Testsuite: add regression test for missing class constraint

The following program is accepted by ghc-7.0 to ghc-7.10, but rejected
by ghc-6.12.3 and HEAD (and rightfully so):

    class Class1 a
    class Class1 a => Class2 a
    class Class2 a => Class3 a
    instance Class3 a => Class2 a

The last line is missing a `Class1 a` constraint. Add a regression test
for this (typechecker/should_fail/tcfail223).

Add similar missing class constraints to T7126 and T5751. I verified
that the these changes don't interfer with the intention of the tests
(they still result in a loop with ghc-7.4.1).

Reviewers: austin, simonpj, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D1078

5 files changed, 22 insertions, 2 deletions

diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index d1bf03b37e..fbbeddbbe0 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -246,6 +246,7 @@ test('tcfail219', normal, multimod_compile_fail, ['tcfail219.hsig', '-sig-of "Sh
 test('tcfail220', normal, multimod_compile_fail, ['tcfail220.hsig', '-sig-of "ShouldFail is base:Prelude"'])
 test('tcfail221', normal, multimod_compile_fail, ['tcfail221.hsig', '-sig-of "ShouldFail is base:Prelude"'])
 test('tcfail222', normal, multimod_compile_fail, ['tcfail222.hsig', '-sig-of "ShouldFail is base:Data.STRef"'])
+test('tcfail223', normal, compile_fail, [''])
 
 test('SilentParametersOverlapping', normal, compile, [''])
 test('FailDueToGivenOverlapping', normal, compile_fail, [''])
diff --git a/testsuite/tests/typecheck/should_fail/tcfail223.hs b/testsuite/tests/typecheck/should_fail/tcfail223.hs
new file mode 100644
index 0000000000..e5e0d5c8f8
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/tcfail223.hs
@@ -0,0 +1,10 @@
+{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
+module ShouldFail where
+
+class Class1 a
+class Class1 a => Class2 a
+class Class2 a => Class3 a
+
+-- This was wrongfully accepted by ghc-7.0 to ghc-7.10.
+-- It is missing a `Class1 a` constraint.
+instance Class3 a => Class2 a
diff --git a/testsuite/tests/typecheck/should_fail/tcfail223.stderr b/testsuite/tests/typecheck/should_fail/tcfail223.stderr
new file mode 100644
index 0000000000..e4a4fcda54
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/tcfail223.stderr
@@ -0,0 +1,9 @@
+
+tcfail223.hs:10:10: error:
+    Could not deduce (Class1 a)
+      arising from the superclasses of an instance declaration
+    from the context: Class3 a
+      bound by the instance declaration at tcfail223.hs:10:10-29
+    Possible fix:
+      add (Class1 a) to the context of the instance declaration
+    In the instance declaration for ‘Class2 a’
diff --git a/testsuite/tests/typecheck/should_run/T5751.hs b/testsuite/tests/typecheck/should_run/T5751.hs
index 423a40736d..7c7d8ab0b9 100644
--- a/testsuite/tests/typecheck/should_run/T5751.hs
+++ b/testsuite/tests/typecheck/should_run/T5751.hs
@@ -25,7 +25,7 @@ main =
 class (Widgets x) => MonadRender x
 class (XMLGenerator m)  => Widgets m
 -- instance Widgets (IdentityT IO) -- if you uncomment this, it will work
-instance MonadRender m => Widgets m
+instance (XMLGenerator m, MonadRender m) => Widgets m
 instance MonadRender (IdentityT IO)
 
 web :: ( MonadIO m
diff --git a/testsuite/tests/typecheck/should_run/T7126.hs b/testsuite/tests/typecheck/should_run/T7126.hs
index ce9792de37..184d5df1f0 100644
--- a/testsuite/tests/typecheck/should_run/T7126.hs
+++ b/testsuite/tests/typecheck/should_run/T7126.hs
@@ -24,7 +24,7 @@ class Class2 a => Class3 a where
 instance Class1 Int where
   func1 = id
 
-instance Class3 a => Class2 a where
+instance (Class1 a, Class3 a) => Class2 a where
   func2 = func3
 
 instance Class3 Int where