haskellPackages.idris: Fix build (new GHC 8.8 & old megaparsec 7)

The hackage-packages.nix change was generated by hackage2nix v2.15.1 from Hackage revision
https://github.com/commercialhaskell/all-cabal-hashes/commit/ad4a70d448051948f2064d0bba5e63eab25f04d8.

pkgs/development/haskell-modules/configuration-common.nix

@@ -716,10 +716,19 @@ self: super: {
     '';
   });
 
-  # The standard libraries are compiled separately
-  idris = generateOptparseApplicativeCompletion "idris" (
-    doJailbreak (dontCheck super.idris)
-  );
+  # The standard libraries are compiled separately.
+  # The megaparsec-7 override is needed because https://github.com/idris-lang/Idris-dev/issues/4826 declares that
+  # idris1 has no plans to migrate to megaparsec-8.
+  # The idris-lang/Idris-dev#4808 patch is for GHC 8.8 compatibility, and can likely be removed with the next release.
+  idris = generateOptparseApplicativeCompletion "idris" (doJailbreak (dontCheck
+    (appendPatches
+      (super.idris.override { megaparsec = self.megaparsec_7_0_5; }) [
+        (pkgs.fetchpatch {
+          url = "https://github.com/idris-lang/Idris-dev/pull/4808.diff";
+          sha256 = "060ib1rczy34ip8xf3bv1pf28655f6s0bvvij19jhh5dpcr0pf71";
+          excludes = [ ".travis.yml" "Makefile" "appveyor.yml" ];
+        })
+      ])));
 
   # https://github.com/bos/math-functions/issues/25
   math-functions = dontCheck super.math-functions;