✅ Cover the discriminant-key popup

src/tagged-union.test.ts

@@ -398,3 +398,55 @@ test("autocomplete: a fallback makes the remaining tags optional", () => {
         assert.deepEqual([...result], ["a?", "b?", "c?"]);
     });
 });
+
+// `Mixed` has two common keys, but `id`'s value is not a tag, so only `kind`
+// may serve as the discriminant.
+const MIXED_SOURCE = `type Mixed = { id: Date; kind: "a"; a: number } | { id: Date; kind: "b"; b: number };`;
+
+const discriminantLabelsFor = ({
+    name,
+    factory,
+    typeName,
+    typeSource,
+}: {
+    readonly name: string;
+    readonly factory: "getTaggedUnionMatcher" | "getTaggedUnionMatcherW";
+    readonly typeName: string;
+    readonly typeSource: string;
+}): Promise<readonly string[]> => {
+    const session = new LspSession(REPO_ROOT);
+    const target: CompletionTarget = {
+        file: `src/__autocomplete_${name}.ts`,
+        source: [
+            `import { ${factory} } from "./index.ts";`,
+            typeSource,
+            `const m = ${factory}<${typeName}>()(/*COMPLETE*/);`,
+            "",
+        ].join("\n"),
+    };
+    return session
+        .completionLabelsAt(target)
+        .then((result) => result.labels)
+        .finally(() => session.close());
+};
+
+test("autocomplete: the discriminant key is offered at the key argument", () => {
+    // Arrange
+    const name = "tagged_union_discriminant";
+
+    // Act
+    const labels = discriminantLabelsFor({
+        name,
+        factory: "getTaggedUnionMatcher",
+        typeName: "Mixed",
+        typeSource: MIXED_SOURCE,
+    });
+
+    // Assert
+    return labels.then((result) => {
+        // The argument position also offers every global identifier, so assert
+        // inclusion of the discriminant and exclusion of the non-tag key.
+        assert.ok(result.includes('"kind"'));
+        assert.ok(!result.includes('"id"'));
+    });
+});