diff --git a/compiler/fir/analysis-tests/testData/resolve/smartcasts/kt10240.kt b/compiler/fir/analysis-tests/testData/resolve/smartcasts/kt10240.kt
new file mode 100644
index 00000000000..afaf177ac05
--- /dev/null
+++ b/compiler/fir/analysis-tests/testData/resolve/smartcasts/kt10240.kt
@@ -0,0 +1,36 @@
+fun a(): Int {
+    fun b(): Int {
+        var c: Int? = null
+        if (c == null ||
+            0 < c // FE 1.0: smart cast impossible, see KT-10240
+        ) c = 0
+        return c ?: 0
+    }
+
+    var c: Int = 0
+    c = 0
+    return c
+}
+
+fun myRun(f: () -> Unit) {
+    f()
+}
+
+fun println(arg: Int) {}
+
+fun d() {
+    myRun {
+        var koko: String? = "Alpha"
+        while (koko != null) {
+            println(koko.length)
+            koko = null
+        }
+    }
+    myRun {
+        var koko: String? = "Omega"
+        while (koko != null) {
+            println(koko.length) // FE 1.0: smart cast impossible, see KT-19446
+            koko = null
+        }
+    }
+}
\ No newline at end of file
diff --git a/compiler/fir/analysis-tests/testData/resolve/smartcasts/kt10240.txt b/compiler/fir/analysis-tests/testData/resolve/smartcasts/kt10240.txt
new file mode 100644
index 00000000000..ecdf1185447
--- /dev/null
+++ b/compiler/fir/analysis-tests/testData/resolve/smartcasts/kt10240.txt
@@ -0,0 +1,42 @@
+FILE: kt10240.kt
+    public final fun a(): R|kotlin/Int| {
+        local final fun b(): R|kotlin/Int| {
+            lvar c: R|kotlin/Int?| = Null(null)
+            when () {
+                ==(R|<local>/c|, Null(null)) || CMP(<, Int(0).R|kotlin/Int.compareTo|(R|<local>/c|)) ->  {
+                    R|<local>/c| = Int(0)
+                }
+            }
+
+            ^b R|<local>/c| ?: Int(0)
+        }
+
+        lvar c: R|kotlin/Int| = Int(0)
+        R|<local>/c| = Int(0)
+        ^a R|<local>/c|
+    }
+    public final fun myRun(f: R|() -> kotlin/Unit|): R|kotlin/Unit| {
+        R|<local>/f|.R|FakeOverride<kotlin/Function0.invoke: R|kotlin/Unit|>|()
+    }
+    public final fun println(arg: R|kotlin/Int|): R|kotlin/Unit| {
+    }
+    public final fun d(): R|kotlin/Unit| {
+        R|/myRun|(<L> = myRun@fun <anonymous>(): R|kotlin/Unit| {
+            lvar koko: R|kotlin/String?| = String(Alpha)
+            while(!=(R|<local>/koko|, Null(null))) {
+                R|/println|(R|<local>/koko|.R|kotlin/String.length|)
+                R|<local>/koko| = Null(null)
+            }
+
+        }
+        )
+        R|/myRun|(<L> = myRun@fun <anonymous>(): R|kotlin/Unit| {
+            lvar koko: R|kotlin/String?| = String(Omega)
+            while(!=(R|<local>/koko|, Null(null))) {
+                R|/println|(R|<local>/koko|.R|kotlin/String.length|)
+                R|<local>/koko| = Null(null)
+            }
+
+        }
+        )
+    }
diff --git a/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsTestGenerated.java b/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsTestGenerated.java
index 8b44fb18910..f27a0c6195d 100644
--- a/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsTestGenerated.java
+++ b/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsTestGenerated.java
@@ -2259,6 +2259,11 @@ public class FirDiagnosticsTestGenerated extends AbstractFirDiagnosticsTest {
             runTest("compiler/fir/analysis-tests/testData/resolve/smartcasts/equalsAndIdentity.kt");
         }
 
+        @TestMetadata("kt10240.kt")
+        public void testKt10240() throws Exception {
+            runTest("compiler/fir/analysis-tests/testData/resolve/smartcasts/kt10240.kt");
+        }
+
         @TestMetadata("kt37327.kt")
         public void testKt37327() throws Exception {
             runTest("compiler/fir/analysis-tests/testData/resolve/smartcasts/kt37327.kt");
diff --git a/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithLightTreeTestGenerated.java b/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithLightTreeTestGenerated.java
index d0232441e28..85ff23a849a 100644
--- a/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithLightTreeTestGenerated.java
+++ b/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithLightTreeTestGenerated.java
@@ -2259,6 +2259,11 @@ public class FirDiagnosticsWithLightTreeTestGenerated extends AbstractFirDiagnos
             runTest("compiler/fir/analysis-tests/testData/resolve/smartcasts/equalsAndIdentity.kt");
         }
 
+        @TestMetadata("kt10240.kt")
+        public void testKt10240() throws Exception {
+            runTest("compiler/fir/analysis-tests/testData/resolve/smartcasts/kt10240.kt");
+        }
+
         @TestMetadata("kt37327.kt")
         public void testKt37327() throws Exception {
             runTest("compiler/fir/analysis-tests/testData/resolve/smartcasts/kt37327.kt");
diff --git a/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/LazyBodyIsNotTouchedTilContractsPhaseTestGenerated.java b/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/LazyBodyIsNotTouchedTilContractsPhaseTestGenerated.java
index 1aed7329553..77229a68ec6 100644
--- a/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/LazyBodyIsNotTouchedTilContractsPhaseTestGenerated.java
+++ b/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/LazyBodyIsNotTouchedTilContractsPhaseTestGenerated.java
@@ -2259,6 +2259,11 @@ public class LazyBodyIsNotTouchedTilContractsPhaseTestGenerated extends Abstract
             runTest("compiler/fir/analysis-tests/testData/resolve/smartcasts/equalsAndIdentity.kt");
         }
 
+        @TestMetadata("kt10240.kt")
+        public void testKt10240() throws Exception {
+            runTest("compiler/fir/analysis-tests/testData/resolve/smartcasts/kt10240.kt");
+        }
+
         @TestMetadata("kt37327.kt")
         public void testKt37327() throws Exception {
             runTest("compiler/fir/analysis-tests/testData/resolve/smartcasts/kt37327.kt");