Fix possible loop in auto implicit search

A local variable can't be applied to itself when searching (otherwise,
for example, we could end up trying something like id id id id id id etc
forever). So remove it from the environment before searching for its
arguments.

This and the previous patch fix #24. (Or, at least, the minimised cases
reported as part of it!)

src/Core/AutoSearch.idr

@@ -217,6 +217,17 @@ searchLocalWith {vars} fc rigc defaults trying depth def top env ((prf, ty) :: r
              findPos defs prf id nty target)
          (searchLocalWith fc rigc defaults trying depth def top env rest target)
   where
+    clearEnvType : {idx : Nat} -> .(IsVar name idx vs) -> 
+                   FC -> Env Term vs -> Env Term vs
+    clearEnvType First fc (b :: env) 
+        = Lam (multiplicity b) Explicit (Erased fc) :: env
+    clearEnvType (Later p) fc (b :: env) = b :: clearEnvType p fc env
+
+    clearEnv : Term vars -> Env Term vars -> Env Term vars
+    clearEnv (Local fc _ idx p) env
+        = clearEnvType p fc env
+    clearEnv _ env = env
+
     findDirect : Defs -> Term vars -> 
                  (Term vars -> Term vars) ->
                  NF vars ->  -- local's type
@@ -225,6 +236,7 @@ searchLocalWith {vars} fc rigc defaults trying depth def top env ((prf, ty) :: r
     findDirect defs prf f ty target
         = do (args, appTy) <- mkArgs fc rigc env ty
              logNF 10 "Trying" env ty
+             logNF 10 "For target" env target
              ures <- unify InTerm fc env target appTy
              let [] = constraints ures
                  | _ => throw (CantSolveGoal fc [] top)
@@ -233,9 +245,12 @@ searchLocalWith {vars} fc rigc defaults trying depth def top env ((prf, ty) :: r
                 then do
                    let candidate = apply fc (f prf) (map metaApp args)
                    logTermNF 10 "Local var candidate " env candidate
+                   -- Clear the local from the environment to avoid getting
+                   -- into a loop repeatedly applying it
+                   let env' = clearEnv prf env
                    -- Work right to left, because later arguments may solve
                    -- earlier ones by unification
-                   traverse (searchIfHole fc defaults trying False depth def top env) 
+                   traverse (searchIfHole fc defaults trying False depth def top env') 
                             (reverse args)
                    pure candidate
                 else do logNF 10 "Can't use " env ty

src/TTImp/ProcessDef.idr

@@ -222,7 +222,8 @@ checkLHS {vars} mult hashit n opts nest env fc lhs_in
          autoImplicits autoimp
          lhs <- implicitsAs defs vars lhs_bound
 
-         log 5 $ "Checking " ++ show lhs
+         log 5 $ "Checking LHS of " ++ show !(getFullName (Resolved n)) ++
+                 " " ++ show lhs
          logEnv 5 "In env" env
          (lhstm, lhstyg) <- 
              wrapError (InLHS fc !(getFullName (Resolved n))) $

tests/Main.idr

@@ -37,7 +37,7 @@ idrisTests
        "interactive005", "interactive006", "interactive007", "interactive008",
        "interactive009", "interactive010", "interactive011", "interactive012",
        "interface001", "interface002", "interface003", "interface004",
-       "interface005", "interface006",
+       "interface005", "interface006", "interface007",
        "lazy001",
        "linear001", "linear002", "linear003", "linear004", "linear005",
        "linear006", "linear007",

tests/idris2/interface007/A.idr

@@ -0,0 +1,7 @@
+module A
+
+-- Check that this doesn't go into a loop when resolving Show. because
+-- f itself is a candidate when elaborating the top level f function!
+public export
+interface F (p : Type -> Type) where
+  f : Show a => p a -> a

tests/idris2/interface007/expected

@@ -0,0 +1 @@
+1/1: Building A (A.idr)

tests/idris2/interface007/run

@@ -0,0 +1,3 @@
+$1 --check A.idr
+
+rm -rf build