[FIR] fix resolution ambiguities between weakly compatible expect and actual

There is a corresponding example inside the stdlib,
see `kotlin.text.startsWith`.

JVM and common counterpart are weakly-compatible
as the actual declaration has default arguments,
which results in `ExpectActualCompatibility.Incompatible.ActualFunctionWithDefaultParameters`

This commit allows such cases.

^KT-61732 fixed

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

@@ -97,6 +97,12 @@ public class FirOldFrontendMPPDiagnosticsWithLightTreeTestGenerated extends Abst
         runTest("compiler/testData/diagnostics/tests/multiplatform/arraySortFixed.kt");
     }
 
+    @Test
+    @TestMetadata("callConflictsOnExpectAndActualWeaklyCompatible.kt")
+    public void testCallConflictsOnExpectAndActualWeaklyCompatible() throws Exception {
+        runTest("compiler/testData/diagnostics/tests/multiplatform/callConflictsOnExpectAndActualWeaklyCompatible.kt");
+    }
+
     @Test
     @TestMetadata("checkNoActualForExpectInLastModule.kt")
     public void testCheckNoActualForExpectInLastModule() throws Exception {

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

@@ -97,6 +97,12 @@ public class FirOldFrontendMPPDiagnosticsWithPsiTestGenerated extends AbstractFi
         runTest("compiler/testData/diagnostics/tests/multiplatform/arraySortFixed.kt");
     }
 
+    @Test
+    @TestMetadata("callConflictsOnExpectAndActualWeaklyCompatible.kt")
+    public void testCallConflictsOnExpectAndActualWeaklyCompatible() throws Exception {
+        runTest("compiler/testData/diagnostics/tests/multiplatform/callConflictsOnExpectAndActualWeaklyCompatible.kt");
+    }
+
     @Test
     @TestMetadata("checkNoActualForExpectInLastModule.kt")
     public void testCheckNoActualForExpectInLastModule() throws Exception {

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/ConeOverloadConflictResolver.kt

@@ -9,6 +9,7 @@ import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.declarations.FirMemberDeclaration
 import org.jetbrains.kotlin.fir.declarations.getSingleExpectForActualOrNull
+import org.jetbrains.kotlin.fir.declarations.getSingleCompatibleOrWeaklyIncompatibleExpectForActualOrNull
 import org.jetbrains.kotlin.fir.declarations.utils.isActual
 import org.jetbrains.kotlin.fir.declarations.utils.isExpect
 import org.jetbrains.kotlin.fir.declarations.utils.modality
@@ -289,7 +290,10 @@ class ConeOverloadConflictResolver(
         val expectForActualSymbols = candidates
             .mapNotNullTo(mutableSetOf()) {
                 val callableSymbol = it.symbol as? FirCallableSymbol<*> ?: return@mapNotNullTo null
-                runIf(callableSymbol.isActual) { callableSymbol.getSingleExpectForActualOrNull() }
+                runIf(callableSymbol.isActual) {
+                    callableSymbol.getSingleExpectForActualOrNull()
+                        ?: callableSymbol.getSingleCompatibleOrWeaklyIncompatibleExpectForActualOrNull()
+                }
             }
 
         return if (expectForActualSymbols.isEmpty()) {

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/ExpectActualAttributes.kt

@@ -11,6 +11,7 @@ import org.jetbrains.kotlin.fir.symbols.impl.FirFunctionSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirRegularClassSymbol
 import org.jetbrains.kotlin.fir.symbols.lazyResolveToPhase
 import org.jetbrains.kotlin.resolve.multiplatform.ExpectActualCompatibility
+import org.jetbrains.kotlin.resolve.multiplatform.isCompatibleOrWeaklyIncompatible
 
 private object ExpectForActualAttributeKey : FirDeclarationDataKey()
 
@@ -26,6 +27,13 @@ fun FirBasedSymbol<*>.getSingleExpectForActualOrNull(): FirBasedSymbol<*>? {
     return expectForActual?.values?.singleOrNull()?.singleOrNull()
 }
 
+fun FirBasedSymbol<*>.getSingleCompatibleOrWeaklyIncompatibleExpectForActualOrNull(): FirBasedSymbol<*>? {
+    val expectForActual = expectForActual ?: return null
+    val compatibleOrWeakCompatible: List<FirBasedSymbol<*>> =
+        expectForActual.entries.singleOrNull { it.key.isCompatibleOrWeaklyIncompatible }?.value ?: return null
+    return compatibleOrWeakCompatible.singleOrNull()
+}
+
 val FirBasedSymbol<*>.expectForActual: ExpectForActualData?
     get() {
         lazyResolveToPhase(FirResolvePhase.EXPECT_ACTUAL_MATCHING)

compiler/testData/diagnostics/tests/multiplatform/callConflictsOnExpectAndActualWeaklyCompatible.fir.kt

@@ -0,0 +1,33 @@
+// ISSUE: KT-61732
+// based on of kotlin.text.startsWith from kotlin-stdlib
+
+// MODULE: common
+// TARGET_PLATFORM: Common
+// FILE: common.kt
+
+expect fun String.foo(prefix: String, ignoreCase: Boolean = false): Boolean
+
+expect fun String.foo(prefix: String, startIndex: Int, ignoreCase: Boolean = false): Boolean
+
+// MODULE: jvm()()(common)
+// TARGET_PLATFORM: JVM
+// FILE: jvm.kt
+
+
+@Suppress(<!ERROR_SUPPRESSION!>"ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS"<!>)
+actual fun String.foo(prefix: String, ignoreCase: Boolean = false): Boolean {
+    return true
+}
+
+@Suppress(<!ERROR_SUPPRESSION!>"ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS"<!>)
+actual fun String.foo(prefix: String, startIndex: Int, ignoreCase: Boolean = false): Boolean {
+    return true
+}
+
+// MODULE: client(jvm)()()
+// TARGET_PLATFORM: JVM
+// FILE: client.kt
+
+fun main() {
+    "".foo("")
+}

compiler/testData/diagnostics/tests/multiplatform/callConflictsOnExpectAndActualWeaklyCompatible.kt

@@ -0,0 +1,33 @@
+// ISSUE: KT-61732
+// based on of kotlin.text.startsWith from kotlin-stdlib
+
+// MODULE: common
+// TARGET_PLATFORM: Common
+// FILE: common.kt
+
+expect fun String.foo(prefix: String, ignoreCase: Boolean = false): Boolean
+
+expect fun String.foo(prefix: String, startIndex: Int, ignoreCase: Boolean = false): Boolean
+
+// MODULE: jvm()()(common)
+// TARGET_PLATFORM: JVM
+// FILE: jvm.kt
+
+
+@Suppress("ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS")
+actual fun String.foo(prefix: String, ignoreCase: Boolean = false): Boolean {
+    return true
+}
+
+@Suppress("ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS")
+actual fun String.foo(prefix: String, startIndex: Int, ignoreCase: Boolean = false): Boolean {
+    return true
+}
+
+// MODULE: client(jvm)()()
+// TARGET_PLATFORM: JVM
+// FILE: client.kt
+
+fun main() {
+    "".foo("")
+}

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

@@ -23020,6 +23020,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                 runTest("compiler/testData/diagnostics/tests/multiplatform/arraySortFixed.kt");
             }
 
+            @Test
+            @TestMetadata("callConflictsOnExpectAndActualWeaklyCompatible.kt")
+            public void testCallConflictsOnExpectAndActualWeaklyCompatible() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/multiplatform/callConflictsOnExpectAndActualWeaklyCompatible.kt");
+            }
+
             @Test
             @TestMetadata("checkNoActualForExpectInLastModule.kt")
             public void testCheckNoActualForExpectInLastModule() throws Exception {

core/compiler.common/src/org/jetbrains/kotlin/resolve/multiplatform/ExpectActualCompatibility.kt

@@ -99,6 +99,10 @@ sealed class ExpectActualCompatibility<out D> {
     object Compatible : ExpectActualCompatibility<Nothing>()
 }
 
+val ExpectActualCompatibility<*>.isCompatibleOrWeaklyIncompatible: Boolean
+    get() = this is ExpectActualCompatibility.Compatible
+            || this is ExpectActualCompatibility.Incompatible.WeakIncompatible
+
 val ExpectActualCompatibility<*>.compatible: Boolean
     get() = this == ExpectActualCompatibility.Compatible