[compiler] repro for dep merging edge case (non-hir)

Found when writing #31037, summary copied from comments:

This is an extreme edge case and not code we'd expect any reasonable developer to write. In most cases e.g. `(a?.b != null ? a.b : DEFAULT)`, we do want to take a dependency on `a?.b`.

I found this trying to come up with edge cases that break the current dependency + CFG merging logic. I think it makes sense to error on the side of correctness. After all, we still take `a` as a dependency if users write `a != null ? a.b : DEFAULT`, and the same fix (understanding the `<hoistable> != null` test expression) works for both. Can be convinced otherwise though!

ghstack-source-id: cc06afda59
Pull Request resolved: https://github.com/facebook/react/pull/31035

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/reduce-reactive-deps/bug-merge-uncond-optional-chain-and-cond.expect.md

@@ -0,0 +1,88 @@
+
+## Input
+
+```javascript
+import {identity} from 'shared-runtime';
+
+/**
+ * Evaluator failure:
+ * Found differences in evaluator results
+ *   Non-forget (expected):
+ *   (kind: ok) {}
+ *   [[ (exception in render) TypeError: Cannot read properties of null (reading 'title_text') ]]
+ *   Forget:
+ *   (kind: ok) {}
+ *   {}
+ */
+/**
+ * Very contrived text fixture showing that it's technically incorrect to merge
+ * a conditional dependency (e.g. dep.path in `cond ? dep.path : ...`) and an
+ * unconditionally evaluated optional chain (`dep?.path`).
+ *
+ *
+ * when screen is non-null, useFoo returns { title: null } or "(not null)"
+ * when screen is null, useFoo throws
+ */
+function useFoo({screen}: {screen: null | undefined | {title_text: null}}) {
+  return screen?.title_text != null
+    ? '(not null)'
+    : identity({title: screen.title_text});
+}
+export const FIXTURE_ENTRYPOINT = {
+  fn: useFoo,
+  params: [{screen: null}],
+  sequentialRenders: [{screen: {title_bar: undefined}}, {screen: null}],
+};
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime";
+import { identity } from "shared-runtime";
+
+/**
+ * Evaluator failure:
+ * Found differences in evaluator results
+ *   Non-forget (expected):
+ *   (kind: ok) {}
+ *   [[ (exception in render) TypeError: Cannot read properties of null (reading 'title_text') ]]
+ *   Forget:
+ *   (kind: ok) {}
+ *   {}
+ */
+/**
+ * Very contrived text fixture showing that it's technically incorrect to merge
+ * a conditional dependency (e.g. dep.path in `cond ? dep.path : ...`) and an
+ * unconditionally evaluated optional chain (`dep?.path`).
+ *
+ *
+ * when screen is non-null, useFoo returns { title: null } or "(not null)"
+ * when screen is null, useFoo throws
+ */
+function useFoo(t0) {
+  const $ = _c(2);
+  const { screen } = t0;
+  let t1;
+  if ($[0] !== screen?.title_text) {
+    t1 =
+      screen?.title_text != null
+        ? "(not null)"
+        : identity({ title: screen.title_text });
+    $[0] = screen?.title_text;
+    $[1] = t1;
+  } else {
+    t1 = $[1];
+  }
+  return t1;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: useFoo,
+  params: [{ screen: null }],
+  sequentialRenders: [{ screen: { title_bar: undefined } }, { screen: null }],
+};
+
+```
+      

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/reduce-reactive-deps/bug-merge-uncond-optional-chain-and-cond.ts

@@ -0,0 +1,31 @@
+import {identity} from 'shared-runtime';
+
+/**
+ * Evaluator failure:
+ * Found differences in evaluator results
+ *   Non-forget (expected):
+ *   (kind: ok) {}
+ *   [[ (exception in render) TypeError: Cannot read properties of null (reading 'title_text') ]]
+ *   Forget:
+ *   (kind: ok) {}
+ *   {}
+ */
+/**
+ * Very contrived text fixture showing that it's technically incorrect to merge
+ * a conditional dependency (e.g. dep.path in `cond ? dep.path : ...`) and an
+ * unconditionally evaluated optional chain (`dep?.path`).
+ *
+ *
+ * when screen is non-null, useFoo returns { title: null } or "(not null)"
+ * when screen is null, useFoo throws
+ */
+function useFoo({screen}: {screen: null | undefined | {title_text: null}}) {
+  return screen?.title_text != null
+    ? '(not null)'
+    : identity({title: screen.title_text});
+}
+export const FIXTURE_ENTRYPOINT = {
+  fn: useFoo,
+  params: [{screen: null}],
+  sequentialRenders: [{screen: {title_bar: undefined}}, {screen: null}],
+};

compiler/packages/snap/src/SproutTodoFilter.ts

@@ -478,6 +478,7 @@ const skipFilter = new Set([
   'fbt/bug-fbt-plural-multiple-function-calls',
   'fbt/bug-fbt-plural-multiple-mixed-call-tag',
   'bug-invalid-hoisting-functionexpr',
+  'reduce-reactive-deps/bug-merge-uncond-optional-chain-and-cond',
   'original-reactive-scopes-fork/bug-nonmutating-capture-in-unsplittable-memo-block',
   'original-reactive-scopes-fork/bug-hoisted-declaration-with-scope',
   'bug-codegen-inline-iife',