FIR: Support SAM conversion for interfaces containing Object methods

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/SamResolution.kt

@@ -265,7 +265,7 @@ private fun FirRegularClass.findSingleAbstractMethodByNames(
                 "${functionSymbol.callableId.callableName} is expected to be _root_ide_package_.org.jetbrains.kotlin.fir.declarations.FirSimpleFunction, but ${functionSymbol::class} was found"
             }
 
-            if (firFunction.modality != Modality.ABSTRACT) return@processFunctionsByName ProcessorAction.NEXT
+            if (firFunction.modality != Modality.ABSTRACT || firFunction.isPublicInObject()) return@processFunctionsByName ProcessorAction.NEXT
 
             if (resultMethod != null) {
                 metIncorrectMember = true
@@ -286,7 +286,7 @@ private fun FirRegularClass.hasMoreThenOneAbstractFunctionOrHasAbstractProperty(
     var wasAbstractFunction = false
     for (declaration in declarations) {
         if (declaration is FirProperty && declaration.modality == Modality.ABSTRACT) return true
-        if (declaration is FirSimpleFunction && declaration.modality == Modality.ABSTRACT) {
+        if (declaration is FirSimpleFunction && declaration.modality == Modality.ABSTRACT && !declaration.isPublicInObject()) {
             if (wasAbstractFunction) return true
             wasAbstractFunction = true
         }
@@ -295,6 +295,17 @@ private fun FirRegularClass.hasMoreThenOneAbstractFunctionOrHasAbstractProperty(
     return false
 }
 
+// From the definition of function interfaces in the Java specification (pt. 9.8):
+// "methods that are members of I that do not have the same signature as any public instance method of the class Object"
+// It means that if an interface declares `int hashCode()` then the method won't be taken into account when
+// checking if the interface is SAM.
+private fun FirSimpleFunction.isPublicInObject(): Boolean {
+    // TODO: We make here a conservative check just filtering out methods by name, check the signature as well
+    return name.asString() in PUBLIC_METHOD_NAMES_IN_OBJECT
+}
+
+private val PUBLIC_METHOD_NAMES_IN_OBJECT = setOf("equals", "hashCode", "getClass", "wait", "notify", "notifyAll", "toString")
+
 private fun FirSimpleFunction.getFunctionTypeForAbstractMethod(): ConeLookupTagBasedType {
     val parameterTypes = valueParameters.map {
         it.returnTypeRef.coneTypeSafe<ConeKotlinType>() ?: ConeKotlinErrorType("No type for parameter $it")

compiler/fir/resolve/testData/resolve/stdlib/javaLangComparator.kt

@@ -0,0 +1,3 @@
+fun test_2(list: List<Int>) {
+    val comp = java.util.Comparator<Int> { x, y -> 1 }
+}

compiler/fir/resolve/testData/resolve/stdlib/javaLangComparator.txt

@@ -0,0 +1,7 @@
+FILE: javaLangComparator.kt
+    public final fun test_2(list: R|kotlin/collections/List<kotlin/Int>|): R|kotlin/Unit| {
+        lval comp: R|java/util/Comparator<kotlin/Int>| = Q|java/util|.R|java/util/Comparator|<R|kotlin/Int|>(<L> = Comparator@fun <anonymous>(x: R|kotlin/Int|, y: R|kotlin/Int|): R|kotlin/Int| {
+            Int(1)
+        }
+        )
+    }

compiler/fir/resolve/testData/resolve/stdlib/problems/unresolvedComparator.kt

@@ -2,14 +2,10 @@ fun test_1() {
     val comp = <!UNRESOLVED_REFERENCE!>Comparator<!><Int> { x, y -> 1 }
 }
 
-fun test_2(list: List<Int>) {
-    val comp = java.util.<!UNRESOLVED_REFERENCE!>Comparator<!><Int> { x, y -> 1 }
-}
-
 fun test_3(comparator: java.util.Comparator<Int>) {
     comparator.compare(1, 2)
 }
 
 fun test_4(comparator: Comparator<Int>) {
     comparator.compare(1, 2)
-}
+}

compiler/fir/resolve/testData/resolve/stdlib/problems/unresolvedComparator.txt

@@ -5,12 +5,6 @@ FILE: unresolvedComparator.kt
         }
         )
     }
-    public final fun test_2(list: R|kotlin/collections/List<kotlin/Int>|): R|kotlin/Unit| {
-        lval comp: <ERROR TYPE REF: Unresolved name: Comparator> = Q|java/util|.<Unresolved name: Comparator>#<R|kotlin/Int|>(<L> = Comparator@fun <anonymous>(x: R|class error: No type for parameter|, y: R|class error: No type for parameter|): R|kotlin/Int| {
-            Int(1)
-        }
-        )
-    }
     public final fun test_3(comparator: R|java/util/Comparator<kotlin/Int>|): R|kotlin/Unit| {
         R|<local>/comparator|.R|FakeOverride<java/util/Comparator.compare: R|kotlin/Int|>|(Int(1), Int(2))
     }

compiler/fir/resolve/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithStdlibTestGenerated.java

@@ -118,6 +118,11 @@ public class FirDiagnosticsWithStdlibTestGenerated extends AbstractFirDiagnostic
         runTest("compiler/fir/resolve/testData/resolve/stdlib/javaEnumSynthetic.kt");
     }
 
+    @TestMetadata("javaLangComparator.kt")
+    public void testJavaLangComparator() throws Exception {
+        runTest("compiler/fir/resolve/testData/resolve/stdlib/javaLangComparator.kt");
+    }
+
     @TestMetadata("listPlusAssign.kt")
     public void testListPlusAssign() throws Exception {
         runTest("compiler/fir/resolve/testData/resolve/stdlib/listPlusAssign.kt");