Use wherefrom-compat

This provides a compatibility shim around the wherefrom function allowing a stable interface to be used for more versions of GHC.

In particular it should insulate users of the library from future breaking changes

nothunks.cabal

@@ -37,6 +37,11 @@ flag vector
   default: True
   manual: True
 
+flag wherefrom
+  description: Access IPE information using wherefrom-compat
+  default: True
+  manual: False
+
 library
     exposed-modules:  NoThunks.Class
 
@@ -54,6 +59,8 @@ library
       build-depends:  text       >= 1.2  && < 1.3 || >= 2 && < 2.2
     if flag(vector)
       build-depends:  vector     >= 0.12 && < 0.14
+    if flag(wherefrom)
+      build-depends:  wherefrom-compat >= 0.1.1.0 && < 0.2.0.0
 
     hs-source-dirs:   src
     default-language: Haskell2010

src/NoThunks/Class.hs

@@ -71,8 +71,8 @@ import GHC.Stack
 import Numeric.Natural
 #endif
 
-#if MIN_VERSION_base(4,18,0)
-import GHC.InfoProv
+#if MIN_VERSION_base(4,16,0)
+import GHC.InfoProv.Compat
 #endif
 
 import qualified Control.Concurrent.MVar       as MVar
@@ -231,7 +231,7 @@ newtype ThunkInfo = ThunkInfo { thunkInfo  :: Either Context Info }
   deriving Show
 
 getThunkInfo :: Context -> a -> IO ThunkInfo
-#if MIN_VERSION_base(4,18,0)
+#if MIN_VERSION_base(4,16,0)
 getThunkInfo ctxt a = ThunkInfo . maybe (Left ctxt) (Right . fmt) <$> whereFrom a
   where
     fmt :: InfoProv -> Info