nixpkgs overlay to mostly disable gcc 9 on my system

gistfile1.txt

  nixpkgs.overlays = [(self: super: { gccFun = self.callPackage <nixpkgs/pkgs/development/compilers/gcc/8>;
                                      gcc = self.gcc8;
                                      gfortran = self.gfortran8;
                                      # I can't get llvm 9.0.1 to pass its tets.
                                      mesa = super.mesa.override { llvmPackages = self.llvmPackages_10; };
                                      # nix 3.0 and 2.4 doesn't seem to build with gcc 8.4
                                       nixUnstable = super.nixUnstable.override { stdenv = self.clangStdenv; };
                                      mkLibsForQt5 = x: super.mkLibsForQt5 x // {
                                       appstream-qt = (super.mkLibsForQt5 x).appstream-qt.overrideAttrs (oldAttr: {
                                        patches = (oldAttr.patches or []) ++ [(self.writeTextFile {
                                         name="appstram-qt-patch-gcc8.patch";
                                         text = 
''
--- source/meson.build
+++ source/meson.build
@@ -54,7 +54,7 @@
 
 # a few compiler warning flags we always want enabled
 add_global_arguments('-Werror=implicit-function-declaration', '-Wno-unused-parameter', language: 'c')
-add_global_arguments('-Wno-unused-parameter', '-Wno-error=deprecated-copy', language: 'cpp')
+add_global_arguments('-Wno-unused-parameter', language: 'cpp')
 add_global_arguments('-DAS_COMPILATION', language : 'c')
 
 #
'';                                 
                                       })];});};
                                    })];