K2: Fix generic inference case with !! synthetic call

Result of the `checkNotNull` calls should always be a non-nullable
values.
The simplest idea how to acheive it is adding not-nullable Any bound
to the type parameter declaration.

Existing comment stating about impossibility of such bound seems to be
not 100% correct because it doesn't take into account presence of
definitely-non-nullable X & Any types that allow described case with
nullable generic.

^KT-55804 Fixed

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirLightTreeOldFrontendDiagnosticsTestGenerated.java

@@ -14366,6 +14366,12 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
                 runTest("compiler/testData/diagnostics/tests/inference/errorsOnImplicitInvokeInSimpleCall.kt");
             }
 
+            @Test
+            @TestMetadata("exclExclInference.kt")
+            public void testExclExclInference() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/inference/exclExclInference.kt");
+            }
+
             @Test
             @TestMetadata("expectedTypeAdditionalTest.kt")
             public void testExpectedTypeAdditionalTest() throws Exception {

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirPsiOldFrontendDiagnosticsTestGenerated.java

@@ -14372,6 +14372,12 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
                 runTest("compiler/testData/diagnostics/tests/inference/errorsOnImplicitInvokeInSimpleCall.kt");
             }
 
+            @Test
+            @TestMetadata("exclExclInference.kt")
+            public void testExclExclInference() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/inference/exclExclInference.kt");
+            }
+
             @Test
             @TestMetadata("expectedTypeAdditionalTest.kt")
             public void testExpectedTypeAdditionalTest() throws Exception {

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirSyntheticCallGenerator.kt

@@ -239,7 +239,10 @@ class FirSyntheticCallGenerator(
         containingDeclarations = components.containingDeclarations
     )
 
-    private fun generateSyntheticSelectTypeParameter(functionSymbol: FirSyntheticFunctionSymbol): Pair<FirTypeParameter, FirResolvedTypeRef> {
+    private fun generateSyntheticSelectTypeParameter(
+        functionSymbol: FirSyntheticFunctionSymbol,
+        isNullableBound: Boolean = true,
+    ): Pair<FirTypeParameter, FirResolvedTypeRef> {
         val typeParameterSymbol = FirTypeParameterSymbol()
         val typeParameter =
             buildTypeParameter {
@@ -251,7 +254,12 @@ class FirSyntheticCallGenerator(
                 containingDeclarationSymbol = functionSymbol
                 variance = Variance.INVARIANT
                 isReified = false
-                addDefaultBoundIfNecessary()
+
+                if (!isNullableBound) {
+                    bounds += moduleData.session.builtinTypes.anyType
+                } else {
+                    addDefaultBoundIfNecessary()
+                }
             }
 
         val typeParameterTypeRef = buildResolvedTypeRef { type = ConeTypeParameterTypeImpl(typeParameterSymbol.toLookupTag(), false) }
@@ -280,13 +288,9 @@ class FirSyntheticCallGenerator(
 
     private fun generateSyntheticCheckNotNullFunction(): FirSimpleFunction {
         // Synthetic function signature:
-        //   fun <K> checkNotNull(arg: K?): K
-        //
-        // Note: The upper bound of `K` cannot be `Any` because of the following case:
-        //   fun <X> test(a: X) = a!!
-        // `X` is not a subtype of `Any` and hence cannot satisfy `K` if it had an upper bound of `Any`.
+        //   fun <K : Any> checkNotNull(arg: K?): K
         val functionSymbol = FirSyntheticFunctionSymbol(SyntheticCallableId.CHECK_NOT_NULL)
-        val (typeParameter, returnType) = generateSyntheticSelectTypeParameter(functionSymbol)
+        val (typeParameter, returnType) = generateSyntheticSelectTypeParameter(functionSymbol, isNullableBound = false)
 
         val argumentType = buildResolvedTypeRef {
             type = returnType.type.withNullability(ConeNullability.NULLABLE, session.typeContext)