testsuite: add a regression test for #10011

Signed-off-by: Austin Seipp <austin@well-typed.com>

testsuite/.gitignore

@@ -1045,6 +1045,7 @@ mk/ghcconfig*_inplace_bin_ghc-stage2.exe.mk
 /tests/numeric/should_run/T7689
 /tests/numeric/should_run/T8726
 /tests/numeric/should_run/T9810
+/tests/numeric/should_run/T10011
 /tests/numeric/should_run/add2
 /tests/numeric/should_run/arith001
 /tests/numeric/should_run/arith002

testsuite/tests/numeric/should_run/T10011.hs

+{-# LANGUAGE ScopedTypeVariables, TypeOperators, GADTs #-}
+module Main
+       ( main -- :: IO ()
+       ) where
+import Data.Data
+import Data.Ratio
+
+main :: IO ()
+main =
+  let bad = gmapT (\(x :: b) ->
+              case eqT :: Maybe (b :~: Integer) of
+                Nothing -> x;
+                Just Refl -> x * 2) (1 % 2) :: Rational
+  in print (bad == numerator bad % denominator bad)

testsuite/tests/numeric/should_run/T10011.stdout

+True

testsuite/tests/numeric/should_run/all.T

@@ -64,3 +64,4 @@ test('NumDecimals', normal, compile_and_run, [''])
 test('T8726', normal, compile_and_run, [''])
 test('CarryOverflow', omit_ways(['ghci']), compile_and_run, [''])
 test('T9810', normal, compile_and_run, [''])
+test('T10011', normal, compile_and_run, [''])