K2: Replicate K1 enhancement behavior for warning-mode top-level types

^KT-56657 Fixed
^KT-57307 Related

compiler/testData/diagnostics/foreignAnnotationsTests/java8Tests/warningModeForHeadType.kt

@@ -0,0 +1,38 @@
+// FIR_IDENTICAL
+// FULL_JDK
+// !DIAGNOSTICS: -UNUSED_VARIABLE -UNUSED_PARAMETER
+
+// FILE: ElementTypesAreNonnullByDefault.java
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+import javax.annotation.Nonnull;
+import javax.annotation.meta.TypeQualifierDefault;
+
+@Retention(RetentionPolicy.RUNTIME)
+@Target({ElementType.TYPE})
+@TypeQualifierDefault({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER})
+@Nonnull
+@interface ElementTypesAreNonnullByDefault {
+}
+
+// FILE: Maps.java
+import org.checkerframework.checker.nullness.qual.Nullable;
+// Here it's important that @ElementTypesAreNonnullByDefault is a JSR-305 default qualifier and disabled by default (resulting in warnings-only)
+// Thus return type (head type) is considered as warningly-annotated as not-nullable and that makes annotations on bounds for K and V
+// be effectively ignored on non-warnings level.
+@ElementTypesAreNonnullByDefault
+public final class Maps {
+    public static <K extends @Nullable Object, V extends @Nullable Object> java.util.HashMap<K,V> newHashMap() { return null; }
+}
+
+// FILE: main.kt
+
+fun foo() {
+    val x = Maps.newHashMap<String, Int>()
+    x.put("", 1)
+    // If there were no @ElementTypesAreNonnullByDefault on the Maps class, there would be an error on `null` argument because the type of `x`
+    // would be `HashMap<String, Int>!`, i.e. with non-flexible type arguments, thus not allowing nulls.
+    x.put("", null)
+}