Fix typechecking time bug for large rationals (#15646)

When desugaring large overloaded literals we now avoid
computing the `Rational` value. Instead prefering to
store the significant and exponent as given where
reasonable and possible.

See Note [FractionalLit representation] for details.

2 files changed, 39 insertions, 0 deletions

diff --git a/testsuite/tests/pmcheck/should_compile/T19384.hs b/testsuite/tests/pmcheck/should_compile/T19384.hs
new file mode 100644
index 0000000000..1cecffa8ad
--- /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T19384.hs
@@ -0,0 +1,34 @@
+{-# OPTIONS_GHC -Wno-missing-methods #-}
+
+-- Pattern match checking is broken for overloaded rationals currently.
+
+module T15646a where
+
+
+foo_small :: ()
+foo_small = case 2e2 :: Rational of
+  2e1 -> () -- redundant
+  2e2 -> ()
+
+-- Large exponents are handled differently so we have an extra check.
+foo :: ()
+foo = case 2e102 :: Rational of
+  2e101 -> () -- redundant
+  2e102 -> ()
+
+-- Any literal of type T will desugar to the same value MkT.
+-- Eg. (1.0 :: T) == MkT
+data T = MkT deriving Eq
+instance Num T where
+instance Fractional T where
+  fromRational _ = MkT
+
+bar :: ()
+bar = case 2e102 :: T of
+  2e101 -> () -- not redundant, pattern evaluates to MkT
+  2e102 -> () -- redundant, pattern also evaluates to MkT
+
+baz :: ()
+baz = case 2e1 :: T of
+  2e1 -> () -- not redundant, pattern evaluates to MkT
+  2e2 -> () -- redundant, pattern also evaluates to MkT
diff --git a/testsuite/tests/pmcheck/should_compile/all.T b/testsuite/tests/pmcheck/should_compile/all.T
index 40b59b2fd3..b922696fae 100644
--- a/testsuite/tests/pmcheck/should_compile/all.T
+++ b/testsuite/tests/pmcheck/should_compile/all.T
@@ -252,3 +252,8 @@ test('EmptyCase009', [],  compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 test('EmptyCase010', [],  compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
+
+# Overloaded patterns and rational pattern matching being broken.
+test('T19384',
+     expect_broken(19384),
+     compile, [''])