From fdceedda005e8587b957dcb18a40e4d07a7c2f54 Mon Sep 17 00:00:00 2001
From: Richard Feldman <oss@rtfeldman.com>
Date: Sat, 18 Jul 2020 14:16:46 -0400
Subject: [PATCH] Fix unifying optional fields

---
 compiler/solve/tests/solve_expr.rs | 30 ++++++++++++++++++++++++++++++
 compiler/unify/src/unify.rs        | 15 +++++++++++++--
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/compiler/solve/tests/solve_expr.rs b/compiler/solve/tests/solve_expr.rs
index df9125ceee..76f8296003 100644
--- a/compiler/solve/tests/solve_expr.rs
+++ b/compiler/solve/tests/solve_expr.rs
@@ -2565,4 +2565,34 @@ mod solve_expr {
             "should fail",
         );
     }
+
+    // OPTIONAL RECORD FIELDS
+
+    #[test]
+    fn optional_field_unifies_with_missing() {
+        infer_eq_without_problem(
+            indoc!(
+                r#"
+                    negatePoint : { x : Int, y : Int, z ? Num c } -> { x : Int, y : Int, z : Num c }
+
+                    negatePoint { x: 1, y: 2 }
+                "#
+            ),
+            "{ x : Int, y : Int, z : Num c }",
+        );
+    }
+
+    #[test]
+    fn optional_field_unifies_with_present() {
+        infer_eq_without_problem(
+            indoc!(
+                r#"
+                    negatePoint : { x : Num a, y : Num b, z ? c } -> { x : Num a, y : Num b, z : c }
+
+                    negatePoint { x: 1, y: 2.1, z: 0x3 }
+                "#
+            ),
+            "{ x : Num a, y : Float, z : Int }",
+        );
+    }
 }
diff --git a/compiler/unify/src/unify.rs b/compiler/unify/src/unify.rs
index 7ff9edfd2d..78c84c1a1f 100644
--- a/compiler/unify/src/unify.rs
+++ b/compiler/unify/src/unify.rs
@@ -614,6 +614,17 @@ fn unify_shared_tags(
     }
 }
 
+fn has_no_required_fields<'a, I, T>(fields: &mut I) -> bool
+where
+    I: Iterator<Item = &'a RecordField<T>>,
+    T: 'a,
+{
+    fields.all(|field| match field {
+        RecordField::Required(_) => false,
+        RecordField::Optional(_) => true,
+    })
+}
+
 #[inline(always)]
 fn unify_flat_type(
     subs: &mut Subs,
@@ -627,11 +638,11 @@ fn unify_flat_type(
     match (left, right) {
         (EmptyRecord, EmptyRecord) => merge(subs, ctx, Structure(left.clone())),
 
-        (Record(fields, ext), EmptyRecord) if fields.is_empty() => {
+        (Record(fields, ext), EmptyRecord) if has_no_required_fields(&mut fields.values()) => {
             unify_pool(subs, pool, *ext, ctx.second)
         }
 
-        (EmptyRecord, Record(fields, ext)) if fields.is_empty() => {
+        (EmptyRecord, Record(fields, ext)) if has_no_required_fields(&mut fields.values()) => {
             unify_pool(subs, pool, ctx.first, *ext)
         }