From 615b20b177f3d20af0b569727c91994798bc4677 Mon Sep 17 00:00:00 2001
From: Mark Probst <mark.probst@gmail.com>
Date: Mon, 20 Jul 2026 12:22:18 -0400
Subject: [PATCH] test: expected-output fixtures for JSON inputs

The JSON fixture harness asserts round-trip output ~ input, with
unconditional leniency: extra null properties in the output are always
allowed, and allowMissingNull languages may drop nulls.  Correct
behaviors where the output legitimately differs from the input were
therefore untestable -- concretely, that Go's omitempty drops null
fields.

Port the JSON Schema fixtures' `.out.<feature>.json` convention to JSON
inputs: an input `foo.json` may come with an expected-output file
`foo.out.<key>.json`, which applies to a run when `<key>` is one of the
language's `features` or the name of a renderer option that the
particular run sets (via a pinned `quickTestRendererOptions` entry).
When it applies, the output must match strictly, without the null
leniency; when absent, behavior is unchanged.  `.out.` files are
excluded from test-input enumeration.

Use it to assert the omit-empty behavior from #2509: a pinned Go run of
priority/omit-empty.json with omit-empty enabled must drop the null
field, per omit-empty.out.omit-empty.json.  The input is in priority/
(not misc/) so it also runs under QUICKTEST in CI.

Salvaged from #2509; the test case is Adam724's.

Co-authored-by: Adam724 <alaspina6@gmail.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
---
 test/fixtures.ts                              | 33 ++++++++++++++++---
 test/inputs/json/priority/omit-empty.json     | 11 +++++++
 .../priority/omit-empty.out.omit-empty.json   | 10 ++++++
 test/languages.ts                             |  7 +++-
 test/utils.ts                                 |  6 +++-
 5 files changed, 61 insertions(+), 6 deletions(-)
 create mode 100644 test/inputs/json/priority/omit-empty.json
 create mode 100644 test/inputs/json/priority/omit-empty.out.omit-empty.json

diff --git a/test/fixtures.ts b/test/fixtures.ts
index dece181ef6..45f51eb937 100644
--- a/test/fixtures.ts
+++ b/test/fixtures.ts
@@ -359,14 +359,39 @@ class JSONFixture extends LanguageFixture {
             return 0;
         }
 
-        compareJsonFileToJson(
-            comparisonArgs(
+        // The analog of the JSON Schema fixture's `.out.<feature>.json`
+        // convention: a JSON input `foo.json` can come with an expected-output
+        // file `foo.out.<key>.json`, which applies when `<key>` is one of the
+        // language's `features`, or the name of a renderer option this
+        // particular run sets (via `quickTestRendererOptions`).  When it
+        // applies, the output must match it exactly, without the usual
+        // round-trip leniency for null properties.  This is how output that
+        // legitimately differs from the input — e.g. Go's `omitempty`
+        // dropping null fields — gets asserted.
+        let expectedFilename = filename;
+        let strict = false;
+        const expectedOutputKeys = [
+            ...this.language.features,
+            ...Object.keys(additionalRendererOptions),
+        ];
+        for (const key of expectedOutputKeys) {
+            const outFilename = filename.replace(/\.json$/, `.out.${key}.json`);
+            if (fs.existsSync(outFilename)) {
+                expectedFilename = outFilename;
+                strict = true;
+                break;
+            }
+        }
+
+        compareJsonFileToJson({
+            ...comparisonArgs(
                 this.language,
                 filename,
-                filename,
+                expectedFilename,
                 additionalRendererOptions,
             ),
-        );
+            strict,
+        });
 
         if (
             this.language.diffViaSchema &&
diff --git a/test/inputs/json/priority/omit-empty.json b/test/inputs/json/priority/omit-empty.json
new file mode 100644
index 0000000000..56425f9b78
--- /dev/null
+++ b/test/inputs/json/priority/omit-empty.json
@@ -0,0 +1,11 @@
+{
+    "results": [
+        {
+            "age": 52,
+            "name": null
+        },
+        {
+            "age": 27
+        }
+    ]
+}
diff --git a/test/inputs/json/priority/omit-empty.out.omit-empty.json b/test/inputs/json/priority/omit-empty.out.omit-empty.json
new file mode 100644
index 0000000000..809e3cfa02
--- /dev/null
+++ b/test/inputs/json/priority/omit-empty.out.omit-empty.json
@@ -0,0 +1,10 @@
+{
+    "results": [
+        {
+            "age": 52
+        },
+        {
+            "age": 27
+        }
+    ]
+}
diff --git a/test/languages.ts b/test/languages.ts
index 6bcfd43611..1aedcb4024 100644
--- a/test/languages.ts
+++ b/test/languages.ts
@@ -500,7 +500,12 @@ export const GoLanguage: Language = {
         "postman-collection.schema",
     ],
     rendererOptions: {},
-    quickTestRendererOptions: [],
+    quickTestRendererOptions: [
+        // Runs against the expected-output file
+        // `omit-empty.out.omit-empty.json`, which asserts that `omitempty`
+        // actually drops the null field.
+        ["omit-empty.json", { "omit-empty": "true" }],
+    ],
     sourceFiles: ["src/language/Golang/index.ts"],
 };
 
diff --git a/test/utils.ts b/test/utils.ts
index db98e3101b..3a6d1f4bc2 100644
--- a/test/utils.ts
+++ b/test/utils.ts
@@ -186,7 +186,11 @@ export async function inDir(dir: string, work: () => Promise<void>) {
 }
 
 export function testsInDir(dir: string, extension: string): string[] {
-    return shell.ls(`${dir}/*.${extension}`);
+    // Expected-output files (`foo.out.<key>.json`) accompany test inputs;
+    // they are never test inputs themselves.
+    return shell
+        .ls(`${dir}/*.${extension}`)
+        .filter((fn) => !/\.out\.[^./]+\.[^./]+$/.test(fn));
 }
 
 export interface Sample {