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 {

compiler/testData/diagnostics/tests/inference/substitutions/hideNullableLocalTypeInPublicPosition.kt

@@ -1,4 +1,6 @@
 // ISSUE: KT-30054
+// FIR_IDENTICAL
+// !LANGUAGE: +KeepNullabilityWhenApproximatingLocalType
 interface I {
     fun foo(): String
 }
@@ -13,6 +15,6 @@ fun bar(condition: Boolean) /*: I? */ =
 
 fun main() {
     bar(false).<!UNRESOLVED_REFERENCE!>baz<!>()
-    bar(false).foo()
-    bar(false)<!UNNECESSARY_SAFE_CALL!>?.<!>foo()
+    bar(false)<!UNSAFE_CALL!>.<!>foo()
+    bar(false)?.foo()
 }

compiler/testData/diagnostics/tests/inference/substitutions/hideNullableLocalTypeInPublicPosition.txt

@@ -1,6 +1,6 @@
 package
 
-public fun bar(/*0*/ condition: kotlin.Boolean): I
+public fun bar(/*0*/ condition: kotlin.Boolean): I?
 public fun main(): kotlin.Unit
 
 public interface I {

compiler/testData/diagnostics/tests/inference/substitutions/hideNullableLocalTypeInPublicPosition_before.kt

@@ -0,0 +1,18 @@
+// ISSUE: KT-30054
+interface I {
+    fun foo(): String
+}
+
+<!APPROXIMATED_LOCAL_TYPE_WILL_BECOME_NULLABLE!>fun bar(condition: Boolean)<!> /*: I? */ =
+    if (condition)
+        object : I {
+            override fun foo() = "should check for null first"
+            fun baz() = "invisible"
+        }
+    else null
+
+fun main() {
+    bar(false).<!UNRESOLVED_REFERENCE!>baz<!>()
+    bar(false).foo()
+    bar(false)<!UNNECESSARY_SAFE_CALL!>?.<!>foo()
+}

compiler/testData/diagnostics/tests/inference/substitutions/hideNullableLocalTypeInPublicPosition_before.txt

@@ -0,0 +1,11 @@
+package
+
+public fun bar(/*0*/ condition: kotlin.Boolean): I
+public fun main(): kotlin.Unit
+
+public interface I {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -278,6 +278,7 @@ enum class LanguageFeature(
     ForbidInferringTypeVariablesIntoEmptyIntersection(KOTLIN_1_9, kind = BUG_FIX), // KT-51221
     ForbidExtensionCallsOnInlineFunctionalParameters(KOTLIN_1_9, kind = BUG_FIX), // KT-52502
     ForbidInferringPostponedTypeVariableIntoDeclaredUpperBound(KOTLIN_1_9, kind = BUG_FIX), // KT-47986
+    KeepNullabilityWhenApproximatingLocalType(KOTLIN_1_9, kind = BUG_FIX), // KT-53982
     SkipStandaloneScriptsInSourceRoots(KOTLIN_1_9, kind = OTHER), // KT-52525
     ModifierNonBuiltinSuspendFunError(KOTLIN_1_9),
     BreakContinueInInlineLambdas(KOTLIN_1_9, defaultState = State.ENABLED), // KT-1436