Keep declarations a removed branch shares with another

packages/compiler/src/frontend/shape-inference/overlay.ts

@@ -502,15 +502,18 @@ export function planShapeOverlay(
     // Dead branches: outermost first; edits inside a replaced range drop.
     const dead = (deadByFile.get(sf) ?? []).sort((a, b) => a.pos - b.pos);
     const ranges: [number, number][] = [];
-    const hoisted = new Map<number, readonly string[]>();
+    const removed = new Set<ts.Node>();
     for (const branch of dead) {
       const start = ts.tokenStartOfNode(branch);
       if (ranges.some(([lo, hi]) => start >= lo && start < hi)) continue;
-      const names = hoistedNames(branch);
-      if (names === null) continue;
-      hoisted.set(start, names);
+      if (hoistedNames(branch, removed) === null) continue;
+      removed.add(branch);
       ranges.push([start, branch.end]);
     }
+    // A var that only removed branches declare stays declared in each.
+    const hoisted = new Map<number, readonly string[]>();
+    for (const branch of removed)
+      hoisted.set(ts.tokenStartOfNode(branch), hoistedNames(branch, removed)!);
     if (ranges.length > 0) {
       const kept = edits.filter((e) => !ranges.some(([lo, hi]) => e.pos > lo && e.pos < hi));
       edits.length = 0;
@@ -664,11 +667,11 @@ function neutralizeComment(comment: string): string {
 }
 
 /** The `var` names a dead branch declares that its function (or file)
- * declares nowhere else: replacing the branch must keep them bound, as
- * undefined. Null when the branch declares a function in a block, whose
- * binding outside the block depends on the file's strictness; such a
- * branch is kept. */
-function hoistedNames(branch: ts.Node): readonly string[] | null {
+ * declares nowhere else outside the `removed` branches: replacing the
+ * branch must keep them bound, as undefined. Null when the branch declares
+ * a function in a block, whose binding outside the block depends on the
+ * file's strictness; such a branch is kept. */
+function hoistedNames(branch: ts.Node, removed: ReadonlySet<ts.Node>): readonly string[] | null {
   let scope: ts.Node | undefined = branch.parent;
   while (scope !== undefined && !ts.isSourceFile(scope) && !functionScope(scope))
     scope = scope.parent;
@@ -702,7 +705,7 @@ function hoistedNames(branch: ts.Node): readonly string[] | null {
   const body = ts.isSourceFile(scope) ? scope : (scope as { body?: ts.Node }).body;
   if (body !== undefined)
     ts.walkPreorder(body, (n) => {
-      if (n === branch) return "skip";
+      if (n === branch || removed.has(n)) return "skip";
       if (n !== body && ts.isFunctionDeclaration(n) && n.name !== undefined)
         elsewhere.add(n.name.text);
       if (n !== body && (functionScope(n) || ts.isClassDeclaration(n) || ts.isClassExpression(n)))

tests/fixtures/npm/cases/shape-overlay-facts/main.ts

@@ -1,6 +1,7 @@
 // The shape overlay's facts: results that include undefined, literals that
-// hold comment-like text, and a var declared in a branch no call takes.
-import { banner, hoisted, lookup, lookupOrReturn } from "shape-facts";
+// hold comment-like text, a var declared in a branch no call takes, and
+// a var declared in two such branches.
+import { banner, hoisted, lookup, lookupOrReturn, shared } from "shape-facts";
 
 console.log(lookup("b"));
 console.log(lookup("z"));
@@ -8,3 +9,4 @@ console.log(lookupOrReturn("a"));
 console.log(lookupOrReturn("y"));
 console.log(banner(7));
 console.log(hoisted("note:"));
+console.log(shared("level:"));

tests/fixtures/npm/cases/shape-overlay-facts/node_modules/shape-facts/index.d.ts

@@ -2,3 +2,4 @@ export declare function lookup(key: string): string;
 export declare function lookupOrReturn(key: string): string;
 export declare function banner(n: number): string;
 export declare function hoisted(label: string): string;
+export declare function shared(label: string): string;

tests/fixtures/npm/cases/shape-overlay-facts/node_modules/shape-facts/index.js

@@ -1,6 +1,7 @@
 // Functions whose results include the undefined of a fall-through or a
-// bare return, comment-like text in literals, and a branch no call takes
-// that declares a var read outside it.
+// bare return, comment-like text in literals, a branch no call takes
+// that declares a var read outside it, and two such branches that declare
+// the same var.
 
 const entries = [
   { key: "a", value: 1 },
@@ -53,3 +54,17 @@ function noteOf(extra) {
 export function hoisted(label) {
   return label + " " + noteOf(undefined);
 }
+
+function levelOf(warn, fail) {
+  if (warn) {
+    var level = "warn";
+  }
+  if (fail) {
+    var level = "fail";
+  }
+  return typeof level + " " + String(level);
+}
+
+export function shared(label) {
+  return label + " " + levelOf(undefined, undefined);
+}

tests/harness/shape-overlay.test.ts

@@ -63,7 +63,8 @@ test("shapeOverlay: false compiles without the overlay", { timeout: 900_000 }, a
 
 // shape-facts: results that include the undefined of a fall-through or a bare
 // return keep the caller's falsy branch, comment-like text in literals keeps
-// its value, and a replaced branch keeps its var declared.
+// its value, and a replaced branch keeps its var declared, also when two
+// replaced branches declare the same var.
 const factsEntry = join(repoRoot, "tests/fixtures/npm/cases/shape-overlay-facts/main.ts");
 
 test(
@@ -78,7 +79,7 @@ test(
       throw new Error(result.diagnostics.map((d) => `${d.code}: ${d.message}`).join("\n"));
     const report = (result.warnings ?? []).find((w) => w.code === "SC6103");
     expect(report?.message).toMatch(/of 1 package \(shape-facts\)/);
-    expect(report?.message).toMatch(/replaces 1 branch it proved unreachable/);
+    expect(report?.message).toMatch(/replaces 3 branches it proved unreachable/);
     const native = await execFileAsync(result.binaryPath, [], {
       cwd: dirname(factsEntry),
       encoding: "utf8",