satallax: Fix build with GCC9

pkgs/applications/science/logic/satallax/default.nix

@@ -9,6 +9,11 @@ stdenv.mkDerivation rec {
     sha256 = "1kvxn8mc35igk4vigi5cp7w3wpxk2z3bgwllfm4n3h2jfs0vkpib";
   };
 
+  patches = [
+    # GCC9 doesn't allow default value in friend declaration.
+    ./fix-declaration-gcc9.patch
+  ];
+
   preConfigure = ''
     mkdir fake-tools
     echo "echo 'Nix-build-host.localdomain'" > fake-tools/hostname

pkgs/applications/science/logic/satallax/fix-declaration-gcc9.patch

@@ -0,0 +1,21 @@
+diff --git i/minisat/core/SolverTypes.h w/minisat/core/SolverTypes.h
+--- i/minisat/core/SolverTypes.h
++++ w/minisat/core/SolverTypes.h
+@@ -47,7 +47,7 @@ struct Lit {
+     int     x;
+
+     // Use this as a constructor:
+-    friend Lit mkLit(Var var, bool sign = false);
++    friend Lit mkLit(Var var, bool sign);
+
+     bool operator == (Lit p) const { return x == p.x; }
+     bool operator != (Lit p) const { return x != p.x; }
+@@ -55,7 +55,7 @@ struct Lit {
+ };
+
+
+-inline  Lit  mkLit     (Var var, bool sign) { Lit p; p.x = var + var + (int)sign; return p; }
++inline  Lit  mkLit     (Var var, bool sign = false) { Lit p; p.x = var + var + (int)sign; return p; }
+ inline  Lit  operator ~(Lit p)              { Lit q; q.x = p.x ^ 1; return q; }
+ inline  Lit  operator ^(Lit p, bool b)      { Lit q; q.x = p.x ^ (unsigned int)b; return q; }
+ inline  bool sign      (Lit p)              { return p.x & 1; }