Use implicit return (undefined)

Making ReturnTerminal.value non-nullable broke our optimization to elide final 
value-less return statements. We now check if a return value is explicitly 
`undefined` and elide the value in this case, which then also propagates to 
allow removing the final `return` statement of a function if the value is 
missing.

compiler/forget/src/ReactiveScopes/CodegenReactiveFunction.ts

@@ -388,6 +388,10 @@ function codegenTerminal(
     }
     case "return": {
       const value = codegenPlace(cx, terminal.value);
+      if (value.type === "Identifier" && value.name === "undefined") {
+        // Use implicit undefined
+        return t.returnStatement();
+      }
       return t.returnStatement(value);
     }
     case "switch": {

compiler/forget/src/__tests__/fixtures/compiler/_bug.computed-call-evaluation-order.expect.md

@@ -26,7 +26,6 @@ function Component() {
 import * as React from "react"; // Should print A, B, arg, original
 function changeF(o) {
   o.f = () => console.log("new");
-  return undefined;
 }
 
 function Component() {

compiler/forget/src/__tests__/fixtures/compiler/_bug.property-call-evaluation-order.expect.md

@@ -26,7 +26,6 @@ import * as React from "react"; // Should print A, arg, original
 
 function changeF(o) {
   o.f = () => console.log("new");
-  return undefined;
 }
 
 function Component() {

compiler/forget/src/__tests__/fixtures/compiler/alias-while.expect.md

@@ -50,9 +50,7 @@ function foo(cond) {
   return a;
 }
 
-function mutate(x, y) {
-  return undefined;
-}
+function mutate(x, y) {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/call.expect.md

@@ -19,9 +19,7 @@ function Component(props) {
 
 ```javascript
 import * as React from "react";
-function foo() {
-  return undefined;
-}
+function foo() {}
 
 function Component(props) {
   const $ = React.unstable_useMemoCache(3);

compiler/forget/src/__tests__/fixtures/compiler/conditional-on-mutable.expect.md

@@ -105,12 +105,8 @@ function ComponentB(props) {
   return t0;
 }
 
-function Foo() {
-  return undefined;
-}
-function mayMutate() {
-  return undefined;
-}
+function Foo() {}
+function mayMutate() {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/constructor.expect.md

@@ -19,9 +19,7 @@ function Component(props) {
 
 ```javascript
 import * as React from "react";
-function Foo() {
-  return undefined;
-}
+function Foo() {}
 
 function Component(props) {
   const $ = React.unstable_useMemoCache(3);

compiler/forget/src/__tests__/fixtures/compiler/early-return.expect.md

@@ -18,9 +18,9 @@ function MyApp(props) {
 ```javascript
 function MyApp(props) {
   if (props.cond) {
-    return undefined;
+    return;
   } else {
-    return undefined;
+    return;
   }
 }
 

compiler/forget/src/__tests__/fixtures/compiler/for-return.expect.md

@@ -15,9 +15,8 @@ function Component(props) {
 ```javascript
 function Component(props) {
   for (const i = 0; 0 < props.count; ) {
-    return undefined;
+    return;
   }
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/frozen-after-alias.expect.md

@@ -35,12 +35,8 @@ function Component() {
   return b;
 }
 
-function useFreeze() {
-  return undefined;
-}
-function foo(x) {
-  return undefined;
-}
+function useFreeze() {}
+function foo(x) {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/hook-call.expect.md

@@ -23,12 +23,8 @@ function Component(props) {
 
 ```javascript
 import * as React from "react";
-function useFreeze() {
-  return undefined;
-}
-function foo() {
-  return undefined;
-}
+function useFreeze() {}
+function foo() {}
 
 function Component(props) {
   const $ = React.unstable_useMemoCache(3);

compiler/forget/src/__tests__/fixtures/compiler/hooks-freeze-arguments.expect.md

@@ -35,12 +35,8 @@ function Component() {
   return a;
 }
 
-function useFreeze(x) {
-  return undefined;
-}
-function call(x) {
-  return undefined;
-}
+function useFreeze(x) {}
+function call(x) {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/hooks-freeze-possibly-mutable-arguments.expect.md

@@ -50,12 +50,8 @@ function Component(props) {
   return a;
 }
 
-function useFreeze(x) {
-  return undefined;
-}
-function call(x) {
-  return undefined;
-}
+function useFreeze(x) {}
+function call(x) {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/independent-across-if.expect.md

@@ -37,18 +37,10 @@ function Component(props) {
 
 ```javascript
 import * as React from "react";
-function compute() {
-  return undefined;
-}
-function mutate() {
-  return undefined;
-}
-function foo() {
-  return undefined;
-}
-function Foo() {
-  return undefined;
-}
+function compute() {}
+function mutate() {}
+function foo() {}
+function Foo() {}
 
 /**
  * Should produce 3 scopes:

compiler/forget/src/__tests__/fixtures/compiler/independent.expect.md

@@ -74,15 +74,9 @@ function Component(props) {
   return t2;
 }
 
-function compute() {
-  return undefined;
-}
-function foo() {
-  return undefined;
-}
-function Foo() {
-  return undefined;
-}
+function compute() {}
+function foo() {}
+function Foo() {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/interdependent-across-if.expect.md

@@ -31,15 +31,9 @@ function Component(props) {
 
 ```javascript
 import * as React from "react";
-function compute() {
-  return undefined;
-}
-function foo() {
-  return undefined;
-}
-function Foo() {
-  return undefined;
-}
+function compute() {}
+function foo() {}
+function Foo() {}
 
 /**
  * Should produce 1 scope:

compiler/forget/src/__tests__/fixtures/compiler/interdependent.expect.md

@@ -69,15 +69,9 @@ function Component(props) {
   return t0;
 }
 
-function compute() {
-  return undefined;
-}
-function foo() {
-  return undefined;
-}
-function Foo() {
-  return undefined;
-}
+function compute() {}
+function foo() {}
+function Foo() {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/issue852.expect.md

@@ -17,7 +17,6 @@ function Component(c) {
 function Component(c) {
   const x = { c };
   mutate(x);
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/mutable-lifetime-loops.expect.md

@@ -42,12 +42,8 @@ function Component(props) {
 ## Code
 
 ```javascript
-function mutate(x, y) {
-  return undefined;
-}
-function cond(x) {
-  return undefined;
-}
+function mutate(x, y) {}
+function cond(x) {}
 
 function Component(props) {
   let a = {};
@@ -75,7 +71,6 @@ function Component(props) {
   }
 
   mutate(d, null);
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/mutable-lifetime-with-aliasing.expect.md

@@ -38,9 +38,7 @@ function Component(props) {
 ## Code
 
 ```javascript
-function mutate(x, y) {
-  return undefined;
-}
+function mutate(x, y) {}
 
 function Component(props) {
   const a = {};
@@ -63,7 +61,6 @@ function Component(props) {
   }
 
   mutate(x, null);
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/mutable-liverange-loop.expect.md

@@ -37,12 +37,8 @@ function Component(props) {
 ## Code
 
 ```javascript
-function mutate() {
-  return undefined;
-}
-function cond() {
-  return undefined;
-}
+function mutate() {}
+function cond() {}
 
 function Component(props) {
   const a = {};
@@ -65,7 +61,6 @@ function Component(props) {
   }
 
   mutate(d, null);
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/object-computed-access-assignment.expect.md

@@ -15,7 +15,6 @@ function foo(a, b, c) {
 function foo(a, b, c) {
   a[b] = c[b];
   a[3] = c[b * 4];
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/object-properties.expect.md

@@ -19,7 +19,6 @@ function foo(a, b, c) {
   const y = b.c.d;
   y.z = c.d.e;
   foo(a.b.c);
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/overlapping-scopes-interleaved-by-terminal.expect.md

@@ -26,7 +26,6 @@ function foo(a, b, c) {
 
   y.push(a);
   x.push(b);
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/overlapping-scopes-interleaved.expect.md

@@ -19,7 +19,6 @@ function foo(a, b) {
   const y = [];
   x.push(a);
   y.push(b);
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/overlapping-scopes-shadowed.expect.md

@@ -19,7 +19,6 @@ function foo(a, b) {
   const y = [];
   y.push(b);
   x.push(a);
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/overlapping-scopes-while.expect.md

@@ -23,7 +23,6 @@ function foo(a, b, c) {
     y.push(b);
     x.push(a);
   }
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/return-undefined.expect.md

@@ -0,0 +1,25 @@
+
+## Input
+
+```javascript
+function Component(props) {
+  if (props.cond) {
+    return undefined;
+  }
+  return props.value;
+}
+
+```
+
+## Code
+
+```javascript
+function Component(props) {
+  if (props.cond) {
+    return;
+  }
+  return props.value;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/compiler/return-undefined.js

@@ -0,0 +1,6 @@
+function Component(props) {
+  if (props.cond) {
+    return undefined;
+  }
+  return props.value;
+}

compiler/forget/src/__tests__/fixtures/compiler/reverse-postorder.expect.md

@@ -54,7 +54,6 @@ function Component(props) {
     } else {
     }
   }
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/sequence-expression.expect.md

@@ -42,9 +42,7 @@ function sequence(props) {
   return x;
 }
 
-function foo() {
-  return undefined;
-}
+function foo() {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/simple-alias.expect.md

@@ -20,9 +20,7 @@ function foo() {
 
 ```javascript
 import * as React from "react";
-function mutate() {
-  return undefined;
-}
+function mutate() {}
 function foo() {
   const $ = React.unstable_useMemoCache(1);
   let c;

compiler/forget/src/__tests__/fixtures/compiler/ssa-call-jsx-2.expect.md

@@ -22,9 +22,7 @@ function Component(props) {
 
 ```javascript
 import * as React from "react"; // @Pass runMutableRangeAnalysis
-function foo() {
-  return undefined;
-}
+function foo() {}
 
 function Component(props) {
   const $ = React.unstable_useMemoCache(3);

compiler/forget/src/__tests__/fixtures/compiler/ssa-call-jsx.expect.md

@@ -19,9 +19,7 @@ function Component(props) {
 
 ```javascript
 import * as React from "react";
-function foo() {
-  return undefined;
-}
+function foo() {}
 
 function Component(props) {
   const $ = React.unstable_useMemoCache(3);

compiler/forget/src/__tests__/fixtures/compiler/ssa-complex-multiple-if.expect.md

@@ -20,9 +20,7 @@ function foo() {
 ## Code
 
 ```javascript
-function foo() {
-  return undefined;
-}
+function foo() {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/ssa-complex-single-if.expect.md

@@ -17,9 +17,7 @@ function foo() {
 ## Code
 
 ```javascript
-function foo() {
-  return undefined;
-}
+function foo() {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/ssa-if-else.expect.md

@@ -18,9 +18,7 @@ function foo() {
 ## Code
 
 ```javascript
-function foo() {
-  return undefined;
-}
+function foo() {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/ssa-nested-partial-phi.expect.md

@@ -26,7 +26,6 @@ function foo(a, b, c) {
     }
     return x;
   }
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/ssa-newexpression.expect.md

@@ -17,9 +17,7 @@ function Component(props) {
 
 ```javascript
 import * as React from "react";
-function Foo() {
-  return undefined;
-}
+function Foo() {}
 
 function Component(props) {
   const $ = React.unstable_useMemoCache(1);

compiler/forget/src/__tests__/fixtures/compiler/ssa-shadowing.expect.md

@@ -20,9 +20,7 @@ function Foo(cond) {
 ## Code
 
 ```javascript
-function log() {
-  return undefined;
-}
+function log() {}
 
 function Foo(cond) {
   let str = "";
@@ -33,7 +31,6 @@ function Foo(cond) {
   }
 
   log(str);
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/ssa-sibling-phis.expect.md

@@ -27,9 +27,7 @@ function foo(a, b, c, d) {
 ## Code
 
 ```javascript
-function foo(a, b, c, d) {
-  return undefined;
-}
+function foo(a, b, c, d) {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/ssa-simple-phi.expect.md

@@ -19,9 +19,7 @@ function foo() {
 ## Code
 
 ```javascript
-function foo() {
-  return undefined;
-}
+function foo() {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/ssa-simple.expect.md

@@ -12,9 +12,7 @@ function foo() {
 ## Code
 
 ```javascript
-function foo() {
-  return undefined;
-}
+function foo() {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/ssa-single-if.expect.md

@@ -16,9 +16,7 @@ function foo() {
 ## Code
 
 ```javascript
-function foo() {
-  return undefined;
-}
+function foo() {}
 
 ```
       

compiler/forget/src/__tests__/fixtures/compiler/ssa-switch.expect.md

@@ -38,7 +38,6 @@ function foo() {
     default: {
     }
   }
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/switch-with-fallthrough.expect.md

@@ -55,7 +55,6 @@ function foo(x) {
     default: {
     }
   }
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/transitive-alias-fields.expect.md

@@ -31,7 +31,6 @@ function component() {
   q.y = p.y;
 
   mutate(q);
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/type-args-test-binary-operator.expect.md

@@ -16,7 +16,6 @@ function component(a, b) {
 function component(a, b) {
   if (a > b) {
   }
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/type-binary-operator.expect.md

@@ -20,7 +20,6 @@ function component() {
   const b = someOther();
   if (a > b) {
   }
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/useMemo-return-empty.expect.md

@@ -18,7 +18,6 @@ function component(a) {
 // @inlineUseMemo
 function component(a) {
   mutate(a);
-  return undefined;
 }
 
 ```

compiler/forget/src/__tests__/fixtures/compiler/while-conditional-continue.expect.md

@@ -28,7 +28,6 @@ function foo(a, b, c, d) {
   }
 
   d();
-  return undefined;
 }
 
 ```