base: Add default implementation for Data.Bits.bitSize

Fixes #12970 and will provide a reasonable migration path for the
eventual remove of this function.

Test Plan: Validate

Reviewers: ekmett, hvr

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #12970

Differential Revision: https://phabricator.haskell.org/D4857

libraries/base/Data/Bits.hs

@@ -190,8 +190,12 @@ class Eq a => Bits a where
     {-| Return the number of bits in the type of the argument.  The actual
         value of the argument is ignored.  The function 'bitSize' is
         undefined for types that do not have a fixed bitsize, like 'Integer'.
+
+        Default implementation based upon 'bitSizeMaybe' provided since
+        4.12.0.0.
         -}
     bitSize           :: a -> Int
+    bitSize b = fromMaybe (error "bitSize is undefined") (bitSizeMaybe b)
 
     {-| Return 'True' if the argument is a signed type.  The actual
         value of the argument is ignored -}

libraries/base/changelog.md

@@ -325,6 +325,9 @@
   * New `Control.Exception.TypeError` datatype, which is thrown when an
     expression fails to typecheck when run using `-fdefer-type-errors` (#10284)
 
+  * The `bitSize` method of `Data.Bits.Bits` now has a (partial!)
+    default implementation based on `bitSizeMaybe`. (#12970)
+
 ### New instances
 
   * `Alt`, `Dual`, `First`, `Last`, `Product`, and `Sum` now have `Data`,