Encode shape information in `PmOracle`

Previously, we had an elaborate mechanism for selecting the warnings to
generate in the presence of different `COMPLETE` matching groups that,
albeit finely-tuned, produced wrong results from an end user's
perspective in some cases (#13363).

The underlying issue is that at the point where the `ConVar` case has to
commit to a particular `COMPLETE` group, there's not enough information
to do so and the status quo was to just enumerate all possible complete
sets nondeterministically.  The `getResult` function would then pick the
outcome according to metrics defined in accordance to the user's guide.
But crucially, it lacked knowledge about the order in which affected
clauses appear, leading to the surprising behavior in #13363.

In !1010 we taught the term oracle to reason about literal values a
variable can certainly not take on. This MR extends that idea to
`ConLike`s and thereby fixes #13363: Instead of committing to a
particular `COMPLETE` group in the `ConVar` case, we now split off the
matching constructor incrementally and record the newly covered case as
a refutable shape in the oracle. Whenever the set of refutable shapes
covers any `COMPLETE` set, the oracle recognises vacuosity of the
uncovered set.

This patch goes a step further: Since at this point the information
in value abstractions is merely a cut down representation of what the
oracle knows, value abstractions degenerate to a single `Id`, the
semantics of which is determined by the oracle state `Delta`.
Value vectors become lists of `[Id]` given meaning to by a single
`Delta`, value set abstractions (of which the uncovered set is an
instance) correspond to a union of `Delta`s which instantiate the
same `[Id]` (akin to models of formula).

Fixes #11528 #13021, #13363, #13965, #14059, #14253, #14851, #15753, #17096, #17149

-------------------------
Metric Decrease:
    ManyAlternatives
    T11195
-------------------------

1 files changed, 4 insertions, 5 deletions

diff --git a/compiler/typecheck/TcRnTypes.hs b/compiler/typecheck/TcRnTypes.hs
index 5d56d33d53..0a2db75a2d 100644
--- a/compiler/typecheck/TcRnTypes.hs
+++ b/compiler/typecheck/TcRnTypes.hs
@@ -167,7 +167,7 @@ import TcType
 import Annotations
 import InstEnv
 import FamInstEnv
-import PmExpr
+import {-# SOURCE #-} PmOracle (Delta)
 import IOEnv
 import RdrName
 import Name
@@ -390,10 +390,9 @@ data DsLclEnv = DsLclEnv {
         dsl_loc     :: RealSrcSpan,      -- To put in pattern-matching error msgs
 
         -- See Note [Note [Type and Term Equality Propagation] in Check.hs
-        -- These two fields are augmented as we walk inwards,
-        -- through each patttern match in turn
-        dsl_dicts   :: Bag EvVar,     -- Constraints from GADT pattern-matching
-        dsl_tm_cs   :: Bag TmVarCt,      -- Constraints form term-level pattern matching
+        -- The oracle state Delta is augmented as we walk inwards,
+        -- through each pattern match in turn
+        dsl_delta   :: Delta,
 
         dsl_pm_iter :: IORef Int  -- Number of iterations for pmcheck so far
                                   -- We fail if this gets too big