Fix #14628: Panic (No skolem Info) in GHCi

This patch implements the [sugggestion from Simon (PJ)](https://gitlab.haskell.org/ghc/ghc/issues/14628#note_146559):
- Make `TcErrors.getSkolemInfo` return a `SkolemInfo` rather than an `Implication`.
- If `getSkolemInfo` gets `RuntimeUnk`s, just return a new data constructor in `SkolemInfo`, called `RuntimeUnkSkol`.
- In `TcErrors.pprSkols` print something sensible for a `RuntimeUnkSkol`.

The `getSkolemInfo` function paniced while formating suggestions to add type annotations (subfunction `suggestAddSig`)
to a *"Couldn't match type ‘x’ with ‘y’"* error message.
The `getSkolemInfo` function didn't find any Implication value and paniced.
With this patch the `getSkolemInfo` function does no longer panic, if it finds `RuntimeUnkSkol`s.

As the panic occured while processing an error message, we don't need to implement any new error message!

7 files changed, 52 insertions, 11 deletions

diff --git a/compiler/typecheck/TcErrors.hs b/compiler/typecheck/TcErrors.hs
index b20fb55e11..9557efa40c 100644
--- a/compiler/typecheck/TcErrors.hs
+++ b/compiler/typecheck/TcErrors.hs
@@ -1762,8 +1762,7 @@ suggestAddSig ctxt ty1 ty2
     inferred_bndrs = nub (get_inf ty1 ++ get_inf ty2)
     get_inf ty | Just tv <- tcGetTyVar_maybe ty
                , isSkolemTyVar tv
-               , (implic, _) : _ <- getSkolemInfo (cec_encl ctxt) [tv]
-               , InferSkol prs <- ic_info implic
+               , ((InferSkol prs, _) : _) <- getSkolemInfo (cec_encl ctxt) [tv]
                = map fst prs
                | otherwise
                = []
@@ -2755,11 +2754,13 @@ pprSkols :: ReportErrCtxt -> [TcTyVar] -> SDoc
 pprSkols ctxt tvs
   = vcat (map pp_one (getSkolemInfo (cec_encl ctxt) tvs))
   where
-    pp_one (Implic { ic_info = skol_info }, tvs)
-      | UnkSkol <- skol_info
+    pp_one (UnkSkol, tvs)
       = hang (pprQuotedList tvs)
            2 (is_or_are tvs "an" "unknown")
-      | otherwise
+    pp_one (RuntimeUnkSkol, tvs)
+      = hang (pprQuotedList tvs)
+           2 (is_or_are tvs "an" "unknown runtime")
+    pp_one (skol_info, tvs)
       = vcat [ hang (pprQuotedList tvs)
                   2 (is_or_are tvs "a"  "rigid" <+> text "bound by")
              , nest 2 (pprSkolInfo skol_info)
@@ -2779,20 +2780,21 @@ getAmbigTkvs ct
     dep_tkv_set = tyCoVarsOfTypes (map tyVarKind tkvs)
 
 getSkolemInfo :: [Implication] -> [TcTyVar]
-              -> [(Implication, [TcTyVar])]
+              -> [(SkolemInfo, [TcTyVar])]                    -- #14628
 -- Get the skolem info for some type variables
--- from the implication constraints that bind them
+-- from the implication constraints that bind them.
 --
--- In the returned (implic, tvs) pairs, the 'tvs' part is non-empty
+-- In the returned (skolem, tvs) pairs, the 'tvs' part is non-empty
 getSkolemInfo _ []
   = []
 
 getSkolemInfo [] tvs
-  = pprPanic "No skolem info:" (ppr tvs)
+  | all isRuntimeUnkSkol tvs = [(RuntimeUnkSkol, tvs)]        -- #14628
+  | otherwise = pprPanic "No skolem info:" (ppr tvs)
 
 getSkolemInfo (implic:implics) tvs
-  | null tvs_here =                      getSkolemInfo implics tvs
-  | otherwise     = (implic, tvs_here) : getSkolemInfo implics tvs_other
+  | null tvs_here =                            getSkolemInfo implics tvs
+  | otherwise   = (ic_info implic, tvs_here) : getSkolemInfo implics tvs_other
   where
     (tvs_here, tvs_other) = partition (`elem` ic_skols implic) tvs
 
diff --git a/compiler/typecheck/TcOrigin.hs b/compiler/typecheck/TcOrigin.hs
index fd260f01ac..df7a39f72e 100644
--- a/compiler/typecheck/TcOrigin.hs
+++ b/compiler/typecheck/TcOrigin.hs
@@ -237,6 +237,8 @@ data SkolemInfo
   | QuantCtxtSkol       -- Quantified context, e.g.
                         --   f :: forall c. (forall a. c a => c [a]) => blah
 
+  | RuntimeUnkSkol      -- Runtime skolem from the GHCi debugger      #14628
+
   | UnkSkol             -- Unhelpful info (until I improve it)
 
 instance Outputable SkolemInfo where
@@ -267,6 +269,7 @@ pprSkolInfo (DataConSkol name)= text "the data constructor" <+> quotes (ppr name
 pprSkolInfo ReifySkol         = text "the type being reified"
 
 pprSkolInfo (QuantCtxtSkol {}) = text "a quantified context"
+pprSkolInfo RuntimeUnkSkol     = text "Unknown type from GHCi runtime"
 
 -- UnkSkol
 -- For type variables the others are dealt with by pprSkolTvBinding.
diff --git a/testsuite/tests/ghci.debugger/scripts/T14628.hs b/testsuite/tests/ghci.debugger/scripts/T14628.hs
new file mode 100644
index 0000000000..b94d9e736e
--- /dev/null
+++ b/testsuite/tests/ghci.debugger/scripts/T14628.hs
@@ -0,0 +1,14 @@
+module T14628 where
+
+import System.IO
+import Control.Monad.IO.Class
+import Control.Monad.Trans.State
+import Text.Printf
+
+putArrayBytes :: Handle       -- ^ output file handle
+              -> [String]     -- ^ byte-strings
+              -> IO Int       -- ^ total number of bytes written
+putArrayBytes outfile xs = do
+  let writeCount x = modify' (+ length x) >> liftIO (putLine x) :: MonadIO m => StateT Int m ()
+  execStateT (mapM_ writeCount xs) 0
+  where putLine = hPutStrLn outfile . ("  "++) . concatMap (printf "0x%02X,")
diff --git a/testsuite/tests/ghci.debugger/scripts/T14628.script b/testsuite/tests/ghci.debugger/scripts/T14628.script
new file mode 100644
index 0000000000..4675b515de
--- /dev/null
+++ b/testsuite/tests/ghci.debugger/scripts/T14628.script
@@ -0,0 +1,4 @@
+:l T14628.hs
+:br 12 46
+:trace putArrayBytes stdout [['1'..'9'],['2'..'8'],['3'..'7']]
+snd $ runStateT _result 0
diff --git a/testsuite/tests/ghci.debugger/scripts/T14628.stderr b/testsuite/tests/ghci.debugger/scripts/T14628.stderr
new file mode 100644
index 0000000000..276d63ff38
--- /dev/null
+++ b/testsuite/tests/ghci.debugger/scripts/T14628.stderr
@@ -0,0 +1,12 @@
+<interactive>:4:7:
+     Couldn't match type ‘m’ with ‘(,) a0’
+        ‘m’ is untouchable
+          inside the constraints: ()
+          bound by the inferred type of it :: ((), Int)
+          at <interactive>:4:1-25
+      ‘m’ is an interactive-debugger skolem
+      Expected type: (a0, ((), Int))
+        Actual type: m ((), Int)
+     In the second argument of ‘($)’, namely ‘runStateT _result 0’
+      In the expression: snd $ runStateT _result 0
+      In an equation for ‘it’: it = snd $ runStateT _result 0
diff --git a/testsuite/tests/ghci.debugger/scripts/T14628.stdout b/testsuite/tests/ghci.debugger/scripts/T14628.stdout
new file mode 100644
index 0000000000..9564271f8f
--- /dev/null
+++ b/testsuite/tests/ghci.debugger/scripts/T14628.stdout
@@ -0,0 +1,5 @@
+Breakpoint 0 activated at T14628.hs:12:46-63
+Stopped in T14628.putArrayBytes.writeCount, T14628.hs:12:46-63
+_result :: StateT Int m () = _
+putLine :: [Char] -> IO () = _
+x :: [Char] = "123456789"
diff --git a/testsuite/tests/ghci.debugger/scripts/all.T b/testsuite/tests/ghci.debugger/scripts/all.T
index 297b4c2b76..01662361c4 100644
--- a/testsuite/tests/ghci.debugger/scripts/all.T
+++ b/testsuite/tests/ghci.debugger/scripts/all.T
@@ -113,6 +113,7 @@ test('T13825-debugger',
       [when(arch('powerpc64'), expect_broken(14455)),
        when(arch('arm'), fragile_for(17557, ['ghci-ext']))],
       ghci_script, ['T13825-debugger.script'])
+test('T14628', normal, ghci_script, ['T14628.script'])
 test('T14690', normal, ghci_script, ['T14690.script'])
 test('T16700', normal, ghci_script, ['T16700.script'])