interfaces: fix mkMethod erasure (#12171)

We erase the call to `mkMethod` in `convertExpr` instead of pattern
matching against the binding. This fixes a bug when the method body
contained statements using typeclass dictionaries such as
`do [2] === [2]`.

CHANGELOG_BEGIN
CHANGELOG_END

compiler/damlc/daml-lf-conversion/src/DA/Daml/LFConversion.hs

@@ -558,7 +558,7 @@ convertModule envLfVersion envEnableScenarios envPkgMap envStablePackages envIsG
         envInterfaceMethodInstances = MS.fromListWith (++)
           [
             ( (mod, mkTypeCon [getOccText iface], mkTypeCon [getOccText tpl])
-            , [(methodName, body)]
+            , [(methodName, untick val)]
             )
           | (name, val) <- binds
           , TypeCon methodNewtype
@@ -569,15 +569,6 @@ convertModule envLfVersion envEnableScenarios envPkgMap envStablePackages envIsG
             ] <- [varType name]
           , NameIn DA_Internal_Desugar "Method" <- [methodNewtype]
           , Just mod <- [nameModule_maybe (getName iface)]
-          , Var (NameIn DA_Internal_Desugar "mkMethod")
-            `App` Type _tpl
-            `App` Type _iface
-            `App` Type _methodName
-            `App` Type _methodType
-            `App` _implDict
-            `App` _hasMethodDict
-            `App` body
-              <- [untick val]
           ]
         envChoiceData = MS.fromListWith (++)
             [ (mkTypeCon [getOccText tplTy], [ChoiceData ty v])
@@ -1220,6 +1211,9 @@ convertExpr env0 e = do
             pure $ ETmLam (v, TStruct fields) $ ERecCon tupleType $ zipWithFrom mkFieldProj (1 :: Int) fields
     go env (VarIn GHC_Types "primitive") (LType (isStrLitTy -> Just y) : LType t : args)
         = fmap (, args) $ convertPrim (envLfVersion env) (unpackFS y) <$> convertType env t
+    -- erase mkMethod calls and leave only the body.
+    go env (VarIn DA_Internal_Desugar "mkMethod") (LType _tpl : LType _iface : LType _methodName : LType _methodTy : LExpr _implDict : LExpr _hasMethodDic : LExpr body : args)
+        = go env body args
     go env (VarIn GHC_Types "primitiveInterface") (LType (isStrLitTy -> Just y) : LType t : args)
         = do
         ty <- convertType env t

compiler/damlc/tests/daml-test-files/Interface.daml

@@ -73,6 +73,7 @@ template Asset
             pure (toInterfaceContractId @Token cid)
 
       let noopImpl = \nothing -> do
+            [1] === [1] -- make sure `mkMethod` calls are properly erased in the presence of polymorphism.
             pure ()
 
 main = scenario do