In deepSplitCprType_maybe, be more forgiving

the ConTag may be out of range (e.g. if the type constructor is imported
via SOURCE and we don't know any of its data constructors); just return
Nothing without complaining in that case. This fixes #8743.

compiler/stranal/WwLib.lhs

@@ -529,7 +529,9 @@ deepSplitCprType_maybe fam_envs con_tag ty
   , Just (tc, tc_args) <- splitTyConApp_maybe ty1
   , isDataTyCon tc
   , let cons = tyConDataCons tc
-        con = ASSERT( cons `lengthAtLeast` con_tag ) cons !! (con_tag - fIRST_TAG)
+  , cons `lengthAtLeast` con_tag -- This might not be true if we import the
+                                 -- type constructor via a .hs-bool file (#8743)
+  , let con  = cons !! (con_tag - fIRST_TAG)
   = Just (con, tc_args, dataConInstArgTys con tc_args, co)
 deepSplitCprType_maybe _ _ _ = Nothing
 \end{code}

testsuite/tests/stranal/should_compile/all.T

@@ -18,5 +18,5 @@ test('newtype', req_profiling, compile, ['-prof -auto-all'])
 test('T1988', normal, compile, [''])
 test('T8467', normal, compile, [''])
 test('T8037', normal, compile, [''])
-test('T8743', [ expect_broken(8743), extra_clean(['T8743.o-boot', 'T8743.hi-boot']) ], multimod_compile, ['T8743', '-v0'])
+test('T8743', [ extra_clean(['T8743.o-boot', 'T8743.hi-boot']) ], multimod_compile, ['T8743', '-v0'])