Lower each closure argument of an implicit-any call once (#2150)

packages/compiler/src/frontend/lowering/generic-functions.ts

@@ -2082,6 +2082,10 @@ function storedImplicitArgumentType(lowerer: Lowerer, arg: ts.Expression): IrTyp
     isJsSourceFile(ts.getSourceFileOfNode(arg)) &&
     (ts.isArrowFunction(arg) || ts.isFunctionExpression(arg))
   ) {
+    // A closure lowered here before (another call site's instance key, or
+    // a repeated demand) keeps its value for the argument's own lowering.
+    const known = lowerer.speculatedClosure(arg);
+    if (known !== null) return known.type;
     const saved = lowerer.diagSink;
     const diagnostics: typeof lowerer.diags = [];
     lowerer.diagSink = diagnostics;
@@ -2092,8 +2096,17 @@ function storedImplicitArgumentType(lowerer: Lowerer, arg: ts.Expression): IrTyp
       lowerer.diagSink = saved;
     }
     // Direct inferred helpers can receive native callbacks even when their
-    // result is an opaque handle that cannot cross checked storage.
-    if (diagnostics.length === 0 && value?.type.kind === "func") return value.type;
+    // result is an opaque handle that cannot cross checked storage. The
+    // argument's own lowering then takes this value instead of lowering
+    // the closure a second time.
+    if (diagnostics.length === 0 && value?.type.kind === "func") {
+      lowerer.speculatedClosures.set(arg, {
+        ctx: lowerer.ctx,
+        bindings: lowerer.typeParamBindings,
+        value,
+      });
+      return value.type;
+    }
   }
   if (arg.kind === ts.SyntaxKind.ThisKeyword) return lowerer.resolveThis()?.type ?? null;
   if (ts.isAwaitExpression(arg)) {

packages/compiler/src/frontend/lowering/lowerer.ts

@@ -3178,6 +3178,14 @@ export class Lowerer {
    * (builtins/fs-paths.ts): each lowers to the hidden local that holds its
    * value, which is null until the path's lowering has saved it. */
   readonly stagedArguments = new Map<ts.Expression, { localId: string; type: IrType } | null>();
+  /** Closure arguments an implicit-any call already lowered to learn their
+   * type (storedImplicitArgumentType), by node: the argument's own lowering
+   * in the same function and with the same type bindings takes that value
+   * once instead of lowering the closure again. */
+  readonly speculatedClosures = new Map<
+    ts.Expression,
+    { ctx: FnCtx; bindings: Map<ts.Symbol, IrType> | null; value: IrExpr }
+  >();
   /** What each fs path argument of a URL-path call lowers to (the plans
    * of builtins/fs-paths.ts). */
   readonly fsPathPlans = new Map<ts.Expression, FsPathPlan>();
@@ -12946,12 +12954,30 @@ export class Lowerer {
   /* ── expressions ──────────────────────────────────────────────────── */
 
   lowerExpr(expr: ts.Expression): IrExpr {
+    if (this.speculatedClosures.size > 0) {
+      const speculated = this.speculatedClosure(expr);
+      if (speculated !== null) {
+        this.speculatedClosures.delete(expr);
+        return speculated;
+      }
+    }
     const streamValue = this.valueStreamReceivers.get(expr);
     if (streamValue)
       return valueStreamOf(varRef(streamValue.localId, streamValue.type, locOf(expr)));
     return this.stagedArgument(expr) ?? lowerExpr(this, expr);
   }
 
+  /** The value storedImplicitArgumentType lowered for `expr` in the active
+   * function and type bindings (speculatedClosures), or null. */
+  speculatedClosure(expr: ts.Expression): IrExpr | null {
+    const entry = this.speculatedClosures.get(expr);
+    return entry !== undefined &&
+      entry.ctx === this.ctx &&
+      entry.bindings === this.typeParamBindings
+      ? entry.value
+      : null;
+  }
+
   /** The saved value of a staged fs argument (stagedArguments, keyed
    * without type-only wrappers), or null for every other expression. */
   private stagedArgument(node: ts.Expression): IrExpr | null {