PropagateScopeDeps treats switch w only default as unconditional

If we have a switch with only a default case, then that code will be executed 
unconditionally. PropagateScopeDeps can take advantage of this to record 
dependencies in these cases as unconditional, which avoids the issue seen in the 
previous PR.

compiler/packages/babel-plugin-react-forget/src/ReactiveScopes/PropagateScopeDependencies.ts

@@ -732,6 +732,15 @@ class PropagationVisitor extends ReactiveFunctionVisitor<Context> {
       }
       case "switch": {
         context.visitOperand(terminal.test);
+        const isDefaultOnly =
+          terminal.cases.length === 1 && terminal.cases[0].test == null;
+        if (isDefaultOnly) {
+          const case_ = terminal.cases[0];
+          if (case_.block != null) {
+            this.visitBlock(case_.block, context);
+            break;
+          }
+        }
         const depsInCases = [];
         let foundDefault = false;
         /*

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.todo-switch-with-only-default.expect.md

@@ -1,21 +0,0 @@
-
-## Input
-
-```javascript
-function Component({ kind, ...props }) {
-  switch (kind) {
-    default:
-      return <Stringify {...props} />;
-  }
-}
-
-```
-
-
-## Error
-
-```
-[ReactForget] Invariant: Expected trees to be at least 2 elements long.
-```
-          
-      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.todo-switch-with-only-default.js

@@ -1,6 +0,0 @@
-function Component({ kind, ...props }) {
-  switch (kind) {
-    default:
-      return <Stringify {...props} />;
-  }
-}

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/switch-with-only-default.expect.md

@@ -0,0 +1,63 @@
+
+## Input
+
+```javascript
+import { Stringify } from "shared-runtime";
+
+function Component({ kind, ...props }) {
+  switch (kind) {
+    default:
+      return <Stringify {...props} />;
+  }
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ kind: "foo", a: 1, b: true, c: "sathya" }],
+};
+
+```
+
+## Code
+
+```javascript
+import { unstable_useMemoCache as useMemoCache } from "react";
+import { Stringify } from "shared-runtime";
+
+function Component(t0) {
+  const $ = useMemoCache(5);
+  let kind;
+  let props;
+  if ($[0] !== t0) {
+    ({ kind, ...props } = t0);
+    $[0] = t0;
+    $[1] = kind;
+    $[2] = props;
+  } else {
+    kind = $[1];
+    props = $[2];
+  }
+  switch (kind) {
+    default: {
+      let t1;
+      if ($[3] !== props) {
+        t1 = <Stringify {...props} />;
+        $[3] = props;
+        $[4] = t1;
+      } else {
+        t1 = $[4];
+      }
+      return t1;
+    }
+  }
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ kind: "foo", a: 1, b: true, c: "sathya" }],
+};
+
+```
+      
+### Eval output
+(kind: ok) <div>{"a":1,"b":true,"c":"sathya"}</div>

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/switch-with-only-default.js

@@ -0,0 +1,13 @@
+import { Stringify } from "shared-runtime";
+
+function Component({ kind, ...props }) {
+  switch (kind) {
+    default:
+      return <Stringify {...props} />;
+  }
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ kind: "foo", a: 1, b: true, c: "sathya" }],
+};