Improve error message suppression

TcErrors has a system for suppressing some type errors if a more
serious one occurs.  But there was a crucial missing case, which
sometimes resulted in a cascade of irrelevant errors overwhelming
the actual cause. This was Trac #11541.

The fix is simple.  Worth merging to 8.0

6 files changed, 56 insertions, 104 deletions

diff --git a/compiler/typecheck/TcErrors.hs b/compiler/typecheck/TcErrors.hs
index 2cbf28d1b2..5f2c908849 100644
--- a/compiler/typecheck/TcErrors.hs
+++ b/compiler/typecheck/TcErrors.hs
@@ -304,14 +304,19 @@ reportImplic ctxt implic@(Implic { ic_skols = tvs, ic_given = given
                      , ic_info  = info' }
     ctxt' = ctxt { cec_tidy     = env1
                  , cec_encl     = implic' : cec_encl ctxt
-                 , cec_suppress = insoluble  -- Suppress inessential errors if there
-                                             -- are are insolubles anywhere in the
-                                             -- tree rooted here
+
+                 , cec_suppress = insoluble || cec_suppress ctxt
+                      -- Suppress inessential errors if there
+                      -- are are insolubles anywhere in the
+                      -- tree rooted here, or we've come across
+                      -- a suppress-worthy constraint higher up (Trac #11541)
+
                  , cec_binds    = cec_binds ctxt *> m_evb }
-                                  -- if cec_binds ctxt is Nothing, that means
-                                  -- we're reporting *all* errors. Don't change
-                                  -- that behavior just because we're going into
-                                  -- an implication.
+                      -- If cec_binds ctxt is Nothing, that means
+                      -- we're reporting *all* errors. Don't change
+                      -- that behavior just because we're going into
+                      -- an implication.
+
     dead_givens = case status of
                     IC_Solved { ics_dead = dead } -> dead
                     _                             -> []
diff --git a/testsuite/tests/rename/should_fail/RnStaticPointersFail02.stderr b/testsuite/tests/rename/should_fail/RnStaticPointersFail02.stderr
index e596a51fb3..b34f4354b3 100644
--- a/testsuite/tests/rename/should_fail/RnStaticPointersFail02.stderr
+++ b/testsuite/tests/rename/should_fail/RnStaticPointersFail02.stderr
@@ -1,9 +1,3 @@
-
-RnStaticPointersFail02.hs:5:5: error:
-    • No instance for (Data.Typeable.Internal.Typeable t0)
-        arising from a static form
-    • In the expression: static T
-      In an equation for ‘f’: f = static T
-
-RnStaticPointersFail02.hs:5:12: error:
-    Data constructor not in scope: T
+

+RnStaticPointersFail02.hs:5:12: error:

+    Data constructor not in scope: T

diff --git a/testsuite/tests/rename/should_fail/mc14.stderr b/testsuite/tests/rename/should_fail/mc14.stderr
index bc7b7df291..4182c8fa8c 100644
--- a/testsuite/tests/rename/should_fail/mc14.stderr
+++ b/testsuite/tests/rename/should_fail/mc14.stderr
@@ -1,16 +1,2 @@
-
-mc14.hs:14:16: error:
-    Ambiguous type variable ‘t0’ arising from a use of ‘fmap’
-    prevents the constraint ‘(Functor t0)’ from being solved.
-    Probable fix: use a type annotation to specify what ‘t0’ should be.
-    These potential instances exist:
-      instance Functor IO -- Defined in ‘GHC.Base’
-      instance Functor Maybe -- Defined in ‘GHC.Base’
-      instance Functor ((->) r) -- Defined in ‘GHC.Base’
-      ...plus two others
-      (use -fprint-potential-instances to see them all)
-    In the expression: fmap
-    In a stmt of a monad comprehension: then group using f
-    In the expression: [() | f <- functions, then group using f]
-
-mc14.hs:14:49: error: Variable not in scope: f :: [a] -> m (t0 a)
+

+mc14.hs:14:49: error: Variable not in scope: f :: [a] -> m (t0 a)

diff --git a/testsuite/tests/typecheck/should_compile/T11254.stderr b/testsuite/tests/typecheck/should_compile/T11254.stderr
index 25cd751aa7..692c72f18d 100644
--- a/testsuite/tests/typecheck/should_compile/T11254.stderr
+++ b/testsuite/tests/typecheck/should_compile/T11254.stderr
@@ -1,25 +1,15 @@
-
-T11254.hs:16:10: warning:
-    • Couldn't match type ‘Frac Int’ with ‘Int’
-        arising from the superclasses of an instance declaration
-    • In the instance declaration for ‘ID Rational’
-
-T11254.hs:16:10: warning:
-    • No instance for (Fractional Int)
-        arising from the superclasses of an instance declaration
-    • In the instance declaration for ‘ID Rational’
-
-T11254.hs:16:10: warning:
-    • No instance for (ID Int)
-        arising from the superclasses of an instance declaration
-    • In the instance declaration for ‘ID Rational’
-
-T11254.hs:18:12: warning:
-    • Couldn't match type ‘GHC.Real.Ratio Integer’ with ‘Int’
-      Expected type: Rational -> Frac Rational
-        Actual type: Rational -> Rational
-    • When checking that instance signature for ‘embed’
-        is more general than its signature in the class
-        Instance sig: Rational -> Rational
-           Class sig: Rational -> Frac Rational
-      In the instance declaration for ‘ID Rational’
+

+T11254.hs:16:10: warning:

+    • Couldn't match type ‘Frac Int’ with ‘Int’

+        arising from the superclasses of an instance declaration

+    • In the instance declaration for ‘ID Rational’

+

+T11254.hs:18:12: warning:

+    • Couldn't match type ‘GHC.Real.Ratio Integer’ with ‘Int’

+      Expected type: Rational -> Frac Rational

+        Actual type: Rational -> Rational

+    • When checking that instance signature for ‘embed’

+        is more general than its signature in the class

+        Instance sig: Rational -> Rational

+           Class sig: Rational -> Frac Rational

+      In the instance declaration for ‘ID Rational’

diff --git a/testsuite/tests/typecheck/should_fail/mc22.stderr b/testsuite/tests/typecheck/should_fail/mc22.stderr
index 5e369d7ffe..955ebe50fc 100644
--- a/testsuite/tests/typecheck/should_fail/mc22.stderr
+++ b/testsuite/tests/typecheck/should_fail/mc22.stderr
@@ -1,27 +1,15 @@
-
-mc22.hs:9:9: error:
-    • No instance for (Functor t) arising from a use of ‘fmap’
-      Possible fix:
-        add (Functor t) to the context of
-          a type expected by the context:
-            (a -> b) -> t a -> t b
-          or the inferred type of foo :: [t [Char]]
-    • In the expression: fmap
-      In a stmt of a monad comprehension: then group using take 5
-      In the expression:
-        [x + 1 | x <- ["Hello", "World"], then group using take 5]
-
-mc22.hs:10:26: error:
-    • Couldn't match type ‘a’ with ‘t a’
-      ‘a’ is a rigid type variable bound by
-        a type expected by the context:
-          forall a. [a] -> [t a]
-        at mc22.hs:9:9
-      Expected type: [a] -> [t a]
-        Actual type: [t a] -> [t a]
-    • In the expression: take 5
-      In a stmt of a monad comprehension: then group using take 5
-      In the expression:
-        [x + 1 | x <- ["Hello", "World"], then group using take 5]
-    • Relevant bindings include
-        foo :: [t [Char]] (bound at mc22.hs:8:1)
+

+mc22.hs:10:26: error:

+    • Couldn't match type ‘a’ with ‘t a’

+      ‘a’ is a rigid type variable bound by

+        a type expected by the context:

+          forall a. [a] -> [t a]

+        at mc22.hs:9:9

+      Expected type: [a] -> [t a]

+        Actual type: [t a] -> [t a]

+    • In the expression: take 5

+      In a stmt of a monad comprehension: then group using take 5

+      In the expression:

+        [x + 1 | x <- ["Hello", "World"], then group using take 5]

+    • Relevant bindings include

+        foo :: [t [Char]] (bound at mc22.hs:8:1)

diff --git a/testsuite/tests/typecheck/should_fail/mc25.stderr b/testsuite/tests/typecheck/should_fail/mc25.stderr
index 406f89e719..7fdb6ff74c 100644
--- a/testsuite/tests/typecheck/should_fail/mc25.stderr
+++ b/testsuite/tests/typecheck/should_fail/mc25.stderr
@@ -1,20 +1,9 @@
-
-mc25.hs:9:10: error:
-    • No instance for (Functor t1) arising from a use of ‘fmap’
-      Possible fix:
-        add (Functor t1) to the context of
-          a type expected by the context:
-            (a -> b) -> t1 a -> t1 b
-          or the inferred type of z :: [t1 t]
-    • In the expression: fmap
-      In a stmt of a monad comprehension: then group by x using take
-      In the expression: [x | x <- [1 .. 10], then group by x using take]
-
-mc25.hs:9:46: error:
-    • Couldn't match type ‘a -> t’ with ‘Int’
-      Expected type: (a -> t) -> [a] -> [t1 a]
-        Actual type: Int -> [t1 a] -> [t1 a]
-    • In the expression: take
-      In a stmt of a monad comprehension: then group by x using take
-      In the expression: [x | x <- [1 .. 10], then group by x using take]
-    • Relevant bindings include z :: [t1 t] (bound at mc25.hs:9:1)
+

+mc25.hs:9:46: error:

+    • Couldn't match type ‘a -> t’ with ‘Int’

+      Expected type: (a -> t) -> [a] -> [t1 a]

+        Actual type: Int -> [t1 a] -> [t1 a]

+    • In the expression: take

+      In a stmt of a monad comprehension: then group by x using take

+      In the expression: [x | x <- [1 .. 10], then group by x using take]

+    • Relevant bindings include z :: [t1 t] (bound at mc25.hs:9:1)