fix: preserve boolean checks through TCO

sjsonnet/src/sjsonnet/Evaluator.scala

@@ -1502,14 +1502,19 @@ class Evaluator(
       }
     }
 
-  // And/Or rhs tail-position helpers — extracted to preserve @tailrec on visitExprWithTailCallSupport.
-  // TailCall sentinels pass through without a boolean type check: this is a deliberate semantic
-  // relaxation matching google/jsonnet behavior (where `&&` is simply `if a then b else false`).
-  // Direct non-boolean rhs values (e.g. `true && "hello"`) are still caught.
+  // And/Or rhs tail-position helpers - extracted to preserve @tailrec on
+  // visitExprWithTailCallSupport. TailCall sentinels keep the boolean type check as delayed
+  // continuation state so the trampoline can remain stack-safe while preserving normal &&/||
+  // result validation.
+  //
+  // Do not resolve the TailCall here and then call .asBoolean. That would re-enter the trampoline
+  // from inside the current function body, restoring recursive stack growth for deep &&/|| chains
+  // and moving the lazy evaluation boundary. Instead, attach the check and let the outer
+  // TailCall.resolve loop apply it once the final value is produced.
   private def visitAndRhsTailPos(rhs: Expr, andPos: Position)(implicit scope: ValScope): Val = {
     visitExprWithTailCallSupport(rhs) match {
       case b: Val.Bool  => b
-      case tc: TailCall => tc
+      case tc: TailCall => tc.withBooleanCheck(new TailCall.BooleanCheck("&&", andPos))
       case unknown      =>
         Error.fail(s"binary operator && does not operate on ${unknown.prettyName}s.", andPos)
     }
@@ -1518,7 +1523,7 @@ class Evaluator(
   private def visitOrRhsTailPos(rhs: Expr, orPos: Position)(implicit scope: ValScope): Val = {
     visitExprWithTailCallSupport(rhs) match {
       case b: Val.Bool  => b
-      case tc: TailCall => tc
+      case tc: TailCall => tc.withBooleanCheck(new TailCall.BooleanCheck("||", orPos))
       case unknown      =>
         Error.fail(s"binary operator || does not operate on ${unknown.prettyName}s.", orPos)
     }
@@ -1530,8 +1535,8 @@ class Evaluator(
    * `TailCall.resolve` in `visitApply*` to iterate rather than grow the JVM stack.
    *
    * Potential tail positions are propagated through: IfElse (both branches), LocalExpr (returned),
-   * AssertExpr (returned), And (rhs), Or (rhs), and Expr.Error (value). All other expression types
-   * delegate to normal `visitExpr`.
+   * AssertExpr (returned), And (rhs), and Or (rhs). All other expression types delegate to normal
+   * `visitExpr`.
    */
   @tailrec
   private def visitExprWithTailCallSupport(e: Expr)(implicit scope: ValScope): Val = e match {

sjsonnet/src/sjsonnet/StaticOptimizer.scala

@@ -658,7 +658,7 @@ class StaticOptimizer(
         // when lhs is true). We must still check rhs for non-recursive exits.
         hasNonRecursiveExit(e.rhs, selfName, selfIdx, paramCount)
       case _: Expr.Error =>
-        // error value is the last thing evaluated before throwing → non-recursive exit
+        // error value is the last thing evaluated before throwing -> non-recursive exit
         true
       case e if isSelfTailCall(e) =>
         false // this path IS a self-recursive tail call → not a non-recursive exit

sjsonnet/src/sjsonnet/Val.scala

@@ -2033,15 +2033,33 @@ final class TailCall(
     val args: Array[Eval],
     val namedNames: Array[String],
     val callSiteExpr: Expr,
-    val strict: Boolean = false)
+    val strict: Boolean = false,
+    private var booleanCheck: TailCall.BooleanCheck = null)
     extends Val {
   private[sjsonnet] def valTag: Byte = -1
   var pos: Position = callSiteExpr.pos
   def prettyName = "tailcall"
   override def exprErrorString: String = callSiteExpr.exprErrorString
+
+  def withBooleanCheck(check: TailCall.BooleanCheck): TailCall = {
+    if (booleanCheck eq null) booleanCheck = check
+    this
+  }
+
+  private[sjsonnet] def pendingBooleanCheck: TailCall.BooleanCheck = booleanCheck
 }
 
 object TailCall {
+  // Delayed result validation for tail-position &&/||. It deliberately lives on the TailCall rather
+  // than resolving nested TailCalls inside Evaluator.visitAndRhsTailPos/visitOrRhsTailPos, because
+  // nested resolution would undermine stack-safety and could force work at a different lazy boundary.
+  final class BooleanCheck(val op: String, val pos: Position) {
+    def check(result: Val)(implicit ev: EvalErrorScope): Val = result match {
+      case b: Val.Bool => b
+      case unknown     =>
+        Error.fail(s"binary operator $op does not operate on ${unknown.prettyName}s.", pos)
+    }
+  }
 
   /**
    * Iteratively resolve a [[TailCall]] chain (trampoline loop). If `current` is not a TailCall, it
@@ -2057,21 +2075,27 @@ object TailCall {
    * Error frames preserve the original call-site expression name (e.g. "Apply2") so that TCO does
    * not alter user-visible stack traces.
    */
+  def resolve(current: Val)(implicit ev: EvalScope): Val = resolve(current, null)
+
   @tailrec
-  def resolve(current: Val)(implicit ev: EvalScope): Val = current match {
-    case tc: TailCall =>
-      val mode: TailstrictMode =
-        if (tc.strict) TailstrictModeEnabled else TailstrictModeAutoTCO
-      val next =
-        try {
-          tc.func.apply(tc.args, tc.namedNames, tc.callSiteExpr.pos)(ev, mode)
-        } catch {
-          case e: Error =>
-            throw e.addFrame(tc.callSiteExpr.pos, tc.callSiteExpr)
-        }
-      resolve(next)
-    case result => result
-  }
+  private def resolve(current: Val, booleanCheck: BooleanCheck)(implicit ev: EvalScope): Val =
+    current match {
+      case tc: TailCall =>
+        val mode: TailstrictMode =
+          if (tc.strict) TailstrictModeEnabled else TailstrictModeAutoTCO
+        val nextCheck =
+          if (tc.pendingBooleanCheck != null) tc.pendingBooleanCheck else booleanCheck
+        val next =
+          try {
+            tc.func.apply(tc.args, tc.namedNames, tc.callSiteExpr.pos)(ev, mode)
+          } catch {
+            case e: Error =>
+              throw e.addFrame(tc.callSiteExpr.pos, tc.callSiteExpr)
+          }
+        resolve(next, nextCheck)
+      case result =>
+        if (booleanCheck == null) result else booleanCheck.check(result)
+    }
 }
 
 /**

sjsonnet/test/resources/new_test_suite/auto_tco_patterns.jsonnet

@@ -89,11 +89,11 @@ local f_four(a, b, c, d) =
 // This tests that hasNonRecursiveExit correctly identifies the outer if's base case
 // (not the &&/|| rhs which always recurses).
 local f_and_with_outer_base(n) =
-  if n <= 0 then 0  // outer base case
+  if n <= 0 then true  // outer base case
   else (true && f_and_with_outer_base(n - 1));  // rhs always recurses, but outer if provides exit
 
 local f_or_with_outer_base(n) =
-  if n <= 0 then 0
+  if n <= 0 then true
   else (false || f_or_with_outer_base(n - 1));
 
 // Run tests
@@ -117,7 +117,7 @@ std.assertEqual(f_deep_if(10000), 2) &&
 std.assertEqual(f_zero(), 42) &&
 std.assertEqual(f_four(10000, 0, 0, 0), 10000) &&
 // Edge case: &&/|| with outer base case — auto-TCO should work because outer if provides exit
-std.assertEqual(f_and_with_outer_base(10000), 0) &&
-std.assertEqual(f_or_with_outer_base(10000), 0) &&
+std.assertEqual(f_and_with_outer_base(10000), true) &&
+std.assertEqual(f_or_with_outer_base(10000), true) &&
 
 true

sjsonnet/test/resources/new_test_suite/error.auto_tco_and_tailcall_bool_check.jsonnet

@@ -0,0 +1,6 @@
+// The rhs returns through a TailCall, so the boolean check must be delayed until
+// the trampoline finishes. This should match direct `true && 0` behavior.
+local f(n) =
+  if n <= 0 then 0
+  else true && f(n - 1);
+f(1000)

sjsonnet/test/resources/new_test_suite/error.auto_tco_and_tailcall_bool_check.jsonnet.golden

@@ -0,0 +1 @@
+sjsonnet.Error: binary operator && does not operate on numbers.

sjsonnet/test/resources/new_test_suite/error.auto_tco_or_tailcall_bool_check.jsonnet

@@ -0,0 +1,6 @@
+// The rhs returns through a TailCall, so the boolean check must be delayed until
+// the trampoline finishes. This should match direct `false || 0` behavior.
+local f(n) =
+  if n <= 0 then 0
+  else false || f(n - 1);
+f(1000)

sjsonnet/test/resources/new_test_suite/error.auto_tco_or_tailcall_bool_check.jsonnet.golden

@@ -0,0 +1 @@
+sjsonnet.Error: binary operator || does not operate on numbers.