Fix #15502 by not casting to Int during TH conversion

Summary:
When turning an `IntegerL` to an `IntegralLit` during TH
conversion, we were stupidly casting an `Integer` to an `Int` in
order to determine how it should be pretty-printed. Unsurprisingly,
this causes problems when the `Integer` doesn't lie within the bounds
of an `Int`, as demonstrated in #15502.

The fix is simple: don't cast to an `Int`.

Test Plan: make test TEST=T15502

Reviewers: bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, carter

GHC Trac Issues: #15502

Differential Revision: https://phabricator.haskell.org/D5089

1 files changed, 12 insertions, 2 deletions

diff --git a/compiler/basicTypes/BasicTypes.hs b/compiler/basicTypes/BasicTypes.hs
index 93010b75f9..ce4696293c 100644
--- a/compiler/basicTypes/BasicTypes.hs
+++ b/compiler/basicTypes/BasicTypes.hs
@@ -1436,9 +1436,12 @@ data IntegralLit
   deriving (Data, Show)
 
 mkIntegralLit :: Integral a => a -> IntegralLit
-mkIntegralLit i = IL { il_text = SourceText (show (fromIntegral i :: Int))
+mkIntegralLit i = IL { il_text = SourceText (show i_integer)
                      , il_neg = i < 0
-                     , il_value = toInteger i }
+                     , il_value = i_integer }
+  where
+    i_integer :: Integer
+    i_integer = toInteger i
 
 negateIntegralLit :: IntegralLit -> IntegralLit
 negateIntegralLit (IL text neg value)
@@ -1463,6 +1466,13 @@ data FractionalLit
 
 mkFractionalLit :: Real a => a -> FractionalLit
 mkFractionalLit r = FL { fl_text = SourceText (show (realToFrac r::Double))
+                           -- Converting to a Double here may technically lose
+                           -- precision (see #15502). We could alternatively
+                           -- convert to a Rational for the most accuracy, but
+                           -- it would cause Floats and Doubles to be displayed
+                           -- strangely, so we opt not to do this. (In contrast
+                           -- to mkIntegralLit, where we always convert to an
+                           -- Integer for the highest accuracy.)
                        , fl_neg = r < 0
                        , fl_value = toRational r }