llvmPackages_*: respect cc for target when choosing C++ flavour

llvmPackages_*.clang should check the default compiler for the package
set it is targeting (targetPackages.stdenv.cc) instead of the compiler
that has been used to build it (stdenv.cc) in order to get some sense of
whether to use libc++ or libstdc++.

Since we are now inspecting targetPackages in the llvmPackages.clang
attribute, we need to avoid using it in the cross stdenv — which just
forces us to explicitly request libcxxClang for darwin instead of
relying on the clang attribute to pick it for us.

We also need to do something similar for targetPackages.stdenv.cc: Here
the llvmPackages.clang logic would work as we want (inspect
targetPackages.stdenv.cc and if it doesn't exist, make the choice based
on stdenv.cc), but it gets locked in a cycle with the previous package.
We can easily break this, however: We know that the previous set had
clang and the next one doesn't exist, so we'd choose libcxxClang any day
of the week.

2 files changed, 8 insertions, 2 deletions

diff --git a/pkgs/stdenv/booter.nix b/pkgs/stdenv/booter.nix
index 51d617354e8..f1d07e6461a 100644
--- a/pkgs/stdenv/booter.nix
+++ b/pkgs/stdenv/booter.nix
@@ -124,7 +124,13 @@ stageFuns: let
       if buildPackages.stdenv.hasCC
       then
         if buildPackages.stdenv.cc.isClang or false
-        then buildPackages.clang
+        # buildPackages.clang checks targetPackages.stdenv.cc (i. e. this
+        # attribute) to get a sense of the its set's default compiler and
+        # chooses between libc++ and libstdc++ based on that. If we hit this
+        # code here, we'll cause an infinite recursion. Since a set with
+        # clang as its default compiler always means libc++, we can infer this
+        # decision statically.
+        then buildPackages.llvmPackages.libcxxClang
         else buildPackages.gcc
       else
         # This will blow up if anything uses it, but that's OK. The `if
diff --git a/pkgs/stdenv/cross/default.nix b/pkgs/stdenv/cross/default.nix
index 613b8d5304c..e01ac74599a 100644
--- a/pkgs/stdenv/cross/default.nix
+++ b/pkgs/stdenv/cross/default.nix
@@ -70,7 +70,7 @@ in lib.init bootStages ++ [
              # when there is a C compiler and everything should be fine.
              then throw "no C compiler provided for this platform"
            else if crossSystem.isDarwin
-             then buildPackages.llvmPackages.clang
+             then buildPackages.llvmPackages.libcxxClang
            else if crossSystem.useLLVM or false
              then buildPackages.llvmPackages.clangUseLLVM
            else buildPackages.gcc;