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

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java

@@ -14366,6 +14366,12 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
                 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 {

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java

@@ -14366,6 +14366,12 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
                 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/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
+        //   fun <K : Any> 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`.
         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)

compiler/testData/diagnostics/tests/collectionLiterals/kt34515.fir.kt

@@ -1,3 +1,3 @@
 fun main() {
-    <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER, NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!><!UNSUPPORTED!>[]<!><!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>!!<!>
+    <!UNSUPPORTED!>[]<!><!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!><!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
 }

compiler/testData/diagnostics/tests/inference/exclExclInference.fir.kt

@@ -0,0 +1,17 @@
+// FIR_DUMP
+// ISSUE: KT-55804
+
+fun foo() {
+    val x: String?
+    x = materialize()!! // Should be treated as non-nullable assignment
+    x.length // Should be allowed
+}
+
+fun <E> materialize(): E = TODO()
+
+fun <F> test(f: F) = f!!
+
+fun main() {
+    test<String>("").length
+    test<String?>(null).length // `.length` should be allowed because return type of "test" should be inferred to `F & Any`
+}

compiler/testData/diagnostics/tests/inference/exclExclInference.fir.txt

@@ -0,0 +1,16 @@
+FILE: exclExclInference.fir.kt
+    public final fun foo(): R|kotlin/Unit| {
+        lval x: R|kotlin/String?|
+        R|<local>/x| = R|/materialize|<R|kotlin/String?|>()!!
+        R|<local>/x|.R|kotlin/String.length|
+    }
+    public final fun <E> materialize(): R|E| {
+        ^materialize R|kotlin/TODO|()
+    }
+    public final fun <F> test(f: R|F|): R|F & Any| {
+        ^test R|<local>/f|!!
+    }
+    public final fun main(): R|kotlin/Unit| {
+        R|/test|<R|kotlin/String|>(String()).R|kotlin/String.length|
+        R|/test|<R|kotlin/String?|>(Null(null)).R|kotlin/String.length|
+    }

compiler/testData/diagnostics/tests/inference/exclExclInference.kt

@@ -0,0 +1,17 @@
+// FIR_DUMP
+// ISSUE: KT-55804
+
+fun foo() {
+    val x: String?
+    x = materialize()!! // Should be treated as non-nullable assignment
+    <!DEBUG_INFO_SMARTCAST!>x<!>.length // Should be allowed
+}
+
+fun <E> materialize(): E = TODO()
+
+fun <F> test(f: F) = f!!
+
+fun main() {
+    test<String>("").length
+    test<String?>(null).length // `.length` should be allowed because return type of "test" should be inferred to `F & Any`
+}

compiler/testData/diagnostics/tests/inference/regressions/kt36342.fir.kt

@@ -4,8 +4,8 @@ import java.lang.Exception
 fun <K> id(arg: K): K = arg
 
 fun test() {
-    <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER, NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>id(<!UNRESOLVED_REFERENCE!>unresolved<!>)!!<!>
+    id(<!UNRESOLVED_REFERENCE!>unresolved<!>)!!
-    <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER, NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!><!UNRESOLVED_REFERENCE!>unresolved<!>!!!!<!>
+    <!UNRESOLVED_REFERENCE!>unresolved<!>!!<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
     <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER, NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER, NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>try {
         id(<!UNRESOLVED_REFERENCE!>unresolved<!>)
     } catch (e: Exception) {

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java

@@ -14372,6 +14372,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                 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 {