Hmm.  Yes, this is an (actually harmless) assertion failure if your build has -DDEBUG on.  I think it’s just a fluke that it’s only just started failing.

I’ll make a ticket for it; I don’t think it’s super-urgent.

Simon

From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Simon Marlow
Sent: 09 November 2016 13:03
To: ghc-devs@haskell.org
Subject: T12041 failing?

I just saw the error below in a validate with -DDEBUG.  Anyone know about this?
--- /tmp/ghctest-uhJ8rt/test   spaces/./indexed-types/should_fail/T12041.run/T12041.stderr.normalised       2016-11-09 12:13:38.206501840 +0000
+++ /tmp/ghctest-uhJ8rt/test   spaces/./indexed-types/should_fail/T12041.run/T12041.comp.stderr.normalised  2016-11-09 12:13:38.206501840 +0000
@@ -1,7 +1,17 @@
+ghc: panic! (the 'impossible' happened)
+  (GHC version 8.1.20161109 for x86_64-unknown-linux):
+      ASSERT failed!
+  i_axb
+  Call stack:
+      CallStack (from HasCallStack):
+        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:<line>:<column> in <package-id>:Outputable
+        callStackDoc, called at compiler/utils/Outputable.hs:<line>:<column> in <package-id>:Outputable
+        assertPprPanic, called at compiler/typecheck/TcType.hs:<line>:<column> in <package-id>:TcType
+  Call stack:
+      CallStack (from HasCallStack):
+        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:<line>:<column> in <package-id>:Outputable
+        callStackDoc, called at compiler/utils/Outputable.hs:<line>:<column> in <package-id>:Outputable
+        pprPanic, called at compiler/utils/Outputable.hs:<line>:<column> in <package-id>:Outputable
+        assertPprPanic, called at compiler/typecheck/TcType.hs:<line>:<column> in <package-id>:TcType
 
-T12041.hs:12:15:
-     Expected kind ‘i -> Constraint’,
-        but ‘(~) Int’ has kind ‘* -> Constraint’
-     In the type ‘(~) Int’
-      In the type instance declaration for ‘Ob’
-      In the instance declaration for ‘Category I’
+Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
*** unexpected failure for T12041(normal)