Refactoring of the cost model:

- Dynamic is now an invalid value
 - introduce a HugeCost constant to be used for runtime-cost values or arbitrarily huge cost
 - add sanity checks for cost values: must be >=0 and not too large
This change provides several benefits:
 - it fixes shortcoming is some cost computation where the Dynamic case was not properly handled.
 - it simplifies cost computation logic, and should avoid future similar shortcomings.
 - it allows to distinguish between different level of dynamic/huge/infinite cost
 - it should enable further simplifications in the computation of costs (save compilation time)

Eigen/src/Core/util/StaticAssert.h

@@ -93,7 +93,8 @@
         THE_STORAGE_ORDER_OF_BOTH_SIDES_MUST_MATCH,
         OBJECT_ALLOCATED_ON_STACK_IS_TOO_BIG,
         IMPLICIT_CONVERSION_TO_SCALAR_IS_FOR_INNER_PRODUCT_ONLY,
-        STORAGE_LAYOUT_DOES_NOT_MATCH
+        STORAGE_LAYOUT_DOES_NOT_MATCH,
+        EIGEN_INTERNAL_ERROR_PLEASE_FILE_A_BUG_REPORT__INVALID_COST_VALUE
       };
     };
 
@@ -200,5 +201,9 @@
                                             >::value), \
                           YOU_CANNOT_MIX_ARRAYS_AND_MATRICES)
 
+// Check that a cost value is positive, and that is stay within a reasonable range
+// TODO this check could be enabled for internal debugging only
+#define EIGEN_INTERNAL_CHECK_COST_VALUE(C) \
+      EIGEN_STATIC_ASSERT((C)>=0 && (C)<2*HugeCost*HugeCost, EIGEN_INTERNAL_ERROR_PLEASE_FILE_A_BUG_REPORT__INVALID_COST_VALUE);
 
 #endif // EIGEN_STATIC_ASSERT_H