GCC: fix compilation on MacOS 10.15

MacOS 10.15 now includes "aligned_alloc".  Disagreement between the
headers and the binaries about whether aligned_alloc exists leads to
a compilation failure (see #73319 and the detailed comment in this
commit).

(cherry picked from commit 3a79681eb45df9c89d1de613114394f49582f0d6)

1 files changed, 32 insertions, 0 deletions

diff --git a/nixpkgs/pkgs/development/compilers/gcc/common/pre-configure.nix b/nixpkgs/pkgs/development/compilers/gcc/common/pre-configure.nix
index 85b854e19b9a..fff490148007 100644
--- a/nixpkgs/pkgs/development/compilers/gcc/common/pre-configure.nix
+++ b/nixpkgs/pkgs/development/compilers/gcc/common/pre-configure.nix
@@ -17,3 +17,35 @@ lib.optionalString (hostPlatform.isSunOS && hostPlatform.is64bit) ''
 '' + lib.optionalString langAda ''
   export PATH=${gnatboot}/bin:$PATH
 ''
+
+# NOTE 2020/3/18: This environment variable prevents configure scripts from
+# detecting the presence of aligned_alloc on Darwin.  There are many facts that
+# collectively make this fix necessary:
+#  - Nix uses a fixed set of standard library headers on all MacOS systems,
+#    regardless of their actual version.  (Nix uses version 10.12 headers.)
+#  - Nix uses the native standard library binaries for the build system.  That
+#    means the standard library binaries may not exactly match the standard
+#    library headers.
+#  - The aligned_alloc procedure is present in MacOS 10.15 (Catalina), but not
+#    in earlier versions.  Therefore on Catalina systems, aligned_alloc is
+#    linkable (i.e. present in the binary libraries) but not present in the
+#    headers.
+#  - Configure scripts detect a procedure's existence by checking whether it is
+#    linkable.  They do not check whether it is present in the headers.
+#  - GCC throws an error during compilation because aligned_alloc is not
+#    defined in the headers---even though the linker can see it.
+#
+# This fix would not be necessary if ANY of the above were false:
+#  - If Nix used native headers for each different MacOS version, aligned_alloc
+#    would be in the headers on Catalina.
+#  - If Nix used the same libary binaries for each MacOS version, aligned_alloc
+#    would not be in the library binaries.
+#  - If Catalina did not include aligned_alloc, this wouldn't be a problem.
+#  - If the configure scripts looked for header presence as well as
+#    linkability, they would see that aligned_alloc is missing.
+#  - If GCC allowed implicit declaration of symbols, it would not fail during
+#    compilation even if the configure scripts did not check header presence.
+#
++ lib.optionalString (hostPlatform.isDarwin) ''
+  export ac_cv_func_aligned_alloc=no
+''