[alias] Don't alias primitives

compiler/forget/src/HIR/HIR.ts

@@ -607,3 +607,7 @@ function phiTypeEquals(tA: Type, tB: Type): boolean {
 export function isObjectType(id: Identifier): boolean {
   return id.type.kind === "Object";
 }
+
+export function isPrimitiveType(id: Identifier): boolean {
+  return id.type.kind === "Primitive";
+}

compiler/forget/src/Inference/InferAlias.ts

@@ -3,6 +3,7 @@ import {
   HIRFunction,
   Identifier,
   Instruction,
+  isPrimitiveType,
   LValue,
   Place,
 } from "../HIR/HIR";
@@ -33,6 +34,9 @@ function inferInstr(instr: Instruction, state: AliasAnalyser) {
   let alias: Place | null = null;
   switch (instrValue.kind) {
     case "Identifier": {
+      if (isPrimitiveType(instrValue.identifier)) {
+        return;
+      }
       alias = instrValue;
       break;
     }

compiler/forget/src/__tests__/fixtures/hir/primitive-alias-mutate.expect.md

@@ -0,0 +1,50 @@
+
+## Input
+
+```javascript
+function component(a) {
+  let x = "foo";
+  if (a) {
+    x = "bar";
+  } else {
+    x = "baz";
+  }
+  let y = x;
+  mutate(y);
+  return y;
+}
+
+```
+
+## Code
+
+```javascript
+function component(a) {
+  const $ = React.useMemoCache();
+  const x = "foo";
+  const c_0 = $[0] !== a;
+  let x$0;
+  if (c_0) {
+    x$0 = undefined;
+
+    if (a) {
+      const x$1 = "bar";
+      x$0 = x$1;
+    } else {
+      const x$2 = "baz";
+      x$0 = x$2;
+    }
+
+    $[0] = a;
+    $[1] = x$0;
+  } else {
+    x$0 = $[1];
+  }
+
+  const y = x$0;
+  mutate(y);
+  return y;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/primitive-alias-mutate.js

@@ -0,0 +1,11 @@
+function component(a) {
+  let x = "foo";
+  if (a) {
+    x = "bar";
+  } else {
+    x = "baz";
+  }
+  let y = x;
+  mutate(y);
+  return y;
+}