[FIR] Report == for identity-less

Since we forbid overriding `equals` for
value classes, and value classes may only
implement interfaces, we know everything
about their `equals`, hence we can
guarantee if it always returns `false`
for different value classes.

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirEqualityCompatibilityChecker.kt

@@ -80,6 +80,7 @@ object FirEqualityCompatibilityChecker : FirEqualityOperatorCallChecker() {
 
     private fun checkEqualityApplicability(l: TypeInfo, r: TypeInfo, context: CheckerContext): Applicability {
         val oneIsBuiltin = l.isBuiltin || r.isBuiltin
+        val oneIsIdentityLess = l.isIdentityLess || r.isIdentityLess
 
         // The compiler should only check comparisons
         // when builtins are involved.
@@ -87,7 +88,7 @@ object FirEqualityCompatibilityChecker : FirEqualityOperatorCallChecker() {
         // the list of special fqNames described in RULES1
 
         return when {
-            oneIsBuiltin && shouldReportAsPerRules1(l, r, context) -> getInapplicabilityFor(l, r)
+            (oneIsBuiltin || oneIsIdentityLess) && shouldReportAsPerRules1(l, r, context) -> getInapplicabilityFor(l, r)
             else -> Applicability.APPLICABLE
         }
     }

compiler/testData/diagnostics/testsWithStdLib/equalityCompatibilityCommonCases.fir.kt

@@ -86,13 +86,13 @@ fun incompatibleIdentityRegardlessNullabilitySmartCast(a: Any?, b: Any?) {
 }
 
 fun incompatibleIdentityRegardlessNullabilityWithValueClasses(c: C?, d: D?) {
-    c == d
+    <!EQUALITY_NOT_APPLICABLE!>c == d<!>
     <!FORBIDDEN_IDENTITY_EQUALS!>c === d<!>
 }
 
 fun incompatibleIdentityRegardlessNullabilityWithValueClassesSmartCast(c: Any?, d: Any?) {
     if (c is C? && d is D?) {
-        c == d
+        <!EQUALITY_NOT_APPLICABLE_WARNING!>c == d<!>
         <!FORBIDDEN_IDENTITY_EQUALS_WARNING!>c === d<!>
     }
 }