From fc1617bf17d8443a09d81c7d307d492b01bf0abd Mon Sep 17 00:00:00 2001
From: Ayaz Hafiz <ayaz@rwx.com>
Date: Tue, 5 Jul 2022 19:07:41 -0400
Subject: [PATCH] Phantom types

Closes #3314
---
 crates/compiler/can/src/def.rs            | 29 +++++++---
 crates/compiler/solve/tests/solve_expr.rs | 51 +++++++++++++++++
 crates/reporting/tests/test_reporting.rs  | 67 +++++++++++++++++++++++
 3 files changed, 139 insertions(+), 8 deletions(-)

diff --git a/crates/compiler/can/src/def.rs b/crates/compiler/can/src/def.rs
index 6463029cc8..91d78f824f 100644
--- a/crates/compiler/can/src/def.rs
+++ b/crates/compiler/can/src/def.rs
@@ -351,15 +351,28 @@ fn canonicalize_alias<'a>(
                     region: loc_lowercase.region,
                 });
             }
-            None => {
-                is_phantom = true;
+            None => match kind {
+                AliasKind::Structural => {
+                    is_phantom = true;
 
-                env.problems.push(Problem::PhantomTypeArgument {
-                    typ: symbol,
-                    variable_region: loc_lowercase.region,
-                    variable_name: loc_lowercase.value.clone(),
-                });
-            }
+                    env.problems.push(Problem::PhantomTypeArgument {
+                        typ: symbol,
+                        variable_region: loc_lowercase.region,
+                        variable_name: loc_lowercase.value.clone(),
+                    });
+                }
+                AliasKind::Opaque => {
+                    // Opaques can have phantom types.
+                    can_vars.push(Loc {
+                        value: AliasVar {
+                            name: loc_lowercase.value.clone(),
+                            var: var_store.fresh(),
+                            opt_bound_ability: None,
+                        },
+                        region: loc_lowercase.region,
+                    });
+                }
+            },
         }
     }
 
diff --git a/crates/compiler/solve/tests/solve_expr.rs b/crates/compiler/solve/tests/solve_expr.rs
index 0f00ca6f47..cfbd38859e 100644
--- a/crates/compiler/solve/tests/solve_expr.rs
+++ b/crates/compiler/solve/tests/solve_expr.rs
@@ -7050,4 +7050,55 @@ mod solve_expr {
             &["fun : {} -[[thunk(5) [A Str]*, thunk(5) { a : Str }]]-> Str",]
         );
     }
+
+    #[test]
+    fn check_phantom_type() {
+        infer_eq_without_problem(
+            indoc!(
+                r#"
+                F a b := b
+
+                foo : F Str Str -> F U8 Str
+
+                x : F Str Str
+
+                foo x
+                "#
+            ),
+            "F U8 Str",
+        );
+    }
+
+    #[test]
+    fn infer_phantom_type_flow() {
+        infer_eq_without_problem(
+            indoc!(
+                r#"
+                F a b := b
+
+                foo : _ -> F U8 Str
+                foo = \it -> it
+
+                foo
+                "#
+            ),
+            "F U8 Str -> F U8 Str",
+        );
+    }
+
+    #[test]
+    fn infer_unbound_phantom_type_star() {
+        infer_eq_without_problem(
+            indoc!(
+                r#"
+                F a b := b
+
+                foo = \@F {} -> @F ""
+
+                foo
+                "#
+            ),
+            "F * {}* -> F * Str",
+        );
+    }
 }
diff --git a/crates/reporting/tests/test_reporting.rs b/crates/reporting/tests/test_reporting.rs
index af5c751023..048ad7aae4 100644
--- a/crates/reporting/tests/test_reporting.rs
+++ b/crates/reporting/tests/test_reporting.rs
@@ -9369,4 +9369,71 @@ All branches in an `if` must have the same type!
             a -> Str
         "###
     );
+
+    test_report!(
+        same_phantom_types_unify,
+        indoc!(
+            r#"
+            F a b := b
+
+            foo : F Str Str -> {}
+
+            x : F Str Str
+
+            foo x
+            "#
+        ),
+        @r"" // okay
+    );
+
+    test_report!(
+        different_phantom_types,
+        indoc!(
+            r#"
+            F a b := b
+
+            foo : F Str Str -> {}
+
+            x : F U8 Str
+
+            foo x
+            "#
+        ),
+        @r###"
+    ── TYPE MISMATCH ───────────────────────────────────────── /code/proj/Main.roc ─
+
+    The 1st argument to `foo` is not what I expect:
+
+    10│      foo x
+                 ^
+
+    This `x` value is a:
+
+        F U8 Str
+
+    But `foo` needs the 1st argument to be:
+
+        F Str Str
+    "###
+    );
+
+    test_report!(
+        #[ignore = "TODO This should be a type error"]
+        phantom_type_bound_to_ability_not_implementing,
+        indoc!(
+            r#"
+            app "test" provides [x] to "./platform"
+
+            Foo has foo : a -> a | a has Foo
+
+            F a b := b | a has Foo
+
+            Hash := {}
+
+            x : F Hash {}
+            "#
+        ),
+        @r###"
+        "###
+    );
 }