Fix infinite loop when specializing instance heads (#723)

* Fix infinite loop when specializing instance heads

The bug can only be triggered from TH, hence why it went un-noticed for
so long.

* Add test for #679 and #710

1 files changed, 24 insertions, 0 deletions

diff --git a/html-test/src/Bug679.hs b/html-test/src/Bug679.hs
new file mode 100644
index 00000000..dba194c4
--- /dev/null
+++ b/html-test/src/Bug679.hs
@@ -0,0 +1,24 @@
+{-# LANGUAGE TemplateHaskell #-}
+
+module Bug679 where
+
+import Language.Haskell.TH
+
+data Bar a = Bar
+
+$(do
+   a <- newName "a"
+   
+   let classN = mkName "Foo"
+   let methodN = mkName "foo"
+
+   methodTy <- [t| $(varT a) -> $(varT a) |]
+   let cla = ClassD [] classN [PlainTV a] [] [SigD methodN methodTy]
+ 
+   -- Note that we are /reusing/ the same type variable 'a' as in the class
+   instanceHead <- [t| $(conT classN) (Bar $(varT a)) |]
+   idCall <- [e| id |]
+   let ins = InstanceD Nothing [] instanceHead [FunD methodN [Clause [] (NormalB idCall) []]]
+    
+   pure [cla,ins])
+