[Git][ghc/ghc][wip/T22742] Fix #22742

[Simon Peyton Jones (@simonpj)]

Simon Peyton Jones pushed to branch wip/T22742 at Glasgow Haskell Compiler / GHC


Commits:
0ff66569 by Simon Peyton Jones at 2023-01-20T08:34:01+00:00
Fix #22742

runtimeRepLevity_maybe was panicing unnecessarily; and
the error printing code made use of the case when it should
return Nothing rather than panicing.

For some bizarre reason perf/compiler/T21839r shows a 10% bump in runtime
peak-megagbytes-used, on a single architecture (alpine). See !9753 for
commentary, but I'm going to accept it.

Metric Increase:
    T21839r

- - - - -


4 changed files:

- compiler/GHC/Core/Type.hs
- + testsuite/tests/polykinds/T22742.hs
- + testsuite/tests/polykinds/T22742.stderr
- testsuite/tests/polykinds/all.T


Changes:

=====================================
compiler/GHC/Core/Type.hs
=====================================
@@ -686,8 +686,8 @@ kindBoxedRepLevity_maybe ty
 --  * False of type variables, type family applications,
 --    and of other reps such as @IntRep :: RuntimeRep at .
 isLiftedRuntimeRep :: RuntimeRepType -> Bool
-isLiftedRuntimeRep rep =
-  runtimeRepLevity_maybe rep == Just Lifted
+isLiftedRuntimeRep rep
+  = runtimeRepLevity_maybe rep == Just Lifted
 
 -- | Check whether a type of kind 'RuntimeRep' is unlifted.
 --
@@ -779,7 +779,8 @@ isBoxedRuntimeRep_maybe rep
   | otherwise
   = Nothing
 
--- | Check whether a type of kind 'RuntimeRep' is lifted, unlifted, or unknown.
+-- | Check whether a type (usually of kind 'RuntimeRep') is lifted, unlifted,
+--   or unknown.  Returns Nothing if the type isn't of kind 'RuntimeRep'.
 --
 -- `runtimeRepLevity_maybe rr` returns:
 --
@@ -793,7 +794,9 @@ runtimeRepLevity_maybe rep
     if (rr_tc `hasKey` boxedRepDataConKey)
     then case args of
             [lev] -> levityType_maybe lev
-            _     -> pprPanic "runtimeRepLevity_maybe" (ppr rep)
+            _     -> Nothing  -- Type isn't of kind RuntimeRep
+                     -- The latter case happens via the call to isLiftedRuntimeRep
+                     -- in GHC.Tc.Errors.Ppr.pprMisMatchMsg (#22742)
     else Just Unlifted
         -- Avoid searching all the unlifted RuntimeRep type cons
         -- In the RuntimeRep data type, only LiftedRep is lifted


=====================================
testsuite/tests/polykinds/T22742.hs
=====================================
@@ -0,0 +1,8 @@
+module T22742 where
+
+import GHC.Exts (TYPE)
+
+data T (a :: TYPE r) = MkT
+
+f :: T @(f a b) () -> ()
+f MkT = ()


=====================================
testsuite/tests/polykinds/T22742.stderr
=====================================
@@ -0,0 +1,6 @@
+
+T22742.hs:7:17: error: [GHC-83865]
+    • Couldn't match kind ‘GHC.Types.BoxedRep’ with ‘f a’
+      Expected kind ‘TYPE (f a b)’, but ‘()’ has kind ‘*’
+    • In the second argument of ‘T’, namely ‘()’
+      In the type signature: f :: T @(f a b) () -> ()


=====================================
testsuite/tests/polykinds/all.T
=====================================
@@ -242,3 +242,4 @@ test('T19739d', normal, compile, [''])
 test('T22379a', normal, compile, [''])
 test('T22379b', normal, compile, [''])
 test('T22743', normal, compile_fail, [''])
+test('T22742', normal, compile_fail, [''])



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/0ff665690e989f58a46df19e6fcbd35cb80eb0e8

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/0ff665690e989f58a46df19e6fcbd35cb80eb0e8
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