Fix type class name propagation issue

We weren't shadowing names correctly when a class was parameterised on a
single variable and one of the methods bound the same name. This almost
never happens, but leads to very odd error messages when it does.

Fixes #2026

idris.cabal

@@ -298,6 +298,9 @@ Extra-source-files:
                        test/reg059/run
                        test/reg059/*.idr
                        test/reg059/expected
+                       test/reg060/run
+                       test/reg060/*.idr
+                       test/reg060/expected
 
                        test/basic001/run
                        test/basic001/*.idr

src/Idris/Elab/Instance.hs

@@ -88,6 +88,7 @@ elabInstance info syn doc argDocs what fc cs n ps t expn ds = do
          -- where block
          wparams <- mapM (\p -> case p of
                                   PApp _ _ args -> getWParams (map getTm args)
+                                  a@(PRef fc f) -> getWParams [a]
                                   _ -> return []) ps
          let pnames = map pname (concat (nub wparams))
          let superclassInstances = map (substInstance ips pnames) (class_default_superclasses ci)
@@ -242,7 +243,9 @@ elabInstance info syn doc argDocs what fc cs n ps t expn ds = do
     decorate ns iname (UN n)        = NS (SN (MethodN (UN n))) ns
     decorate ns iname (NS (UN n) s) = NS (SN (MethodN (UN n))) ns
 
-    mkTyDecl (n, op, t, _) = PTy emptyDocstring [] syn fc op n t
+    mkTyDecl (n, op, t, _) 
+        = PTy emptyDocstring [] syn fc op n 
+               (mkUniqueNames [] t)
 
     conbind :: [(Name, PTerm)] -> PTerm -> PTerm
     conbind ((c,ty) : ns) x = PPi constraint c ty (conbind ns x)

test/reg060/reg060.idr

@@ -0,0 +1,12 @@
+
+class MyFunctor (f : Type -> Type) where
+  mymap : (m : a -> b) -> f a -> f b
+
+data Foo x y = Bar y
+
+instance MyFunctor (Foo m) where
+  mymap m x = ?wibble
+
+instance [foo] Functor m => MyFunctor m where
+  mymap m x = map m x
+

test/reg060/run

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+idris $@ reg060.idr --check
+rm -f *.ibc