FIR: Add problematic test case

^KT-37318 Open

compiler/fir/resolve/testData/resolve/problems/flexibleTypeVarAgainstNull.kt

@@ -0,0 +1,16 @@
+// FILE: JavaClass.java
+public class JavaClass<T> {
+    public JavaClass(T t) {}
+
+    public T foo() {}
+}
+
+// FILE: main.kt
+fun main() {
+    // See https://github.com/JetBrains/kotlin/commit/437a26684d3529ee2cfdbe54e59d50f4a6f0a611#diff-ba68311bbe28b71196c36a6246000382L176
+    // In simplifyLowerConstraint, we add constraint Nothing? <: T!, approximating it with Nothing? <: T
+    // But before it we already had T <: String from the explicit types
+    // That lead us to constraint contradiction: Nothing? !<: String
+    // We need to discuss the commit above at some point
+    <!INAPPLICABLE_CANDIDATE!>JavaClass<!><String>(null).<!UNRESOLVED_REFERENCE!>foo<!>().<!UNRESOLVED_REFERENCE!>length<!>
+}

compiler/fir/resolve/testData/resolve/problems/flexibleTypeVarAgainstNull.txt

@@ -0,0 +1,4 @@
+FILE: main.kt
+    public final fun main(): R|kotlin/Unit| {
+        <Inapplicable(INAPPLICABLE): [/JavaClass.JavaClass]>#<R|kotlin/String|>(Null(null)).<Unresolved name: foo>#().<Unresolved name: length>#
+    }

compiler/fir/resolve/tests/org/jetbrains/kotlin/fir/FirDiagnosticsTestGenerated.java

@@ -1407,6 +1407,11 @@ public class FirDiagnosticsTestGenerated extends AbstractFirDiagnosticsTest {
             runTest("compiler/fir/resolve/testData/resolve/problems/definitelyNotNullAndOriginalType.kt");
         }
 
+        @TestMetadata("flexibleTypeVarAgainstNull.kt")
+        public void testFlexibleTypeVarAgainstNull() throws Exception {
+            runTest("compiler/fir/resolve/testData/resolve/problems/flexibleTypeVarAgainstNull.kt");
+        }
+
         @TestMetadata("javaAccessorConversion.kt")
         public void testJavaAccessorConversion() throws Exception {
             runTest("compiler/fir/resolve/testData/resolve/problems/javaAccessorConversion.kt");

compiler/fir/resolve/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithLightTreeTestGenerated.java

@@ -1407,6 +1407,11 @@ public class FirDiagnosticsWithLightTreeTestGenerated extends AbstractFirDiagnos
             runTest("compiler/fir/resolve/testData/resolve/problems/definitelyNotNullAndOriginalType.kt");
         }
 
+        @TestMetadata("flexibleTypeVarAgainstNull.kt")
+        public void testFlexibleTypeVarAgainstNull() throws Exception {
+            runTest("compiler/fir/resolve/testData/resolve/problems/flexibleTypeVarAgainstNull.kt");
+        }
+
         @TestMetadata("javaAccessorConversion.kt")
         public void testJavaAccessorConversion() throws Exception {
             runTest("compiler/fir/resolve/testData/resolve/problems/javaAccessorConversion.kt");