From 31a694c978115df5d7f60920803625162cbf67f4 Mon Sep 17 00:00:00 2001
From: Sofia Faro <sofia.faro@digitalasset.com>
Date: Wed, 2 Sep 2020 12:40:33 +0100
Subject: [PATCH] Preserve empty variant constructor in data-deps.

This fixes #7207 and adds a regression test. In a
separate PR I'll add a warning for variants with
single argument of unit type and add a changelog
entry.

changelog_begin
changelog_end
---
 .../src/DA/Daml/Compiler/DataDependencies.hs  |  3 ++
 compiler/damlc/tests/src/DA/Test/Packaging.hs | 43 +++++++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/compiler/damlc/daml-compiler/src/DA/Daml/Compiler/DataDependencies.hs b/compiler/damlc/daml-compiler/src/DA/Daml/Compiler/DataDependencies.hs
index ced5b5408c26..2fd4db06b04c 100644
--- a/compiler/damlc/daml-compiler/src/DA/Daml/Compiler/DataDependencies.hs
+++ b/compiler/damlc/daml-compiler/src/DA/Daml/Compiler/DataDependencies.hs
@@ -479,6 +479,9 @@ generateSrcFromLf env = noLoc mod
 
         convConDetails :: LF.Type -> Gen (HsConDeclDetails GhcPs)
         convConDetails = \case
+            -- empty variant constructor (see issue #7207)
+            LF.TUnit ->
+                pure $ PrefixCon []
 
             -- variant record constructor
             LF.TConApp LF.Qualified{..} _
diff --git a/compiler/damlc/tests/src/DA/Test/Packaging.hs b/compiler/damlc/tests/src/DA/Test/Packaging.hs
index f256cf8bb858..c4b0449639b1 100644
--- a/compiler/damlc/tests/src/DA/Test/Packaging.hs
+++ b/compiler/damlc/tests/src/DA/Test/Packaging.hs
@@ -1667,6 +1667,49 @@ dataDependencyTests Tools{damlc,repl,validate,davlDar,oldProjDar} = testGroup "D
         withCurrentDirectory (tmpDir </> "proj") $
             callProcessSilent damlc ["build"]
 
+    , testCaseSteps "Empty variant constructors" $ \step -> withTempDir $ \tmpDir -> do
+        -- This test checks that variant constructors without argument
+        -- are preserved. This is a regression test for issue #7207.
+        step "building project with type definition"
+        createDirectoryIfMissing True (tmpDir </> "type")
+        writeFileUTF8 (tmpDir </> "type" </> "daml.yaml") $ unlines
+            [ "sdk-version: " <> sdkVersion
+            , "name: type"
+            , "source: ."
+            , "version: 0.1.0"
+            , "dependencies: [daml-prim, daml-stdlib]"
+            ]
+        writeFileUTF8 (tmpDir </> "type" </> "Foo.daml") $ unlines
+            [ "module Foo where"
+            , "data A = B | C Int"
+            ]
+        withCurrentDirectory (tmpDir </> "type") $
+            callProcessSilent damlc ["build", "-o", "type.dar"]
+
+        step "building project that uses it via data-dependencies"
+        createDirectoryIfMissing True (tmpDir </> "proj")
+        writeFileUTF8 (tmpDir </> "proj" </> "daml.yaml") $ unlines
+            [ "sdk-version: " <> sdkVersion
+            , "name: proj"
+            , "source: ."
+            , "version: 0.1.0"
+            , "dependencies: [daml-prim, daml-stdlib]"
+            , "data-dependencies: "
+            , "  - " <> (tmpDir </> "type" </> "type.dar")
+            ]
+        writeFileUTF8 (tmpDir </> "proj" </> "Main.daml") $ unlines
+            [ "module Main where"
+            , "import Foo"
+            , "mkA : A"
+            , "mkA = B"
+            , "matchA : A -> Int"
+            , "matchA a ="
+            , "  case a of"
+            , "    B -> 0"
+            , "    C n -> n"
+            ]
+        withCurrentDirectory (tmpDir </> "proj") $
+            callProcessSilent damlc ["build"]
     ]
 
 -- | Check that the given file exists in the dar in the given directory.