[NI] Report unsafe invoke properly for flexible types

`isNullable` check returns `true` for flexible types,
which leads to false positive unsafe invoke error, when
receiver for invoke's property is flexible.
^KT-30695 Fixed

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/components/ResolutionParts.kt

@@ -343,7 +343,7 @@ private fun KotlinResolutionCandidate.checkUnsafeImplicitInvokeAfterSafeCall(arg
         }
     } ?: error("Receiver kind does not match receiver argument")
 
-    if (receiverArgument.receiver.stableType.isNullable()) {
+    if (receiverArgument.isSafeCall && receiverArgument.receiver.stableType.isNullable()) {
         addDiagnostic(UnsafeCallError(argument, isForImplicitInvoke = true))
         return ImplicitInvokeCheckStatus.UNSAFE_INVOKE_REPORTED
     }

compiler/testData/diagnostics/tests/resolve/invoke/kt30695_2.fir.kt

@@ -1,3 +1,13 @@
+// FILE: JavaClass.java
+
+public class JavaClass {
+    public static Cls createFlexible() {
+        return new Cls();
+    }
+}
+
+// FILE: test.kt
+
 class MemberInvokeOwner {
     operator fun invoke() {}
 }
@@ -23,3 +33,11 @@ fun testNotNullableReceiver(notNullable: Cls) {
     notNullable.<!INAPPLICABLE_CANDIDATE!>nullableExtensionProperty<!>()
     notNullable?.extensionProperty()
 }
+
+fun testFlexibleReceiver() {
+    val flexible = JavaClass.createFlexible()
+    flexible.extensionProperty()
+    flexible?.extensionProperty()
+    flexible.<!INAPPLICABLE_CANDIDATE!>nullableExtensionProperty<!>()
+    flexible?.nullableExtensionProperty()
+}

compiler/testData/diagnostics/tests/resolve/invoke/kt30695_2.kt

@@ -1,3 +1,13 @@
+// FILE: JavaClass.java
+
+public class JavaClass {
+    public static Cls createFlexible() {
+        return new Cls();
+    }
+}
+
+// FILE: test.kt
+
 class MemberInvokeOwner {
     operator fun invoke() {}
 }
@@ -23,3 +33,11 @@ fun testNotNullableReceiver(notNullable: Cls) {
     notNullable.<!UNSAFE_IMPLICIT_INVOKE_CALL!>nullableExtensionProperty<!>()
     notNullable<!UNNECESSARY_SAFE_CALL!>?.<!>extensionProperty()
 }
+
+fun testFlexibleReceiver() {
+    val flexible = JavaClass.createFlexible()
+    flexible.extensionProperty()
+    flexible?.<!UNSAFE_IMPLICIT_INVOKE_CALL!>extensionProperty<!>()
+    flexible.<!UNSAFE_IMPLICIT_INVOKE_CALL!>nullableExtensionProperty<!>()
+    flexible?.<!UNSAFE_IMPLICIT_INVOKE_CALL!>nullableExtensionProperty<!>()
+}

compiler/testData/diagnostics/tests/resolve/invoke/kt30695_2.txt

@@ -2,6 +2,7 @@ package
 
 public val Cls.extensionProperty: MemberInvokeOwner
 public val Cls.nullableExtensionProperty: MemberInvokeOwner?
+public fun testFlexibleReceiver(): kotlin.Unit
 public fun testNotNullableReceiver(/*0*/ notNullable: Cls): kotlin.Unit
 public fun testNullableReceiver(/*0*/ nullable: Cls?): kotlin.Unit
 
@@ -13,6 +14,16 @@ public final class Cls {
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 }
 
+public open class JavaClass {
+    public constructor JavaClass()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    public open fun createFlexible(): Cls!
+}
+
 public final class MemberInvokeOwner {
     public constructor MemberInvokeOwner()
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean