FE1.0: keep nullability when approximating local types (in 1.9)

Report an error that inference will change and type has to be provided
manually in other language versions, since the current behavior is an
unsoundness that can cause runtime NPEs while the new behavior may
silently change overload resolution.

^KT-30054 Fixed

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -619,6 +619,8 @@ public interface Errors {
 
     DiagnosticFactory1<KtDeclaration, Collection<KotlinType>> AMBIGUOUS_ANONYMOUS_TYPE_INFERRED =
             DiagnosticFactory1.create(ERROR, DECLARATION_SIGNATURE);
+    DiagnosticFactory1<KtDeclaration, KotlinType> APPROXIMATED_LOCAL_TYPE_WILL_BECOME_NULLABLE =
+            DiagnosticFactory1.create(WARNING, DECLARATION_SIGNATURE);
 
     DiagnosticFactory1<KtNamedDeclaration, TypeParameterDescriptor>
             KCLASS_WITH_NULLABLE_TYPE_PARAMETER_IN_SIGNATURE = DiagnosticFactory1.create(ERROR, PositioningStrategies.DECLARATION_NAME);

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -1067,6 +1067,8 @@ public class DefaultErrorMessages {
         MAP.put(CATCH_PARAMETER_WITH_DEFAULT_VALUE, "Catch clause parameter may not have a default value");
 
         MAP.put(AMBIGUOUS_ANONYMOUS_TYPE_INFERRED, "Right-hand side has anonymous type. Please specify type explicitly", TO_STRING);
+        MAP.put(APPROXIMATED_LOCAL_TYPE_WILL_BECOME_NULLABLE,
+                "Declaration return type inferred as ''{0}'' instead of ''{0}?''. This will be fixed in Kotlin 1.9. Please specify the type explicitly to avoid future errors or unexpected changes in behavior. See https://youtrack.jetbrains.com/issue/KT-53982 for details.", TO_STRING);
         MAP.put(KCLASS_WITH_NULLABLE_TYPE_PARAMETER_IN_SIGNATURE,
                 "Declaration has an inconsistent return type. Please add upper bound Any for type parameter ''{0}'' or specify return type explicitly", NAME);
 

compiler/frontend/src/org/jetbrains/kotlin/resolve/DescriptorResolver.java

@@ -1094,6 +1094,11 @@ public class DescriptorResolver {
                     substitutedSuperType = approximatingSuperType;
                 }
 
+                if (languageVersionSettings.supportsFeature(LanguageFeature.KeepNullabilityWhenApproximatingLocalType)) {
+                    return TypeUtils.makeNullableIfNeeded(substitutedSuperType, type.isMarkedNullable());
+                } else if (type.isMarkedNullable()) {
+                    trace.report(APPROXIMATED_LOCAL_TYPE_WILL_BECOME_NULLABLE.on(declaration, substitutedSuperType));
+                }
                 return substitutedSuperType;
             }
             else {