Initialise cec_suppress properly

In TcErrors, cec_suppress is used to suppress low-priority
errors in favour of truly insoluble ones.

But I was failing to initialise it correcly at top level, which
resulted in Trac #15539.  Easy to fix.

A few regression tests have fewer errors reported, but that seems to
be an improvement.

7 files changed, 28 insertions, 75 deletions

diff --git a/compiler/typecheck/TcErrors.hs b/compiler/typecheck/TcErrors.hs
index bc5b061a9e..7ef0754344 100644
--- a/compiler/typecheck/TcErrors.hs
+++ b/compiler/typecheck/TcErrors.hs
@@ -212,7 +212,12 @@ report_unsolved mb_binds_var type_errors expr_holes
                             , cec_expr_holes = expr_holes
                             , cec_type_holes = type_holes
                             , cec_out_of_scope_holes = out_of_scope_holes
-                            , cec_suppress = False -- See Note [Suppressing error messages]
+                            , cec_suppress = insolubleWC wanted
+                                 -- See Note [Suppressing error messages]
+                                 -- Suppress low-priority errors if there
+                                 -- are insolule errors anywhere;
+                                 -- See Trac #15539 and c.f. setting ic_status
+                                 -- in TcSimplify.setImplicationStatus
                             , cec_warn_redundant = warn_redundant
                             , cec_binds    = mb_binds_var }
 
diff --git a/testsuite/tests/ghci/scripts/Defer02.stderr b/testsuite/tests/ghci/scripts/Defer02.stderr
index e76727efa5..63dbc9b042 100644
--- a/testsuite/tests/ghci/scripts/Defer02.stderr
+++ b/testsuite/tests/ghci/scripts/Defer02.stderr
@@ -12,11 +12,6 @@ Defer01.hs:14:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
     • In the expression: 'p'
       In an equation for ‘a’: a = 'p'
 
-Defer01.hs:18:7: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • No instance for (Eq B) arising from a use of ‘==’
-    • In the expression: x == x
-      In an equation for ‘b’: b x = x == x
-
 Defer01.hs:25:1: warning: [-Woverlapping-patterns (in -Wdefault)]
     Pattern match has inaccessible right hand side
     In an equation for ‘c’: c (C2 x) = ...
@@ -29,12 +24,6 @@ Defer01.hs:25:4: warning: [-Winaccessible-code (in -Wdefault)]
     • In the pattern: C2 x
       In an equation for ‘c’: c (C2 x) = True
 
-Defer01.hs:28:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • No instance for (Num (a -> a)) arising from the literal ‘1’
-        (maybe you haven't applied a function to enough arguments?)
-    • In the expression: 1
-      In an equation for ‘d’: d = 1
-
 Defer01.hs:31:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
     • Couldn't match expected type ‘Char -> t’ with actual type ‘Char’
     • The function ‘e’ is applied to one argument,
@@ -65,26 +54,6 @@ Defer01.hs:39:17: warning: [-Wdeferred-type-errors (in -Wdefault)]
         a :: a (bound at Defer01.hs:39:3)
         i :: a -> () (bound at Defer01.hs:39:1)
 
-Defer01.hs:43:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • No instance for (MyClass a1) arising from a use of ‘myOp’
-    • In the expression: myOp 23
-      In an equation for ‘j’: j = myOp 23
-
-Defer01.hs:43:10: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • Ambiguous type variable ‘a1’ arising from the literal ‘23’
-      prevents the constraint ‘(Num a1)’ from being solved.
-      Probable fix: use a type annotation to specify what ‘a1’ should be.
-      These potential instances exist:
-        instance Num Integer -- Defined in ‘GHC.Num’
-        instance Num Double -- Defined in ‘GHC.Float’
-        instance Num Float -- Defined in ‘GHC.Float’
-        ...plus two others
-        ...plus one instance involving out-of-scope types
-        (use -fprint-potential-instances to see them all)
-    • In the first argument of ‘myOp’, namely ‘23’
-      In the expression: myOp 23
-      In an equation for ‘j’: j = myOp 23
-
 Defer01.hs:47:7: warning: [-Wdeferred-type-errors (in -Wdefault)]
     • Couldn't match expected type ‘Bool’ with actual type ‘Int’
     • In the expression: x
diff --git a/testsuite/tests/rename/should_fail/T15539.hs b/testsuite/tests/rename/should_fail/T15539.hs
new file mode 100644
index 0000000000..d1b5a37e7b
--- /dev/null
+++ b/testsuite/tests/rename/should_fail/T15539.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE RankNTypes #-}
+module T15539 where
+
+foo :: String
+foo = show a
+  where a = baz
+
+-- We get top level constraints
+--     Show a
+--     forall . <not in scope baz>
+-- We want the insoluble non-in-scope error to suppress
+-- the Show a, just as it does if the whole things is nested
+
+bar :: Int
+bar = 1
+
+bam = putStrLn foo
diff --git a/testsuite/tests/rename/should_fail/T15539.stderr b/testsuite/tests/rename/should_fail/T15539.stderr
new file mode 100644
index 0000000000..c3de7780d4
--- /dev/null
+++ b/testsuite/tests/rename/should_fail/T15539.stderr
@@ -0,0 +1,4 @@
+
+T15539.hs:6:13: error:
+    • Variable not in scope: baz
+    • Perhaps you meant one of these: ‘bar’ (line 15), ‘bam’ (line 17)
diff --git a/testsuite/tests/rename/should_fail/all.T b/testsuite/tests/rename/should_fail/all.T
index f2bf30ea3f..2eef29f1cc 100644
--- a/testsuite/tests/rename/should_fail/all.T
+++ b/testsuite/tests/rename/should_fail/all.T
@@ -132,3 +132,4 @@ test('T13847', normal, multimod_compile_fail, ['T13847','-v0'])
 test('T14307', normal, compile_fail, [''])
 test('T14591', normal, compile_fail, [''])
 test('T15214', normal, compile_fail, [''])
+test('T15539', normal, compile_fail, [''])
diff --git a/testsuite/tests/typecheck/should_compile/valid_hole_fits.stderr b/testsuite/tests/typecheck/should_compile/valid_hole_fits.stderr
index 6dde6ea5cf..93de053f54 100644
--- a/testsuite/tests/typecheck/should_compile/valid_hole_fits.stderr
+++ b/testsuite/tests/typecheck/should_compile/valid_hole_fits.stderr
@@ -88,21 +88,6 @@ valid_hole_fits.hs:27:5: warning: [-Wtyped-holes (in -Wdefault)]
           (imported from ‘Prelude’ at valid_hole_fits.hs:3:1-40
            (and originally defined in ‘GHC.Base’))
 
-valid_hole_fits.hs:30:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • Ambiguous type variable ‘a1’ arising from a use of ‘show’
-      prevents the constraint ‘(Show a1)’ from being solved.
-      Probable fix: use a type annotation to specify what ‘a1’ should be.
-      These potential instances exist:
-        instance (Show a, Show b) => Show (Either a b)
-          -- Defined in ‘Data.Either’
-        instance Show Ordering -- Defined in ‘GHC.Show’
-        instance Show Integer -- Defined in ‘GHC.Show’
-        ...plus 23 others
-        ...plus 70 instances involving out-of-scope types
-        (use -fprint-potential-instances to see them all)
-    • In the expression: show _
-      In an equation for ‘f’: f = show _
-
 valid_hole_fits.hs:30:10: warning: [-Wtyped-holes (in -Wdefault)]
     • Found hole: _ :: a1
       Where: ‘a1’ is an ambiguous type variable
@@ -138,21 +123,6 @@ valid_hole_fits.hs:30:10: warning: [-Wtyped-holes (in -Wdefault)]
           (imported from ‘Prelude’ at valid_hole_fits.hs:3:1-40
            (and originally defined in ‘GHC.Float’))
 
-valid_hole_fits.hs:34:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
-    • Ambiguous type variable ‘a0’ arising from a use of ‘show’
-      prevents the constraint ‘(Show a0)’ from being solved.
-      Probable fix: use a type annotation to specify what ‘a0’ should be.
-      These potential instances exist:
-        instance (Show a, Show b) => Show (Either a b)
-          -- Defined in ‘Data.Either’
-        instance Show Ordering -- Defined in ‘GHC.Show’
-        instance Show Integer -- Defined in ‘GHC.Show’
-        ...plus 23 others
-        ...plus 70 instances involving out-of-scope types
-        (use -fprint-potential-instances to see them all)
-    • In the expression: show (_ (_ :: Bool))
-      In an equation for ‘h’: h = show (_ (_ :: Bool))
-
 valid_hole_fits.hs:34:11: warning: [-Wtyped-holes (in -Wdefault)]
     • Found hole: _ :: Bool -> a0
       Where: ‘a0’ is an ambiguous type variable
diff --git a/testsuite/tests/typecheck/should_fail/tcfail078.stderr b/testsuite/tests/typecheck/should_fail/tcfail078.stderr
index a0816a746c..014d589bf6 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail078.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail078.stderr
@@ -2,16 +2,3 @@
 tcfail078.hs:5:6: error:
     • Expected kind ‘* -> Constraint’, but ‘Integer’ has kind ‘*’
     • In the type signature: f :: Integer i => i
-
-tcfail078.hs:6:19: error:
-    • Could not deduce (Num i) arising from the literal ‘0’
-      from the context: Integer i
-        bound by the type signature for:
-                   f :: forall i. Integer i => i
-        at tcfail078.hs:5:1-19
-      Possible fix:
-        add (Num i) to the context of
-          the type signature for:
-            f :: forall i. Integer i => i
-    • In the expression: 0
-      In an equation for ‘f’: f = 0