Introduce UnsafeVariance annotation

compiler/frontend/src/org/jetbrains/kotlin/resolve/VarianceChecker.kt

@@ -26,6 +26,7 @@ import org.jetbrains.kotlin.descriptors.Visibilities
 import org.jetbrains.kotlin.resolve.BindingTrace
 import org.jetbrains.kotlin.resolve.typeBinding.TypeBinding
 import com.intellij.psi.PsiElement
+import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.resolve.typeBinding.createTypeBinding
 import org.jetbrains.kotlin.resolve.typeBinding.createTypeBindingForReturnType
 import org.jetbrains.kotlin.psi.JetCallableDeclaration
@@ -161,7 +162,8 @@ class VarianceChecker(private val trace: BindingTrace) {
             val classifierDescriptor = jetType.getConstructor().getDeclarationDescriptor()
             if (classifierDescriptor is TypeParameterDescriptor) {
                 val declarationVariance = classifierDescriptor.getVariance()
-                if (!declarationVariance.allowsPosition(position)) {
+                if (!declarationVariance.allowsPosition(position)
+                        && !jetType.annotations.hasAnnotation(KotlinBuiltIns.FQ_NAMES.unsafeVariance)) {
                     diagnosticSink.report(
                             Errors.TYPE_VARIANCE_CONFLICT.on(
                                     psiElement,

compiler/testData/builtin-classes.txt

@@ -1272,6 +1272,10 @@ public object Unit {
     /*primary*/ private constructor Unit()
 }
 
+@kotlin.annotation.Target(allowedTargets = {AnnotationTarget.TYPE}) @kotlin.annotation.Retention(value = AnnotationRetention.SOURCE) @kotlin.annotation.MustBeDocumented() @kotlin.annotation.annotation() public final class UnsafeVariance : kotlin.Annotation {
+    /*primary*/ public constructor UnsafeVariance()
+}
+
 @kotlin.annotation.Target(allowedTargets = {AnnotationTarget.VALUE_PARAMETER}) @kotlin.annotation.Retention(value = AnnotationRetention.RUNTIME) @kotlin.annotation.MustBeDocumented() @kotlin.annotation.annotation() private final class crossinline : kotlin.Annotation {
     /*primary*/ public constructor crossinline()
 }

compiler/testData/diagnostics/tests/generics/suppressVarianceConflict.kt

@@ -0,0 +1,17 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_VARIABLE
+
+class A<out T, in E> {
+    fun foo(x: @UnsafeVariance T) {}
+    fun foo(x: @UnsafeVariance T, y: List<@UnsafeVariance T>): @UnsafeVariance E = null!!
+
+    fun bar(): List<@UnsafeVariance E> = null!!
+}
+
+fun foo(x: A<String, Any?>, cs: CharSequence, ls: List<CharSequence>) {
+    val y: A<CharSequence, String> = x
+
+    y.foo(cs)
+    val s: String = y.foo(cs, ls)
+
+    val ls2: List<String> = y.bar()
+}

compiler/testData/diagnostics/tests/generics/suppressVarianceConflict.txt

@@ -0,0 +1,13 @@
+package
+
+public fun foo(/*0*/ x: A<kotlin.String, kotlin.Any?>, /*1*/ cs: kotlin.CharSequence, /*2*/ ls: kotlin.List<kotlin.CharSequence>): kotlin.Unit
+
+public final class A</*0*/ out T, /*1*/ in E> {
+    public constructor A</*0*/ out T, /*1*/ in E>()
+    public final fun bar(): kotlin.List<@kotlin.UnsafeVariance() E>
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final fun foo(/*0*/ x: @kotlin.UnsafeVariance() T): kotlin.Unit
+    public final fun foo(/*0*/ x: @kotlin.UnsafeVariance() T, /*1*/ y: kotlin.List<@kotlin.UnsafeVariance() T>): @kotlin.UnsafeVariance() E
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/tests/org/jetbrains/kotlin/checkers/JetDiagnosticsTestGenerated.java

@@ -6399,6 +6399,12 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("suppressVarianceConflict.kt")
+            public void testSuppressVarianceConflict() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/generics/suppressVarianceConflict.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("compiler/testData/diagnostics/tests/generics/nullability")
             @TestDataPath("$PROJECT_ROOT")
             @RunWith(JUnit3RunnerWithInners.class)

core/builtins/src/kotlin/Annotations.kt

@@ -119,6 +119,14 @@ public annotation class HiddenDeclaration
 @MustBeDocumented
 private annotation class external
 
+/**
+ * Suppresses errors about variance conflict
+ */
+@Target(TYPE)
+@Retention(SOURCE)
+@MustBeDocumented
+public annotation class UnsafeVariance
+
 /**
  * Specifies that the corresponding type should be ignored during type inference.
  */

core/descriptors/src/org/jetbrains/kotlin/builtins/KotlinBuiltIns.java

@@ -151,6 +151,7 @@ public abstract class KotlinBuiltIns {
         public final FqName retention = annotationName("Retention");
         public final FqName repeatable = annotationName("Repeatable");
         public final FqName mustBeDocumented = annotationName("MustBeDocumented");
+        public final FqName unsafeVariance = fqName("UnsafeVariance");
 
         public final FqName mutableList = fqName("MutableList");
         public final FqName mutableSet = fqName("MutableSet");

core/descriptors/src/org/jetbrains/kotlin/descriptors/annotations/Annotations.kt

@@ -26,6 +26,8 @@ public interface Annotations : Iterable<AnnotationDescriptor> {
 
     public fun findAnnotation(fqName: FqName): AnnotationDescriptor?
 
+    public fun hasAnnotation(fqName: FqName) = findAnnotation(fqName) != null
+
     public fun findExternalAnnotation(fqName: FqName): AnnotationDescriptor?
 
     public fun getUseSiteTargetedAnnotations(): List<AnnotationWithTarget>