Emit MultiPlatformProjects diagnostic on all declarations

where expect/actual modifier is present

^KT-31281

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirExpectActualDeclarationChecker.kt

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.diagnostics.SourceElementPositioningStrategies
 import org.jetbrains.kotlin.diagnostics.reportOn
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.checkers.getModifierList
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.utils.isActual
@@ -26,6 +27,9 @@ import org.jetbrains.kotlin.fir.scopes.impl.declaredMemberScope
 import org.jetbrains.kotlin.fir.symbols.FirBasedSymbol
 import org.jetbrains.kotlin.fir.symbols.SymbolInternals
 import org.jetbrains.kotlin.fir.symbols.impl.*
+import org.jetbrains.kotlin.fir.types.coneType
+import org.jetbrains.kotlin.fir.types.toSymbol
+import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.resolve.multiplatform.ExpectActualCompatibility
 import org.jetbrains.kotlin.resolve.multiplatform.ExpectActualCompatibility.*
 
@@ -34,7 +38,7 @@ object FirExpectActualDeclarationChecker : FirBasicDeclarationChecker() {
     override fun check(declaration: FirDeclaration, context: CheckerContext, reporter: DiagnosticReporter) {
         if (declaration !is FirMemberDeclaration) return
         if (!context.session.languageVersionSettings.supportsFeature(LanguageFeature.MultiPlatformProjects)) {
-            if ((declaration.isExpect || declaration.isActual) && context.containingDeclarations.lastOrNull() is FirFile) {
+            if ((declaration.isExpect || declaration.isActual) && containsExpectOrActualModifier(declaration)) {
                 reporter.reportOn(
                     declaration.source,
                     FirErrors.UNSUPPORTED_FEATURE,
@@ -53,6 +57,12 @@ object FirExpectActualDeclarationChecker : FirBasicDeclarationChecker() {
         }
     }
 
+    private fun containsExpectOrActualModifier(declaration: FirMemberDeclaration): Boolean {
+        return declaration.source.getModifierList()?.let { modifiers ->
+            KtTokens.EXPECT_KEYWORD in modifiers || KtTokens.ACTUAL_KEYWORD in modifiers
+        } ?: false
+    }
+
     private fun checkExpectDeclarationModifiers(
         declaration: FirMemberDeclaration,
         context: CheckerContext,

compiler/testData/diagnostics/tests/sourceCompatibility/noMultiplatformProjects.kt

@@ -10,3 +10,8 @@
 <!UNSUPPORTED_FEATURE!>actual<!> fun foo2() = 42
 <!MUST_BE_INITIALIZED!><!UNSUPPORTED_FEATURE!>actual<!> val bar2: Int<!>
 <!UNSUPPORTED_FEATURE!>actual<!> interface Baz2
+
+<!UNSUPPORTED_FEATURE!>actual<!> class Baz1 {
+    <!UNSUPPORTED_FEATURE!>actual<!> fun foo() {
+    }
+}

compiler/testData/diagnostics/tests/sourceCompatibility/noMultiplatformProjects.txt

@@ -3,6 +3,14 @@ package
 public actual val bar2: kotlin.Int
 public actual fun foo2(): kotlin.Int
 
+public final actual class Baz1 {
+    public constructor Baz1()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final actual fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
 public actual interface Baz2 {
     public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int