Care with implicit-parameter superclasses

Two bugs, #18627 and #18649, had the same cause: we were not
account for the fact that a constaint tuple might hide an implicit
parameter.

The solution is not hard: look for implicit parameters in
superclasses.  See Note [Local implicit parameters] in
GHC.Core.Predicate.

Then we use this new function in two places

* The "short-cut solver" in GHC.Tc.Solver.Interact.shortCutSolver
  which simply didn't handle implicit parameters properly at all.
  This fixes #18627

* The specialiser, which should not specialise on implicit parameters
  This fixes #18649

There are some lingering worries (see Note [Local implicit
parameters]) but things are much better.

6 files changed, 52 insertions, 0 deletions

diff --git a/testsuite/tests/simplCore/should_compile/T18649.hs b/testsuite/tests/simplCore/should_compile/T18649.hs
new file mode 100644
index 0000000000..de8b59bf86
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T18649.hs
@@ -0,0 +1,26 @@
+{-# LANGUAGE ImplicitParams #-}
+{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE FlexibleContexts #-}
+
+module Test where
+
+import Prelude
+
+type Hidden a =
+  ( ?enable :: a
+  , Eq a  -- removing this "fixes" the issue
+  )
+
+{-# NOINLINE a #-}
+a :: Hidden Bool => Integer -> Bool
+a _ = ?enable
+
+system :: Hidden Bool => Bool
+system = a 0
+
+topEntity :: Bool -> Bool
+topEntity ena = let ?enable = ena
+                in system
+
+someVar = let ?enable = True
+          in system
diff --git a/testsuite/tests/simplCore/should_compile/T18649.stderr b/testsuite/tests/simplCore/should_compile/T18649.stderr
new file mode 100644
index 0000000000..079367fbee
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T18649.stderr
@@ -0,0 +1,4 @@
+
+==================== Tidy Core rules ====================
+
+
diff --git a/testsuite/tests/simplCore/should_compile/all.T b/testsuite/tests/simplCore/should_compile/all.T
index 771798ee18..d377cfd06b 100644
--- a/testsuite/tests/simplCore/should_compile/all.T
+++ b/testsuite/tests/simplCore/should_compile/all.T
@@ -333,3 +333,7 @@ test('T18347', normal, compile, ['-dcore-lint -O'])
 test('T18355', [ grep_errmsg(r'OneShot') ], compile, ['-O -ddump-simpl -dsuppress-uniques'])
 test('T18399', normal, compile, ['-dcore-lint -O'])
 test('T18589', normal, compile, ['-dcore-lint -O'])
+
+# T18649 should /not/ generate a specialisation rule
+test('T18649', normal, compile, ['-O -ddump-rules -Wno-simplifiable-class-constraints'])
+
diff --git a/testsuite/tests/typecheck/should_run/T18627.hs b/testsuite/tests/typecheck/should_run/T18627.hs
new file mode 100644
index 0000000000..22dd87c83a
--- /dev/null
+++ b/testsuite/tests/typecheck/should_run/T18627.hs
@@ -0,0 +1,16 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE ImplicitParams #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+module Main where
+
+import GHC.Classes
+
+instance IP "x" Int where
+  ip = 21
+
+baz :: (?x :: Int) => Int
+baz = ?x
+
+main :: IO ()
+main  = let ?x = 42
+        in print baz
diff --git a/testsuite/tests/typecheck/should_run/T18627.stdout b/testsuite/tests/typecheck/should_run/T18627.stdout
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/testsuite/tests/typecheck/should_run/T18627.stdout
@@ -0,0 +1 @@
+42
diff --git a/testsuite/tests/typecheck/should_run/all.T b/testsuite/tests/typecheck/should_run/all.T
index 5afc7e3725..f69cc71352 100755
--- a/testsuite/tests/typecheck/should_run/all.T
+++ b/testsuite/tests/typecheck/should_run/all.T
@@ -146,3 +146,4 @@ test('UnliftedNewtypesDependentFamilyRun', normal, compile_and_run, [''])
 test('UnliftedNewtypesIdentityRun', normal, compile_and_run, [''])
 test('UnliftedNewtypesCoerceRun', normal, compile_and_run, [''])
 test('T17104', normal, compile_and_run, [''])
+test('T18627', normal, compile_and_run, ['-O'])  # Optimisation shows up the bug