Fix bug in unification of tag args in polymorphic contexts

crates/compiler/test_mono/generated/polymorphic_expression_unification.txt

@@ -0,0 +1,35 @@
+procedure Bool.11 (#Attr.2, #Attr.3):
+    let Bool.23 : Int1 = lowlevel Eq #Attr.2 #Attr.3;
+    ret Bool.23;
+
+procedure Str.3 (#Attr.2, #Attr.3):
+    let Str.266 : Str = lowlevel StrConcat #Attr.2 #Attr.3;
+    ret Str.266;
+
+procedure Test.2 (Test.7):
+    let Test.24 : Str = ".trace(\"";
+    let Test.26 : Str = "\")";
+    let Test.25 : Str = CallByName Str.3 Test.7 Test.26;
+    dec Test.26;
+    let Test.23 : Str = CallByName Str.3 Test.24 Test.25;
+    dec Test.25;
+    let Test.22 : [<r>C List *self, C Str] = TagId(1) Test.23;
+    let Test.21 : List [<r>C List *self, C Str] = Array [Test.22];
+    let Test.8 : [<r>C List *self, C Str] = TagId(0) Test.21;
+    let Test.20 : List [<r>C List *self, C Str] = Array [Test.8];
+    let Test.19 : [<r>C List *self, C Str] = TagId(0) Test.20;
+    ret Test.19;
+
+procedure Test.3 ():
+    let Test.18 : Str = "interface_header";
+    let Test.17 : [<r>C List *self, C Str] = CallByName Test.2 Test.18;
+    ret Test.17;
+
+procedure Test.0 ():
+    let Test.14 : [<r>C List *self, C Str] = CallByName Test.3;
+    let Test.16 : Str = "";
+    let Test.15 : [<r>C List *self, C Str] = TagId(1) Test.16;
+    let Test.13 : Int1 = CallByName Bool.11 Test.14 Test.15;
+    dec Test.15;
+    dec Test.14;
+    ret Test.13;

crates/compiler/test_mono/src/tests.rs

@@ -1925,6 +1925,28 @@ fn encode_derived_tag_one_field_string() {
     )
 }
 
+#[mono_test(no_check)]
+fn polymorphic_expression_unification() {
+    indoc!(
+        r#"
+        app "test" provides [main] to "./platform"
+
+        RenderTree : [
+            Text Str,
+            Indent (List RenderTree),
+        ]
+        parseFunction : Str -> RenderTree
+        parseFunction = \name ->
+            last = Indent [Text ".trace(\"\(name)\")" ]
+            Indent [last]
+
+        values = parseFunction "interface_header"
+
+        main = values == Text ""
+        "#
+    )
+}
+
 #[mono_test]
 fn encode_derived_tag_two_payloads_string() {
     indoc!(

crates/compiler/unify/src/unify.rs

@@ -2979,7 +2979,18 @@ fn unify_flat_type<M: MetaCollector>(
             let mut outcome = unify_zip_slices(env, pool, *l_args, *r_args, ctx.mode);
 
             if outcome.mismatches.is_empty() {
-                outcome.union(merge(env, ctx, Structure(Apply(*r_symbol, *r_args))));
+                let chosen_args = SubsSlice::reserve_into_subs(env.subs, l_args.len());
+                for ((store, var1), var2) in chosen_args
+                    .into_iter()
+                    .zip(l_args.into_iter())
+                    .zip(r_args.into_iter())
+                {
+                    let var1 = env.subs[var1];
+                    let var2 = env.subs[var2];
+                    env.subs[store] = choose_merged_var(env.subs, var1, var2);
+                }
+
+                outcome.union(merge(env, ctx, Structure(Apply(*r_symbol, chosen_args))));
             }
 
             outcome