Introduce Concrete# for representation polymorphism checks

PHASE 1: we never rewrite Concrete# evidence.

This patch migrates all the representation polymorphism checks to
the typechecker, using a new constraint form

  Concrete# :: forall k. k -> TupleRep '[]

Whenever a type `ty` must be representation-polymorphic
(e.g. it is the type of an argument to a function), we emit a new
`Concrete# ty` Wanted constraint. If this constraint goes
unsolved, we report a representation-polymorphism error to the user.
The 'FRROrigin' datatype keeps track of the context of the
representation-polymorphism check, for more informative error messages.

This paves the way for further improvements, such as
allowing type families in RuntimeReps and improving the soundness
of typed Template Haskell. This is left as future work (PHASE 2).

fixes #17907 #20277 #20330 #20423 #20426

updates haddock submodule

-------------------------
Metric Decrease:
    T5642
-------------------------

1 files changed, 7 insertions, 0 deletions

diff --git a/testsuite/tests/rep-poly/RepPolyMatch.stderr b/testsuite/tests/rep-poly/RepPolyMatch.stderr
new file mode 100644
index 0000000000..f52a699002
--- /dev/null
+++ b/testsuite/tests/rep-poly/RepPolyMatch.stderr
@@ -0,0 +1,7 @@
+
+RepPolyMatch.hs:11:9: error:
+    • The type of the first pattern in the case alternative
+      does not have a fixed runtime representation:
+        a :: TYPE rep
+    • In the expression: \case
+      In an equation for ‘match’: match = \case