Call the receiver copy for direct calls whose result is discarded

A direct call to an untyped method that returns `this` on every path,
used as a statement (`builder.add(part);`), boxed its receiver into a
capsule that nothing read. Such a call now runs the method's copy that
returns nothing, as the class dispatch helpers already do.

packages/compiler/src/frontend/lowering/receiver-returns.ts

@@ -7,8 +7,10 @@
  * (`%dyn.class.call.N`) that calls such a method already holds a capsule
  * of the same instance at its exact class: its receiver after
  * normalization. The helper now calls a copy of the method that returns
- * nothing and answers its own receiver, so the call boxes nothing. Other
- * callers (vtables, typed code, method values) keep the method itself. */
+ * nothing and answers its own receiver, so the call boxes nothing. A
+ * direct call whose result nothing reads calls the copy too. Other callers
+ * (vtables, method values, calls that use the result) keep the method
+ * itself. */
 import type { IrExpr, IrFunction, IrStmt } from "../../ir/ir.js";
 import { VOID } from "../../ir/ir.js";
 import { varRef } from "../../ir/build.js";
@@ -57,6 +59,20 @@ export function returnDispatchReceivers(lowerer: Lowerer, functions: IrFunction[
       ];
     }
   }
+  // A direct call whose result nothing reads (`builder.set(1);` in typed
+  // code) needs no capsule at all.
+  for (const fn of [...functions]) {
+    everyStmtList(fn.body, {
+      stmt: (s) => {
+        if (s.kind === "exprStmt" && s.expr.kind === "call") {
+          const copy = copyOf(s.expr.callee);
+          if (copy) s.expr = { ...s.expr, callee: copy.name, type: VOID };
+        }
+        return true;
+      },
+      expr: () => true,
+    });
+  }
 }
 
 /** The helper's receiver is assigned only by its normalization, so it is

packages/compiler/test/ts7/baselines/order-parity/tests/corpus/dynamic-method-receiver-statements.mjs.json

@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/dynamic-method-receiver-statements.mjs"
+ ],
+ "diags": []
+}

tests/corpus/dynamic-method-receiver-statements.mjs

@@ -0,0 +1,46 @@
+// Untyped methods that return their receiver, called as statements on
+// receivers whose class is known: side effects, throws, overrides, and
+// results that are used.
+class Builder {
+  constructor() {
+    this.parts = [];
+    this.count = 0;
+  }
+  add(part) {
+    if (part === "") throw new TypeError("empty part");
+    this.parts.push(part);
+    this.count++;
+    return this;
+  }
+  reset() {
+    this.parts.length = 0;
+    return this;
+  }
+}
+class LoudBuilder extends Builder {
+  add(part) {
+    super.add(part.toUpperCase());
+    return this;
+  }
+}
+const b = new Builder();
+for (let i = 0; i < 1000; i++) {
+  b.add("p" + (i % 3));
+  if (i % 100 === 99) b.reset();
+}
+console.log(b.count, b.parts.join(","));
+b.add("x").add("y");
+const same = b.add("z");
+console.log(same === b, b.parts.join(","));
+try {
+  b.add("");
+} catch (e) {
+  console.log(e instanceof TypeError, e.message, b.count);
+}
+const loud = new LoudBuilder();
+loud.add("a");
+loud.add("b").add("c");
+console.log(loud.parts.join(","), loud.count, loud instanceof Builder);
+const builders = [new Builder(), new LoudBuilder()];
+for (const each of builders) each.add("q");
+console.log(builders.map((each) => each.parts.join("")).join("|"));