ASSERT failed in compiler/typecheck/TcErrors.hs

One of the commits in the range https://github.com/ghc/ghc/compare/a6675a93efe7...de4723fd6d97 which were commited by SPJ, broke running the test suite with a -DDEBUG compiler:

+++ /tmp/ghctest-sw9832/test   spaces/./indexed-types/should_fail/T2627b.run/T2627b.comp.stderr.normalised	2017-03-27 16:28:10.448037112 +0000
@@ -1,13 +1,17 @@
+ghc: panic! (the 'impossible' happened)
+  (GHC version 8.3.20170327 for x86_64-unknown-linux):
+	ASSERT failed!
+  b0_a1gy[tau:4]
+  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/TcErrors.hs:<line>:<column> in <package-id>:TcErrors
+  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/TcErrors.hs:<line>:<column> in <package-id>:TcErrors
 
-T2627b.hs:20:24:
-     Couldn't match type ‘b0’ with ‘Dual (Dual b0)’
-        arising from a use of ‘conn’
-        ‘b0’ is untouchable
-          inside the constraints: b ~ W e f
-          bound by a pattern with constructor:
-                     Wr :: forall e f. e -> Comm f -> Comm (W e f),
-                   in an equation for ‘conn’
-          at T2627b.hs:20:14-19
-     In the expression: conn undefined undefined
-      In an equation for ‘conn’:
-          conn (Rd k) (Wr a r) = conn undefined undefined
+Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

See https://s3.amazonaws.com/archive.travis-ci.org/jobs/215578687/log.txt for a full log.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonpj
Operating system
Architecture