default.nix

@@ -3,7 +3,10 @@
   ghcSrc ? /home/ninjatrappeur/Code/perso/haskell/ghc
 }:
 let
-  ghcOverlay = import ./overlays/ghc.nix ghcSrc;
+  ghcOverlay =
+    import ./overlays/ghc.nix {
+      inherit ghcSrc pkgs;
+    };
   nixpkgsConfig = { overlays = [ ghcOverlay ]; };
   nixpkgsSrc = (import ./nix/sources.nix).nixpkgs;
   pkgs = import nixpkgsSrc nixpkgsConfig;

overlays/ghc.nix

@@ -1,7 +1,12 @@
-ghcSrc:
+{ ghcSrc, pkgs }:
 let
   ghcOverride = self: super: {
-    ghc = super.ghc.overrideDerivation (old: { src = ghcSrc; });
+    ghc = super.ghc.overrideDerivation (old: {
+      src = ghcSrc;
+      rev = "head";
+      # Necessary to use a non-fetchgit src
+      nativeBuildInputs = old.nativeBuildInputs ++ [ pkgs.git ];
+    });
   };
 in self: super: {
   customGhcHEAD = super.haskell.packages.ghcHEAD.extend ghcOverride;