Print alias's underlying structures if OIOP vars are material

crates/compiler/solve/tests/solve_expr.rs

@@ -8167,4 +8167,33 @@ mod solve_expr {
             @"walkHelp {} : [Break [], Continue {}]"
         );
     }
+
+    #[test]
+    fn contextual_openness_for_type_alias() {
+        infer_queries!(
+            indoc!(
+                r#"
+                app "test" provides [accum] to "./platform"
+
+                Q : [Green, Blue]
+
+                f : Q -> Q
+                f = \q -> when q is
+                #^{-1}
+                    Green -> Green
+                    Blue -> Blue
+
+                accum = \q -> when q is
+                #^^^^^{-1}
+                    A -> f Green
+                    B -> Yellow
+                    C -> Orange
+                "#
+            ),
+        @r###"
+        f : Q -[[f(2)]]-> Q
+        accum : [A, B, C] -[[accum(0)]]-> [Blue, Green, Orange, Yellow]*
+        "###
+        );
+    }
 }

crates/compiler/types/src/pretty_print.rs

@@ -757,10 +757,16 @@ fn write_content<'a>(
                     pol,
                 ),
 
-                _ if env.debug.print_only_under_alias => write_parens!(write_parens, buf, {
-                    let content = subs.get_content_without_compacting(*actual);
-                    write_content(env, ctx, content, subs, buf, parens, pol)
-                }),
+                _ if env.debug.print_only_under_alias
+                    // If any infer-open-in-output-position extension variable is now material, we
+                    // cannot keep the alias as-is - we have to print its underlying type!
+                    || args.any_infer_ext_var_is_material(subs) =>
+                {
+                    write_parens!(write_parens, buf, {
+                        let content = subs.get_content_without_compacting(*actual);
+                        write_content(env, ctx, content, subs, buf, parens, pol)
+                    })
+                }
 
                 _ => write_parens!(write_parens, buf, {
                     write_symbol(env, *symbol, buf);

crates/compiler/types/src/subs.rs

@@ -2470,6 +2470,16 @@ impl AliasVariables {
             all_variables_len,
         }
     }
+
+    /// Checks whether any inferred ext var in this alias has been resolved to a material type.
+    pub fn any_infer_ext_var_is_material(&self, subs: &Subs) -> bool {
+        subs.get_subs_slice(self.infer_ext_in_output_variables())
+            .iter()
+            .any(|v| match subs.get_content_unchecked(*v) {
+                Content::FlexVar(None) | Content::Structure(FlatType::EmptyTagUnion) => false,
+                _ => true,
+            })
+    }
 }
 
 impl IntoIterator for AliasVariables {