Work around a nasty compiler bug in GCC 6.3

GCC 6.3 errors out with an ICE or simply hangs when instantiating a certain variant of operator^= on
BitSetVectorReference. This patch works around the problem by forcing the actual XOR instruction
into a separate function for that compiler. The workaround is probably horribly slow, but it
shouldn't have any impact on unaffected compilers.

This probably needs backporting.

dune/common/bitsetvector.hh

@@ -315,11 +315,31 @@ namespace Dune {
       return *this;
     }
 
+  private:
+
+    // For some reason, the following variant of operator^= triggers an ICE or a hanging
+    // compiler on Debian 9 with GCC 6.3 and full optimizations enabled (-O3).
+    // The only way to reliably avoid the issue is by making sure that the compiler does not
+    // see the XOR in the context of the function, so here is a little helper that will normally
+    // be inlined, but not on the broken compiler. This incurs a substantial overhead (a function
+    // call), but until someone else has a better idea, it's the only way to make it work reliably.
+
+    static bool xor_helper(bool a, bool b)
+#if defined(__GNUC__) && ! defined(__clang__) && __GNUC__ == 6 && __GNUC_MINOR__ == 3 && __cplusplus \
+  == 201402L
+      __attribute__((noinline))
+#endif
+      ;
+
+  public:
+
     //! Bitwise exclusive or (for BitSetVectorConstReference and BitSetVectorReference)
     BitSetVectorReference& operator^=(const BitSetVectorConstReference& x)
     {
+      // This uses the helper from above to hoist the actual XOR computation out of the function for
+      // the buggy version of GCC.
       for (size_type i=0; i<block_size; i++)
-        getBit(i) = (test(i) ^ x.test(i));
+        getBit(i) = xor_helper(test(i),x.test(i));
       return *this;
     }
 
@@ -399,6 +419,14 @@ namespace Dune {
     }
   };
 
+  // implementation of helper - I put it into the template to avoid having
+  // to compile it in a dedicated compilation unit
+  template<int block_size, class Alloc>
+  bool BitSetVectorReference<block_size,Alloc>::xor_helper(bool a, bool b)
+  {
+    return a ^ b;
+  }
+
   /**
      typetraits for BitSetVectorReference
    */