Constraint simplification loop now depends on `ExpansionFuel`

instead of a boolean flag for `CDictCan.cc_pend_sc`.
Pending givens get a fuel of 3 while Wanted and quantified constraints get a fuel of 1.
This helps pending given constraints to keep up with pending wanted constraints in case of
`UndecidableSuperClasses` and superclass expansions while simplifying the infered type.

Adds 3 dynamic flags for controlling the fuels for each type of constraints
`-fgivens-expansion-fuel` for givens `-fwanteds-expansion-fuel` for wanteds and `-fqcs-expansion-fuel` for quantified constraints

Fixes #21909
Added Tests T21909, T21909b
Added Note [Expanding Recursive Superclasses and ExpansionFuel]

3 files changed, 38 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T21909.hs b/testsuite/tests/typecheck/should_compile/T21909.hs
new file mode 100644
index 0000000000..2485134703
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T21909.hs
@@ -0,0 +1,24 @@
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE UndecidableSuperClasses #-}
+
+module T21909 where
+
+import Data.Kind
+
+class (Monad m, MyMonad (Inner m)) => MyMonad m where
+  type Inner m :: Type -> Type
+  foo :: m Int
+
+works :: MyMonad m => m String
+works = show <$> ((+ 1) <$> foo)
+
+fails :: MyMonad m => m String
+fails = show <$> fooPlusOne
+  where
+    fooPlusOne = (+ 1) <$> foo
+
+alsoFails :: MyMonad m => m String
+alsoFails =
+  let fooPlusOne = (+ 1) <$> foo
+   in show <$> fooPlusOne
diff --git a/testsuite/tests/typecheck/should_compile/T21909b.hs b/testsuite/tests/typecheck/should_compile/T21909b.hs
new file mode 100644
index 0000000000..3e02ae45f5
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T21909b.hs
@@ -0,0 +1,12 @@
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE UndecidableSuperClasses, FunctionalDependencies, MultiParamTypeClasses, GADTs #-}
+
+module T21909b where
+
+class C [a] => C a where
+  foo :: a -> Int
+
+bar :: C a => a -> Int
+bar x = foolocal x
+  where
+    foolocal a = foo a
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index 2c6debc98f..327dd93675 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -864,3 +864,5 @@ test('T22924', normal, compile, [''])
 test('T22985a', normal, compile, ['-O'])
 test('T22985b', normal, compile, [''])
 test('T23018', normal, compile, [''])
+test('T21909', normal, compile, [''])
+test('T21909b', normal, compile, [''])
\ No newline at end of file