GHCi supports not-necessarily-lifted join points

Fixes #16509.

See Note [Not-necessarily-lifted join points] in ByteCodeGen,
which tells the full story.

This commit also adds some comments and cleans some code
in the byte-code generator, as I was exploring around trying
to understand it.

(This commit removes an old test -- this is really a GHCi problem,
not a pattern-synonym problem.)

test case: ghci/scripts/T16509

1 files changed, 10 insertions, 0 deletions

diff --git a/compiler/coreSyn/CoreSyn.hs b/compiler/coreSyn/CoreSyn.hs
index 725e8da826..16123e7b3a 100644
--- a/compiler/coreSyn/CoreSyn.hs
+++ b/compiler/coreSyn/CoreSyn.hs
@@ -655,6 +655,16 @@ invariant 3 does still need to be checked.) For the rigorous definition of
 
 Invariant 4 is subtle; see Note [The polymorphism rule of join points].
 
+Invariant 6 is to enable code like this:
+
+  f = \(r :: RuntimeRep) (a :: TYPE r) (x :: T).
+      join j :: a
+           j = error @r @a "bloop"
+      in case x of
+           A -> j
+           B -> j
+           C -> error @r @a "blurp"
+
 Core Lint will check these invariants, anticipating that any binder whose
 OccInfo is marked AlwaysTailCalled will become a join point as soon as the
 simplifier (or simpleOptPgm) runs.