[FIR] Fix K2 behavior according to RULES1

The compiler should only report diagnostics for
comparisons over builtins and identity-less types,
other incompatibilities should be reported
via inspections.

It's ok that in `equalityChecksOnIntegerTypes`
instead of `EQUALITY_NOT_APPLICABLE_WARNING` we get
`EQUALITY_NOT_APPLICABLE`, because
`ProperEqualityChecksInBuilderInferenceCalls`
is already active by default.

This change also replaces the notion of a representative superclass
with the least upper bound.
This makes complex types like
intersection/flexible transparent to
RULES1-based compatibility checks.
One way to look at it is to think
that this is an automatic way of handling
type parameters: automatic picking of
"interesting" bounds, and checking them against one another.

Note that `TypeIntersector.intersectTypes`
for `Int` and `T` where `T` is a type parameter
may return both `{Int & T}` or `null`
depending on `T`-s bounds. At the same time,
for type parameters `T` and `K` it will
always return `{T & K}`.

`ConeTypeIntersector.intersectTypes`, on the
other hand, will always return `{Int & T}`
irrespectively of the bounds. Meaning, the two
intersectors differ in corner cases.

`lowerBoundIfFlexible` call in `isLiterallyTypeParameter` is backed by
the `equalityOfFlexibleTypeParameters` test.

^KT-35134 #fixed-in-k2
^KT-22499 #fixed-in-k2
^KT-46383 #fixed-in-k2

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -3848,6 +3848,46 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.INCOMPATIBLE_ENUM_COMPARISON) { firDiagnostic ->
+        IncompatibleEnumComparisonImpl(
+            firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.a),
+            firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.b),
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
+    add(FirErrors.FORBIDDEN_IDENTITY_EQUALS) { firDiagnostic ->
+        ForbiddenIdentityEqualsImpl(
+            firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.a),
+            firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.b),
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
+    add(FirErrors.FORBIDDEN_IDENTITY_EQUALS_WARNING) { firDiagnostic ->
+        ForbiddenIdentityEqualsWarningImpl(
+            firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.a),
+            firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.b),
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
+    add(FirErrors.DEPRECATED_IDENTITY_EQUALS) { firDiagnostic ->
+        DeprecatedIdentityEqualsImpl(
+            firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.a),
+            firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.b),
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
+    add(FirErrors.IMPLICIT_BOXING_IN_IDENTITY_EQUALS) { firDiagnostic ->
+        ImplicitBoxingInIdentityEqualsImpl(
+            firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.a),
+            firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.b),
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.INC_DEC_SHOULD_NOT_RETURN_UNIT) { firDiagnostic ->
         IncDecShouldNotReturnUnitImpl(
             firDiagnostic as KtPsiDiagnostic,

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -2684,6 +2684,36 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val rightType: KtType
     }
 
+    abstract class IncompatibleEnumComparison : KtFirDiagnostic<KtElement>() {
+        override val diagnosticClass get() = IncompatibleEnumComparison::class
+        abstract val leftType: KtType
+        abstract val rightType: KtType
+    }
+
+    abstract class ForbiddenIdentityEquals : KtFirDiagnostic<KtElement>() {
+        override val diagnosticClass get() = ForbiddenIdentityEquals::class
+        abstract val leftType: KtType
+        abstract val rightType: KtType
+    }
+
+    abstract class ForbiddenIdentityEqualsWarning : KtFirDiagnostic<KtElement>() {
+        override val diagnosticClass get() = ForbiddenIdentityEqualsWarning::class
+        abstract val leftType: KtType
+        abstract val rightType: KtType
+    }
+
+    abstract class DeprecatedIdentityEquals : KtFirDiagnostic<KtElement>() {
+        override val diagnosticClass get() = DeprecatedIdentityEquals::class
+        abstract val leftType: KtType
+        abstract val rightType: KtType
+    }
+
+    abstract class ImplicitBoxingInIdentityEquals : KtFirDiagnostic<KtElement>() {
+        override val diagnosticClass get() = ImplicitBoxingInIdentityEquals::class
+        abstract val leftType: KtType
+        abstract val rightType: KtType
+    }
+
     abstract class IncDecShouldNotReturnUnit : KtFirDiagnostic<KtExpression>() {
         override val diagnosticClass get() = IncDecShouldNotReturnUnit::class
     }

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -3235,6 +3235,41 @@ internal class IncompatibleEnumComparisonErrorImpl(
     override val token: KtLifetimeToken,
 ) : KtFirDiagnostic.IncompatibleEnumComparisonError(), KtAbstractFirDiagnostic<KtElement>
 
+internal class IncompatibleEnumComparisonImpl(
+    override val leftType: KtType,
+    override val rightType: KtType,
+    override val firDiagnostic: KtPsiDiagnostic,
+    override val token: KtLifetimeToken,
+) : KtFirDiagnostic.IncompatibleEnumComparison(), KtAbstractFirDiagnostic<KtElement>
+
+internal class ForbiddenIdentityEqualsImpl(
+    override val leftType: KtType,
+    override val rightType: KtType,
+    override val firDiagnostic: KtPsiDiagnostic,
+    override val token: KtLifetimeToken,
+) : KtFirDiagnostic.ForbiddenIdentityEquals(), KtAbstractFirDiagnostic<KtElement>
+
+internal class ForbiddenIdentityEqualsWarningImpl(
+    override val leftType: KtType,
+    override val rightType: KtType,
+    override val firDiagnostic: KtPsiDiagnostic,
+    override val token: KtLifetimeToken,
+) : KtFirDiagnostic.ForbiddenIdentityEqualsWarning(), KtAbstractFirDiagnostic<KtElement>
+
+internal class DeprecatedIdentityEqualsImpl(
+    override val leftType: KtType,
+    override val rightType: KtType,
+    override val firDiagnostic: KtPsiDiagnostic,
+    override val token: KtLifetimeToken,
+) : KtFirDiagnostic.DeprecatedIdentityEquals(), KtAbstractFirDiagnostic<KtElement>
+
+internal class ImplicitBoxingInIdentityEqualsImpl(
+    override val leftType: KtType,
+    override val rightType: KtType,
+    override val firDiagnostic: KtPsiDiagnostic,
+    override val token: KtLifetimeToken,
+) : KtFirDiagnostic.ImplicitBoxingInIdentityEquals(), KtAbstractFirDiagnostic<KtElement>
+
 internal class IncDecShouldNotReturnUnitImpl(
     override val firDiagnostic: KtPsiDiagnostic,
     override val token: KtLifetimeToken,