K2: Fix false-negative RETURN_TYPE_MISMATCH

^KT-53987 Fixed
^KT-55932 Fixed

compiler/fir/analysis-tests/testData/resolve/diagnostics/returnTypeMismatchOnOverride.fir.txt

@@ -180,7 +180,7 @@ FILE: returnTypeMismatchOnOverride.kt
         }
 
         public open fun kek(): R|Z| {
-            ^kek R|/Z.Z<CS errors: /Z.Z>#|()
+            ^kek R|/Z.Z|()
         }
 
     }

compiler/fir/analysis-tests/testData/resolve/extendedCheckers/canBeReplacedWithOperatorAssignment/mutableList.fir.txt

@@ -1,5 +1,5 @@
 FILE: mutableList.kt
     public final fun foo(): R|kotlin/Unit| {
         lvar listVar: R|kotlin/collections/MutableList<kotlin/Int>| = R|kotlin/collections/mutableListOf|<R|kotlin/Int|>(vararg(Int(1), Int(2), Int(3)))
-        R|<local>/listVar| = R|<local>/listVar|.R|kotlin/collections/plus<CS errors: kotlin/collections/plus>#|<R|kotlin/Int|>(Int(4))
+        R|<local>/listVar| = R|<local>/listVar|.R|kotlin/collections/plus|<R|kotlin/Int|>(Int(4))
     }

compiler/fir/analysis-tests/testData/resolve/kt54775.kt

@@ -2,7 +2,6 @@ fun <T> bar(): T {
     return null <!UNCHECKED_CAST!>as T<!>
 }
 
-class X() : <!UNRESOLVED_REFERENCE, UNRESOLVED_REFERENCE!>B<!> by <!ASSIGNMENT_IN_EXPRESSION_CONTEXT!><!VARIABLE_EXPECTED!><!UNRESOLVED_REFERENCE!>get<!>()<!> = bar()<!> {
+class X() : <!UNRESOLVED_REFERENCE, UNRESOLVED_REFERENCE!>B<!> by <!ASSIGNMENT_IN_EXPRESSION_CONTEXT!><!VARIABLE_EXPECTED!><!UNRESOLVED_REFERENCE!>get<!>()<!> = <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>bar<!>()<!> {
     val prop = <!ASSIGNMENT_IN_EXPRESSION_CONTEXT!><!VARIABLE_EXPECTED!><!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>bar<!>()<!> = 2<!>
 }
-

compiler/fir/analysis-tests/testData/resolveWithStdlib/diagnostics/functionReturnTypeMismatchChecker/basic.fir.txt

@@ -20,7 +20,7 @@ FILE: basic.kt
         ^okOneLineFunction Int(10).R|kotlin/Int.plus|(Int(1))
     }
     public final fun errorOneLineFunction(): R|kotlin/String| {
-        ^errorOneLineFunction Int(10).R|kotlin/Int.plus<CS errors: kotlin/Int.plus>#|(Int(1))
+        ^errorOneLineFunction Int(10).R|kotlin/Int.plus|(Int(1))
     }
     public final class A : R|kotlin/Any| {
         public constructor(): R|A| {

compiler/fir/analysis-tests/testData/resolveWithStdlib/diagnostics/functionReturnTypeMismatchChecker/basic.kt

@@ -1,6 +1,6 @@
 // bug: type of the expression in return statement is Char
 fun illegalReturnIf(): Char {
-    return if (1 < 2) 'a' else { 1 }
+    return <!RETURN_TYPE_MISMATCH!>if (1 < 2) 'a' else { 1 }<!>
 }
 
 fun foo(): String {

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

@@ -6566,6 +6566,12 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirDiagnosti
                 runTest("compiler/testData/diagnostics/tests/controlStructures/improperElseInExpression.kt");
             }
 
+            @Test
+            @TestMetadata("incorrectElvis.kt")
+            public void testIncorrectElvis() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/controlStructures/incorrectElvis.kt");
+            }
+
             @Test
             @TestMetadata("jumpAcrossFunctionBoundary.kt")
             public void testJumpAcrossFunctionBoundary() throws Exception {

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

@@ -6560,6 +6560,12 @@ public class FirOldFrontendDiagnosticsWithLightTreeTestGenerated extends Abstrac
                 runTest("compiler/testData/diagnostics/tests/controlStructures/improperElseInExpression.kt");
             }
 
+            @Test
+            @TestMetadata("incorrectElvis.kt")
+            public void testIncorrectElvis() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/controlStructures/incorrectElvis.kt");
+            }
+
             @Test
             @TestMetadata("jumpAcrossFunctionBoundary.kt")
             public void testJumpAcrossFunctionBoundary() throws Exception {

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/coneDiagnosticToFirDiagnostic.kt

@@ -379,10 +379,6 @@ private fun ConstraintSystemError.toDiagnostic(
 
             when (position) {
                 is ConeExpectedTypeConstraintPosition -> {
-                    if (position.expectedTypeMismatchIsReportedInChecker) {
-                        errorsToIgnore.add(this)
-                        return null
-                    }
                     val inferredType =
                         if (!lowerConeType.isNullableNothing)
                             lowerConeType

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirCallCompleter.kt

@@ -166,30 +166,35 @@ class FirCallCompleter(
         mayBeCoercionToUnitApplied: Boolean
     ) {
         val expectedType = expectedTypeRef?.coneTypeSafe<ConeKotlinType>() ?: return
-        val expectedTypeConstraintPosition = ConeExpectedTypeConstraintPosition(expectedTypeMismatchIsReportedInChecker)
 
         val system = candidate.system
         when {
-            !shouldEnforceExpectedType -> {
-                system.addSubtypeConstraintIfCompatible(initialType, expectedType, expectedTypeConstraintPosition)
+            // If type mismatch is assumed to be reported in the checker, we should not add a subtyping constraint that leads to error.
+            // Because it might make resulting type correct while, it's hopefully would be more clear if we let the call be inferred without
+            // the expected type, and then would report diagnostic in the checker.
+            // It's assumed to be safe & sound, because if constraint system has contradictions when expected type is added,
+            // the resulting expression type cannot be inferred to something that is a subtype of `expectedType`,
+            // thus the diagnostic should be reported.
+            !shouldEnforceExpectedType || expectedTypeMismatchIsReportedInChecker -> {
+                system.addSubtypeConstraintIfCompatible(initialType, expectedType, ConeExpectedTypeConstraintPosition)
             }
             isFromCast -> {
                 if (candidate.isFunctionForExpectTypeFromCastFeature()) {
                     system.addSubtypeConstraint(
                         initialType, expectedType,
-                        ConeExpectedTypeConstraintPosition(expectedTypeMismatchIsReportedInChecker = false),
+                        ConeExpectedTypeConstraintPosition,
                     )
                 }
             }
             !expectedType.isUnitOrFlexibleUnit || (!mayBeCoercionToUnitApplied && !expectedTypeMismatchIsReportedInChecker) -> {
-                system.addSubtypeConstraint(initialType, expectedType, expectedTypeConstraintPosition)
+                system.addSubtypeConstraint(initialType, expectedType, ConeExpectedTypeConstraintPosition)
             }
             system.notFixedTypeVariables.isEmpty() -> return
             expectedType.isUnit -> {
-                system.addEqualityConstraintIfCompatible(initialType, expectedType, expectedTypeConstraintPosition)
+                system.addEqualityConstraintIfCompatible(initialType, expectedType, ConeExpectedTypeConstraintPosition)
             }
             else -> {
-                system.addSubtypeConstraintIfCompatible(initialType, expectedType, expectedTypeConstraintPosition)
+                system.addSubtypeConstraintIfCompatible(initialType, expectedType, ConeExpectedTypeConstraintPosition)
             }
         }
     }

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/inference/model/FirConstraintPositionAndErrors.kt

@@ -18,9 +18,7 @@ class ConeFixVariableConstraintPosition(variable: TypeVariableMarker) : FixVaria
 
 class ConeArgumentConstraintPosition(argument: FirElement) : ArgumentConstraintPosition<FirElement>(argument)
 
-class ConeExpectedTypeConstraintPosition(
-    val expectedTypeMismatchIsReportedInChecker: Boolean
-) : ExpectedTypeConstraintPosition<Nothing?>(null)
+object ConeExpectedTypeConstraintPosition : ExpectedTypeConstraintPosition<Nothing?>(null)
 
 class ConeExplicitTypeParameterConstraintPosition(
     typeArgument: FirTypeProjection,