Fix remaining compiler and VM review findings

lynglib/src/commonMain/kotlin/net/sergeych/lyng/Compiler.kt

@@ -6446,7 +6446,7 @@ class Compiler(
             WhenStatement(value, cases, elseCase, whenPos)
         } else {
             // when { cond -> ... }
-            TODO("when without object is not yet implemented")
+            throw ScriptError(t.pos, "when without subject is not implemented")
         }
         return wrapBytecode(stmt)
     }

lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdRuntime.kt

@@ -3930,7 +3930,7 @@ class CmdFrame(
                     } else {
                         val raw = frame.getRawObj(localIndex)
                         if (raw == null && name != null) {
-                            val record = scope.get(name)
+                            val record = findNamedExistingRecord(scope, name)
                             if (record != null) {
                                 val value = record.value
                                 return@mapIndexed when (value) {
@@ -3939,6 +3939,12 @@ class CmdFrame(
                                     else -> ObjRecord(value, isMutable)
                                 }
                             }
+                            if (hasNamedScopeBinding(scope, name)) {
+                                throw ScriptError(
+                                    ensureScope().pos,
+                                    "captured binding '$name' is not available in the execution scope; prepare the script imports/module bindings explicitly"
+                                )
+                            }
                         }
                         when (raw) {
                             is FrameSlotRef -> ObjRecord(raw, isMutable)
@@ -3952,22 +3958,27 @@ class CmdFrame(
                     val target = moduleScope
                     val name = captureNames?.getOrNull(index)
                     if (name != null) {
-                        target.tryGetLocalRecord(target, name, target.currentClassCtx)?.let { return@mapIndexed it }
-                        target.getSlotIndexOf(name)?.let { return@mapIndexed target.getSlotRecord(it) }
-                        target.get(name)?.let { return@mapIndexed it }
+                        findNamedExistingRecord(target, name)?.let { return@mapIndexed it }
                         // Fallback to current scope in case the module scope isn't in the parent chain
                         // or doesn't carry the imported symbol yet.
-                        scope.tryGetLocalRecord(scope, name, scope.currentClassCtx)?.let { return@mapIndexed it }
-                        scope.get(name)?.let { return@mapIndexed it }
+                        findNamedExistingRecord(scope, name)?.let { return@mapIndexed it }
                     }
                     if (slotId < target.slotCount) {
-                        return@mapIndexed target.getSlotRecord(slotId)
+                        val existing = target.getSlotRecord(slotId)
+                        if (name == null || existing.value !== ObjUnset || hasResolvedNamedScopeBinding(target, name)) {
+                            return@mapIndexed existing
+                        }
                     }
                     if (name != null) {
-                        target.applySlotPlan(mapOf(name to slotId))
-                        return@mapIndexed target.getSlotRecord(slotId)
+                        throw ScriptError(
+                            ensureScope().pos,
+                            "module capture '$name' is not available in the execution scope; prepare the script imports/module bindings explicitly"
+                        )
                     }
-                    error("Missing module capture slot $slotId")
+                    throw ScriptError(
+                        ensureScope().pos,
+                        "missing module capture slot $slotId"
+                    )
                 }
             }
         }
@@ -4776,12 +4787,13 @@ class CmdFrame(
 
     private suspend fun getScopeSlotValue(slot: Int): Obj {
         val target = scopeTarget(slot)
+        val name = fn.scopeSlotNames[slot]
+        val hadNamedBinding = name != null && hasResolvedNamedScopeBinding(target, name)
         val index = ensureScopeSlot(target, slot)
         val record = target.getSlotRecord(index)
         val direct = record.value
         if (direct is FrameSlotRef) return direct.read()
         if (direct is RecordSlotRef) return direct.read()
-        val name = fn.scopeSlotNames[slot]
         if (name != null && record.memberName != null && record.memberName != name) {
             val resolved = target.get(name)
             if (resolved != null) {
@@ -4802,9 +4814,15 @@ class CmdFrame(
             return direct
         }
         if (name == null) return record.value
-        val resolved = target.get(name) ?: return record.value
+        val resolved = target.get(name)
+        if (resolved == null) {
+            failMissingPreparedModuleBinding(slot, name, hadNamedBinding, record)
+            return record.value
+        }
         if (resolved.value !== ObjUnset) {
             target.updateSlotFor(name, resolved)
+        } else {
+            failMissingPreparedModuleBinding(slot, name, hadNamedBinding, resolved)
         }
         return resolved.value
     }
@@ -4812,12 +4830,13 @@ class CmdFrame(
     private suspend fun getScopeSlotValueAtAddr(addrSlot: Int): Obj {
         val target = addrScopes[addrSlot] ?: error("Address slot $addrSlot is not resolved")
         val index = addrIndices[addrSlot]
+        val slotId = addrScopeSlots[addrSlot]
+        val name = fn.scopeSlotNames.getOrNull(slotId)
+        val hadNamedBinding = name != null && hasResolvedNamedScopeBinding(target, name)
         val record = target.getSlotRecord(index)
         val direct = record.value
         if (direct is FrameSlotRef) return direct.read()
         if (direct is RecordSlotRef) return direct.read()
-        val slotId = addrScopeSlots[addrSlot]
-        val name = fn.scopeSlotNames.getOrNull(slotId)
         if (name != null && record.memberName != null && record.memberName != name) {
             val resolved = target.get(name)
             if (resolved != null) {
@@ -4838,9 +4857,15 @@ class CmdFrame(
             return direct
         }
         if (name == null) return record.value
-        val resolved = target.get(name) ?: return record.value
+        val resolved = target.get(name)
+        if (resolved == null) {
+            failMissingPreparedModuleBinding(slotId, name, hadNamedBinding, record)
+            return record.value
+        }
         if (resolved.value !== ObjUnset) {
             target.updateSlotFor(name, resolved)
+        } else {
+            failMissingPreparedModuleBinding(slotId, name, hadNamedBinding, resolved)
         }
         return resolved.value
     }
@@ -4875,6 +4900,41 @@ class CmdFrame(
         return index
     }
 
+    private fun hasNamedScopeBinding(target: Scope, name: String): Boolean {
+        if (target.tryGetLocalRecord(target, name, target.currentClassCtx) != null) return true
+        if (target.getSlotIndexOf(name) != null) return true
+        if (target.get(name) != null) return true
+        return false
+    }
+
+    private fun hasResolvedNamedScopeBinding(target: Scope, name: String): Boolean =
+        findNamedExistingRecord(target, name) != null
+
+    private fun findNamedExistingRecord(target: Scope, name: String): ObjRecord? {
+        target.tryGetLocalRecord(target, name, target.currentClassCtx)?.let { return it }
+        target.get(name)?.let { record ->
+            if (record.value !== ObjUnset || record.type == ObjRecord.Type.Delegated || record.type == ObjRecord.Type.Property) {
+                return record
+            }
+        }
+        return null
+    }
+
+    private fun failMissingPreparedModuleBinding(
+        slot: Int,
+        name: String,
+        hadNamedBinding: Boolean,
+        record: ObjRecord
+    ) {
+        if (hadNamedBinding) return
+        if (record.value !== ObjUnset) return
+        if (fn.scopeSlotIsModule.getOrNull(slot) != true) return
+        throw ScriptError(
+            ensureScope().pos,
+            "module binding '$name' is not available in the execution scope; prepare the script imports/module bindings explicitly"
+        )
+    }
+
     private fun ensureLocalMutable(localIndex: Int) {
         val name = fn.localSlotNames.getOrNull(localIndex) ?: return
         val isMutable = fn.localSlotMutables.getOrNull(localIndex) ?: true

lynglib/src/commonTest/kotlin/CompilerVmReviewRegressionTest.kt

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2026 Sergey S. Chernov
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import kotlinx.coroutines.test.runTest
+import net.sergeych.lyng.Compiler
+import net.sergeych.lyng.Scope
+import net.sergeych.lyng.Script
+import net.sergeych.lyng.ScriptError
+import net.sergeych.lyng.Source
+import net.sergeych.lyng.asFacade
+import net.sergeych.lyng.obj.ObjString
+import net.sergeych.lyng.obj.toInt
+import net.sergeych.lyng.pacman.ImportManager
+import kotlin.test.Test
+import kotlin.test.assertContains
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+
+class CompilerVmReviewRegressionTest {
+
+    @Test
+    fun missingModuleCaptureFailsFastInsteadOfBecomingUnset() = runTest {
+        val manager = ImportManager()
+        manager.addTextPackages(
+            """
+            package foo
+
+            val answer = 42
+            """.trimIndent()
+        )
+        val script = Compiler.compile(
+            Source(
+                "<missing-module-capture>",
+                """
+                import foo
+                fun make() = { answer }
+                make()
+                """.trimIndent()
+            ),
+            manager
+        )
+
+        val prepared = manager.newModule()
+        script.importInto(prepared)
+        val preparedLambda = script.execute(prepared)
+        assertEquals(42, prepared.asFacade().call(preparedLambda).toInt())
+
+        val rawModule = manager.newModule()
+        val hostScope = Scope(parent = rawModule, thisObj = ObjString("receiver"))
+
+        val lambda = script.execute(hostScope)
+        val ex = assertFailsWith<ScriptError> {
+            hostScope.asFacade().call(lambda)
+        }
+        assertContains(ex.errorMessage, "module binding 'answer'")
+    }
+
+    @Test
+    fun subjectlessWhenReportsScriptError() = runTest {
+        val ex = assertFailsWith<ScriptError> {
+            Compiler.compile(
+                Source(
+                    "<when-without-subject>",
+                    """
+                    when {
+                        true -> 1
+                    }
+                    """.trimIndent()
+                ),
+                Script.defaultImportManager
+            )
+        }
+        assertContains(ex.errorMessage, "when without subject")
+    }
+}