Introduce ConcreteTv metavariables

  This patch introduces a new kind of metavariable, by adding the
  constructor `ConcreteTv` to `MetaInfo`. A metavariable with
  `ConcreteTv` `MetaInfo`, henceforth a concrete metavariable, can only
  be unified with a type that is concrete (that is, a type that answers
  `True` to `GHC.Core.Type.isConcrete`).

  This solves the problem of dangling metavariables in `Concrete#`
  constraints: instead of emitting `Concrete# ty`, which contains a
  secret existential metavariable, we simply emit a primitive equality
  constraint `ty ~# concrete_tv` where `concrete_tv` is a fresh concrete
  metavariable.

  This means we can avoid all the complexity of canonicalising
  `Concrete#` constraints, as we can just re-use the existing machinery
  for `~#`.

  To finish things up, this patch then removes the `Concrete#` special
  predicate, and instead introduces the special predicate `IsRefl#`
  which enforces that a coercion is reflexive.
  Such a constraint is needed because the canonicaliser is quite happy
  to rewrite an equality constraint such as `ty ~# concrete_tv`, but
  such a rewriting is not handled by the rest of the compiler currently,
  as we need to make use of the resulting coercion, as outlined in the
  FixedRuntimeRep plan.

  The big upside of this approach (on top of simplifying the code)
  is that we can now selectively implement PHASE 2 of FixedRuntimeRep,
  by changing individual calls of `hasFixedRuntimeRep_MustBeRefl` to
  `hasFixedRuntimeRep` and making use of the obtained coercion.

1 files changed, 13 insertions, 0 deletions

diff --git a/testsuite/tests/rep-poly/T17021.stderr b/testsuite/tests/rep-poly/T17021.stderr
new file mode 100644
index 0000000000..accd9c1560
--- /dev/null
+++ b/testsuite/tests/rep-poly/T17021.stderr
@@ -0,0 +1,13 @@
+
+T17021.hs:18:9: error:
+    • The function argument
+        ‘42’
+      does not have a fixed runtime representation.
+      Its type is:
+        Int :: TYPE (Id LiftedRep)
+      NB: GHC does not (yet) support rewriting in runtime representations.
+      Please comment on GHC ticket #13105 if this is causing you trouble.
+      <https://gitlab.haskell.org/ghc/ghc/-/issues/13105>
+    • In the first argument of ‘MkT’, namely ‘42’
+      In the expression: MkT 42
+      In an equation for ‘f’: f = MkT 42