Yield spine in Bit's deepErrorX

Fixes #2978

Author: martijnbastiaan

changelog/2026-02-08T21_14_13+01_00_fix_2978

@@ -0,0 +1 @@
+FIXED: `ensureSpine` for `Bit` now yields its data constructor instead of `XException` [#2978](https://github.com/clash-lang/clash-compiler/issues/2978)

clash-prelude/src/Clash/Sized/Internal/BitVector.hs

@@ -277,7 +277,7 @@ instance ShowX Bit where
   showsPrecX = showsPrecXWith showsPrec
 
 instance NFDataX Bit where
-  deepErrorX = errorX
+  deepErrorX s = unpack# (deepErrorX s)
   ensureSpine = unpack# . xToBV . pack#
   rnfX = rwhnfX
   hasUndefined bv = isLeft (isX bv) || unsafeMask# bv /= 0

clash-prelude/tests/Clash/Tests/BitVector.hs

@@ -25,7 +25,8 @@ import qualified Test.Tasty.QuickCheck as Q
 
 import Clash.Prelude
   (Bit, high, low, bitPattern, type (<=), type (-), natToInteger, msb, bLit, hLit, oLit, rotateL, rotateR)
-import Clash.Sized.Internal.BitVector (BitVector (..))
+import Clash.Sized.Internal.BitVector (BitVector (..), Bit(..))
+import Clash.XException (ensureSpine, errorX, deepErrorX)
 
 import Clash.Tests.SizedNum
 
@@ -135,6 +136,32 @@ tests = localOption (Q.QuickCheckMaxRatio 2) $ testGroup "All"
     , testCase "show17" $ show @(BitVector 12) $(oLit "5324") @?= "0b1010_1101_0100"
     , testCase "show17" $ show @(BitVector 12) $(oLit ".324") @?= "0b...0_1101_0100"
     ]
+  , testGroup "NFDataX"
+    [ testCase
+        "ensureSpine should yield BV constructor"
+        (case ensureSpine (errorX "" :: BitVector 2) of
+          BV m v -> do
+            m @?= 3
+            v @?= 0)
+    , testCase
+        "deepErrorX should yield BV constructor"
+        (case deepErrorX "" :: BitVector 2 of
+          BV m v -> do
+            m @?= 3
+            v @?= 0)
+    , testCase
+        "ensureSpine should yield Bit constructor"
+        (case ensureSpine (errorX "" :: Bit) of
+          Bit m v -> do
+            m @?= 1
+            v @?= 0)
+    , testCase
+        "deepErrorX should yield Bit constructor"
+        (case deepErrorX "" :: Bit of
+          Bit m v -> do
+            m @?= 1
+            v @?= 0)
+    ]
   ]
 
 fromIntegerProp :: forall m. KnownNat m => Proxy m -> Integer -> Q.Property