a bleeding-edge gcc, so I think we should be OK.
  GPC: gcc9
diff: @@ -47,6 +47,13 @@ build_unflags =
  -std=gnu++11
  -std=gnu++14
  -fpermissive
  +# Use a more recent GCC than default; some designated-initializer features are
  +# missing in platformio's default version. The version named below corresponds
  +# to gcc 9.2.1 20191025.
  +#
  +# All versions are listed at
  +# https://bintray.com/platformio/dl-packages/toolchain-gccarmnoneeabi
  +platform_packages = toolchain-gccarmnoneeabi@>1.80301.190214
  [env:stm32]
  platform = ststm32
  