Natural: fix left shift of 0 (fix #19170)

libraries/ghc-bignum/src/GHC/Num/Natural.hs

@@ -239,7 +239,8 @@ naturalShiftR x (W# n) = naturalShiftR# x n
 
 -- | Left shift
 naturalShiftL# :: Natural -> Word# -> Natural
-naturalShiftL# (NS x) n
+naturalShiftL# v@(NS x) n
+   | 0## <- x                     = v
    | isTrue# (clz# x `geWord#` n) = NS (x `uncheckedShiftL#` word2Int# n)
    | True                         = NB (bigNatFromWord# x `bigNatShiftL#` n)
 naturalShiftL# (NB x) n = NB (x `bigNatShiftL#` n)

testsuite/tests/lib/integer/T19170.hs

+module Main where
+
+import Data.Bits
+import GHC.Natural
+import GHC.Num.Natural
+
+main :: IO ()
+main = do
+    print $ (shiftL 0 65 :: Natural)
+    print $ naturalCheck (shiftL 0 65 :: Natural)
+    print $ shiftL 0 65 == (0 :: Natural)

testsuite/tests/lib/integer/T19170.stdout

+0
+True
+True

testsuite/tests/lib/integer/all.T

@@ -12,6 +12,7 @@ test('integerGcdExt', [], compile_and_run, [''])
 test('integerRecipMod', [], compile_and_run, [''])
 test('bignumMatch', [], compile, [''])
 test('T18813', [], compile_and_run, [''])
+test('T19170', [], compile_and_run, [''])
 
 # skip ghci as it doesn't support unboxed tuples
 test('integerImportExport', [omit_ways(['ghci'])], compile_and_run, [''])