Use tcInferFRR to prevent bad generalisation

Fixes #23176

compiler/GHC/Tc/Gen/Bind.hs

@@ -1210,20 +1210,10 @@ This check, mkInferredPolyId, is really in the wrong place:
 `inferred_poly_ty` doesn't obey the PKTI and it would be better not to
 generalise it in the first place; see #20686.  But for now it works.
 
-How else could we avoid generalising over escaping type variables? I
-considered:
-
-* Adjust the generalisation in GHC.Tc.Solver to directly check for
-  escaping kind variables; instead, promote or default them. But that
-  gets into the defaulting swamp and is a non-trivial and unforced
-  change, so I have left it alone for now.
-
-* When inferring the type of a binding, in `tcMonoBinds`, we create
-  an ExpSigmaType with `tcInfer`.  If we simply gave it an ir_frr field
-  that said "must have fixed runtime rep", then the kind would be made
-  Concrete; and we never generalise over Concrete variables.  A bit
-  more indirect, but we need the "don't generalise over Concrete variables"
-  stuff anyway.
+I considered adjusting the generalisation in GHC.Tc.Solver to directly check for
+escaping kind variables; instead, promoting or defaulting them. But that
+gets into the defaulting swamp and is a non-trivial and unforced
+change, so I have left it alone for now.
 
 Note [Impedance matching]
 ~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -1312,7 +1302,9 @@ tcMonoBinds is_rec sig_fn no_gen
   , Nothing <- sig_fn name   -- ...with no type signature
   = setSrcSpanA b_loc    $
     do  { ((co_fn, matches'), rhs_ty')
-            <- tcInfer $ \ exp_ty ->
+            <- tcInferFRR (FRRBinder name) $ \ exp_ty ->
+                          -- tcInferFRR: the type of a let-binder must have
+                          -- a fixed runtime rep. See #23176
                        tcExtendBinderStack [TcIdBndr_ExpType name exp_ty NotTopLevel] $
                           -- We extend the error context even for a non-recursive
                           -- function so that in type error messages we show the
@@ -1334,7 +1326,9 @@ tcMonoBinds is_rec sig_fn no_gen
   | NonRecursive <- is_rec   -- ...binder isn't mentioned in RHS
   , all (isNothing . sig_fn) bndrs
   = addErrCtxt (patMonoBindsCtxt pat grhss) $
-    do { (grhss', pat_ty) <- tcInfer $ \ exp_ty ->
+    do { (grhss', pat_ty) <- tcInferFRR FRRPatBind $ \ exp_ty ->
+                          -- tcInferFRR: the type of each let-binder must have
+                          -- a fixed runtime rep. See #23176
                              tcGRHSsPat grhss exp_ty
 
        ; let exp_pat_ty :: Scaled ExpSigmaTypeFRR

testsuite/tests/polykinds/T22743.stderr

 
-T22743.hs:10:1: error: [GHC-31147]
-    • Quantified type's kind mentions quantified type variable
-        type: ‘forall {f :: * -> RuntimeRep} {g} {a :: TYPE (f g)}. a’
-      where the body of the forall has this kind: ‘TYPE (f g)’
-    • When checking the inferred type
-        x :: forall {f :: * -> RuntimeRep} {g} {a :: TYPE (f g)}. a
+T22743.hs:10:1: error: [GHC-52083]
+    The binder ‘x’
+    cannot be assigned a fixed runtime representation, not even by defaulting.
+    Suggested fix: Add a type signature.
+
+T22743.hs:10:1: error: [GHC-52083]
+    The binder ‘x’
+    cannot be assigned a fixed runtime representation, not even by defaulting.
+    Suggested fix: Add a type signature.

testsuite/tests/rep-poly/T23176.hs

+module T23176 where
+
+import GHC.Exts
+
+f = outOfScope :: (_ :: TYPE (r s))
+(g :: _) = outOfScope :: (_ :: TYPE (r s))

testsuite/tests/rep-poly/T23176.stderr

+
+T23176.hs:5:1: error: [GHC-52083]
+    The binder ‘f’
+    cannot be assigned a fixed runtime representation, not even by defaulting.
+    Suggested fix: Add a type signature.
+
+T23176.hs:5:1: error: [GHC-52083]
+    The binder ‘f’
+    cannot be assigned a fixed runtime representation, not even by defaulting.
+    Suggested fix: Add a type signature.
+
+T23176.hs:5:1: error: [GHC-52083]
+    The binder ‘f’
+    cannot be assigned a fixed runtime representation, not even by defaulting.
+    Suggested fix: Add a type signature.
+
+T23176.hs:6:1: error: [GHC-52083]
+    The pattern binding
+    cannot be assigned a fixed runtime representation, not even by defaulting.
+    Suggested fix: Add a type signature.
+
+T23176.hs:6:1: error: [GHC-52083]
+    The pattern binding
+    cannot be assigned a fixed runtime representation, not even by defaulting.
+    Suggested fix: Add a type signature.
+
+T23176.hs:6:1: error: [GHC-52083]
+    The pattern binding
+    cannot be assigned a fixed runtime representation, not even by defaulting.
+    Suggested fix: Add a type signature.

testsuite/tests/rep-poly/all.T

@@ -118,3 +118,4 @@ test('T21650_b', normal, compile_fail, ['-Wno-deprecated-flags'])            ##
 test('T23051', normal, compile_fail, [''])
 test('T23153', normal, compile_fail, [''])
 test('T23154', normal, compile_fail, [''])
+test('T23176', normal, compile_fail, ['-XPartialTypeSignatures -fdefer-out-of-scope-variables'])