K2: Fix incorrect inference of delegated property type

It was working incorrectly, because we've been trying to fix
P1 variable to intersectTypes(String?, StubForP2) that should result
to String? because we've got stubEqualsToAnything enabled there,
but nullability was being chosen incorrectly because
`StubForP2.isNullableType()` returned false

NB: The code inside `is ConeTypeVariable` case wasn't working properly
because it always `lookupTag.toSymbol(session)` always returned null,
thus there was effectively five dead lines of code there.

^KT-57814 Fixed
^KT-57921 Related

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

@@ -8829,6 +8829,12 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
                     runTest("compiler/testData/diagnostics/tests/delegatedProperty/inference/callableReferenceArgumentInDelegatedExpression.kt");
                 }
 
+                @Test
+                @TestMetadata("decoratedLambda.kt")
+                public void testDecoratedLambda() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/delegatedProperty/inference/decoratedLambda.kt");
+                }
+
                 @Test
                 @TestMetadata("delegateExpressionAsLambda.kt")
                 public void testDelegateExpressionAsLambda() throws Exception {

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

@@ -8829,6 +8829,12 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
                     runTest("compiler/testData/diagnostics/tests/delegatedProperty/inference/callableReferenceArgumentInDelegatedExpression.kt");
                 }
 
+                @Test
+                @TestMetadata("decoratedLambda.kt")
+                public void testDecoratedLambda() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/delegatedProperty/inference/decoratedLambda.kt");
+                }
+
                 @Test
                 @TestMetadata("delegateExpressionAsLambda.kt")
                 public void testDelegateExpressionAsLambda() throws Exception {

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

@@ -8829,6 +8829,12 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
                     runTest("compiler/testData/diagnostics/tests/delegatedProperty/inference/callableReferenceArgumentInDelegatedExpression.kt");
                 }
 
+                @Test
+                @TestMetadata("decoratedLambda.kt")
+                public void testDecoratedLambda() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/delegatedProperty/inference/decoratedLambda.kt");
+                }
+
                 @Test
                 @TestMetadata("delegateExpressionAsLambda.kt")
                 public void testDelegateExpressionAsLambda() throws Exception {

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

@@ -8835,6 +8835,12 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
                     runTest("compiler/testData/diagnostics/tests/delegatedProperty/inference/callableReferenceArgumentInDelegatedExpression.kt");
                 }
 
+                @Test
+                @TestMetadata("decoratedLambda.kt")
+                public void testDecoratedLambda() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/delegatedProperty/inference/decoratedLambda.kt");
+                }
+
                 @Test
                 @TestMetadata("delegateExpressionAsLambda.kt")
                 public void testDelegateExpressionAsLambda() throws Exception {

compiler/fir/providers/src/org/jetbrains/kotlin/fir/resolve/LookupTagUtils.kt

@@ -31,6 +31,7 @@ fun ConeClassifierLookupTag.toSymbol(useSiteSession: FirSession): FirClassifierS
     when (this) {
         is ConeClassLikeLookupTag -> toSymbol(useSiteSession)
         is ConeClassifierLookupTagWithFixedSymbol -> this.symbol
+        // TODO: replace null with error(), see KT-57921
         else -> null
     }
 
@@ -136,4 +137,3 @@ private fun Collection<ConeKotlinType>.findClassRepresentationThatIsSubtypeOf(
     supertype: ConeKotlinType,
     session: FirSession
 ): ConeClassLikeLookupTag? = firstOrNull { it.isSubtypeOf(supertype, session) }?.findClassRepresentation(supertype, session)
-

compiler/fir/providers/src/org/jetbrains/kotlin/fir/types/ConeTypeContext.kt

@@ -480,13 +480,12 @@ interface ConeTypeContext : TypeSystemContext, TypeSystemOptimizationContext, Ty
         return when (this) {
             is ConeFlexibleType -> this.upperBound.isNullableType()
             is ConeTypeParameterType -> lookupTag.symbol.allBoundsAreNullableOrUnresolved()
-            is ConeTypeVariableType -> {
-                val symbol = lookupTag.toSymbol(session) ?: return false
-                when (symbol) {
-                    is FirClassSymbol -> false
-                    is FirTypeAliasSymbol -> symbol.fir.expandedConeType?.isNullableType() ?: false
-                    is FirTypeParameterSymbol -> symbol.allBoundsAreNullableOrUnresolved()
-                }
+            // NB: There's no branch for ConeTypeVariableType, i.e. it always returns false for them
+            // And while it seems reasonable to have similar semantics as for stub types, it would make some diagnostic test failing
+            // Thus, we leave the same semantics only for stubs (similar to TypeUtils.isNullableType)
+            is ConeStubType -> {
+                val symbol = (this.constructor.variable.defaultType.lookupTag.originalTypeParameter as? ConeTypeParameterLookupTag)?.symbol
+                symbol == null || symbol.allBoundsAreNullableOrUnresolved()
             }
             is ConeIntersectionType -> intersectedTypes.all { it.isNullableType() }
             is ConeClassLikeType -> directExpansionType(session)?.isNullableType() ?: false

compiler/testData/diagnostics/tests/delegatedProperty/inference/decoratedLambda.kt

@@ -0,0 +1,21 @@
+// FIR_IDENTICAL
+// ISSUE: KT-57814
+import kotlin.reflect.KProperty
+
+fun <P2> xComponent(
+    builder: (prop1: P2) -> Unit
+): (prop1: P2) -> Unit = {}
+
+operator fun <P1> ((prop1: P1) -> Unit).getValue(
+    thisRef: Any?,
+    property: KProperty<*>
+): (prop1: P1) -> Unit = this
+
+val pdfDocumentViewer by xComponent { _: String? ->
+}
+
+fun xPDFDocumentViewer(
+    href: String?
+) = pdfDocumentViewer(
+    href // Should be OK to pass nullable String? there
+)

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

@@ -8835,6 +8835,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                     runTest("compiler/testData/diagnostics/tests/delegatedProperty/inference/callableReferenceArgumentInDelegatedExpression.kt");
                 }
 
+                @Test
+                @TestMetadata("decoratedLambda.kt")
+                public void testDecoratedLambda() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/delegatedProperty/inference/decoratedLambda.kt");
+                }
+
                 @Test
                 @TestMetadata("delegateExpressionAsLambda.kt")
                 public void testDelegateExpressionAsLambda() throws Exception {