Names containing only '_' are deprecated (declarations, parameters, import aliases, labels, but not backquoted names)

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

@@ -522,6 +522,8 @@ public interface Errors {
 
     DiagnosticFactory1<JetSimpleNameExpression, JetType> COMPARE_TO_TYPE_MISMATCH = DiagnosticFactory1.create(ERROR);
 
+    DiagnosticFactory0<PsiElement> UNDESCORE_IS_DEPRECATED = DiagnosticFactory0.create(WARNING);
+
     // Labels
 
     DiagnosticFactory0<JetSimpleNameExpression> LABEL_NAME_CLASH = DiagnosticFactory0.create(WARNING);

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

@@ -352,6 +352,8 @@ public class DefaultErrorMessages {
 
         MAP.put(COMPARE_TO_TYPE_MISMATCH, "''compareTo()'' must return kotlin.Int, but returns {0}", RENDER_TYPE);
 
+        MAP.put(UNDESCORE_IS_DEPRECATED, "Names _, __, ___, ..., are deprecated");
+
         MAP.put(RETURN_IN_FUNCTION_WITH_EXPRESSION_BODY,
                 "Returns are not allowed for functions with expression body. Use block body in '{...}'");
         MAP.put(NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY, "A 'return' expression required in a function with a block body ('{...}')");

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

@@ -200,6 +200,7 @@ public class ModifiersChecker {
             for (JetMultiDeclarationEntry multiEntry: multiDeclaration.getEntries()) {
                 annotationChecker.check(multiEntry, trace, null);
                 ModifierCheckerCore.INSTANCE$.check(multiEntry, trace, null);
+                UnderscoreChecker.INSTANCE$.checkNamed(multiEntry, trace);
             }
         }
 

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

@@ -38,7 +38,7 @@ public abstract class TargetPlatform(
     }
 }
 
-private val DEFAULT_DECLARATION_CHECKERS = listOf(DataClassAnnotationChecker(), ConstModifierChecker)
+private val DEFAULT_DECLARATION_CHECKERS = listOf(DataClassAnnotationChecker(), ConstModifierChecker, UnderscoreChecker)
 private val DEFAULT_CALL_CHECKERS = listOf(CapturingInClosureChecker(), InlineCheckerWrapper(), ReifiedTypeParameterSubstitutionChecker())
 private val DEFAULT_TYPE_CHECKERS = emptyList<AdditionalTypeChecker>()
 private val DEFAULT_VALIDATORS = listOf(DeprecatedSymbolValidator(), AccessToPrivateTopLevelSymbolValidator())

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

@@ -0,0 +1,60 @@
+/*
+ * Copyright 2010-2015 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jetbrains.kotlin.resolve
+
+import com.intellij.psi.PsiElement
+import org.jetbrains.kotlin.descriptors.DeclarationDescriptor
+import org.jetbrains.kotlin.diagnostics.DiagnosticSink
+import org.jetbrains.kotlin.diagnostics.Errors
+import org.jetbrains.kotlin.psi.JetCallableDeclaration
+import org.jetbrains.kotlin.psi.JetDeclaration
+import org.jetbrains.kotlin.psi.JetNamedDeclaration
+import org.jetbrains.kotlin.psi.JetTypeParameterListOwner
+
+object UnderscoreChecker : DeclarationChecker {
+
+    fun checkIdentifier(identifier: PsiElement?, diagnosticHolder: DiagnosticSink) {
+        if (identifier == null || identifier.text.isEmpty()) return
+        if (identifier.text.all { it == '_' }) {
+            diagnosticHolder.report(Errors.UNDESCORE_IS_DEPRECATED.on(identifier))
+        }
+    }
+
+    fun checkNamed(declaration: JetNamedDeclaration, diagnosticHolder: DiagnosticSink) {
+        checkIdentifier(declaration.nameIdentifier, diagnosticHolder)
+    }
+
+    override fun check(
+            declaration: JetDeclaration,
+            descriptor: DeclarationDescriptor,
+            diagnosticHolder: DiagnosticSink,
+            bindingContext: BindingContext
+    ) {
+        if (declaration is JetCallableDeclaration) {
+            for (parameter in declaration.valueParameters) {
+                checkNamed(parameter, diagnosticHolder)
+            }
+        }
+        if (declaration is JetTypeParameterListOwner) {
+            for (typeParameter in declaration.typeParameters) {
+                checkNamed(typeParameter, diagnosticHolder)
+            }
+        }
+        if (declaration !is JetNamedDeclaration) return
+        checkNamed(declaration, diagnosticHolder)
+    }
+}

compiler/frontend/src/org/jetbrains/kotlin/resolve/lazy/LazyImportScope.kt

@@ -100,6 +100,11 @@ class LazyImportResolver(
                 }
             }
         }
+        for ((alias, import) in explicitClassImports.entries()) {
+            if (alias.all { it == '_' }) {
+                traceForImportResolve.report(Errors.UNDESCORE_IS_DEPRECATED.on(import))
+            }
+        }
         for (alias in explicitClassImports.keySet()) {
             val imports = explicitClassImports.get(alias)
             if (imports.size() > 1) {

compiler/frontend/src/org/jetbrains/kotlin/types/expressions/BasicExpressionTypingVisitor.java

@@ -867,6 +867,11 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
             @NotNull ExpressionTypingContext context,
             boolean isStatement
     ) {
+        JetSimpleNameExpression labelExpression = expression.getTargetLabel();
+        if (labelExpression != null) {
+            PsiElement labelIdentifier = labelExpression.getIdentifier();
+            UnderscoreChecker.INSTANCE$.checkIdentifier(labelIdentifier, context.trace);
+        }
         JetExpression baseExpression = expression.getBaseExpression();
         if (baseExpression == null) return TypeInfoFactoryPackage.noTypeInfo(context);
 

compiler/testData/diagnostics/tests/Underscore.kt

@@ -0,0 +1,16 @@
+// !DIAGNOSTICS: -DEPRECATED_SYMBOL_WITH_MESSAGE
+
+<!UNDESCORE_IS_DEPRECATED!>import kotlin.Deprecated as ___<!>
+
+@___("") data class Pair(val x: Int, val y: Int)
+
+class <!UNDESCORE_IS_DEPRECATED!>_<!><<!UNDESCORE_IS_DEPRECATED!>________<!>>
+val <!UNDESCORE_IS_DEPRECATED!>______<!> = _<Int>()
+
+fun <!UNDESCORE_IS_DEPRECATED!>__<!>(<!UNDESCORE_IS_DEPRECATED!>___<!>: Int, y: _<Int>?): Int {
+    val (x, <!UNDESCORE_IS_DEPRECATED!>__________<!>) = Pair(___ - 1, 42)
+    val <!UNDESCORE_IS_DEPRECATED!>____<!> = x
+    // in backquotes: allowed
+    val `_` = __________
+    <!UNDESCORE_IS_DEPRECATED!>__<!>@ return if (y != null) __(____, y) else __(`_`, ______)
+}

compiler/testData/diagnostics/tests/Underscore.txt

@@ -0,0 +1,23 @@
+package
+
+public val ______: _<kotlin.Int>
+public fun __(/*0*/ ___: kotlin.Int, /*1*/ y: _<kotlin.Int>?): kotlin.Int
+
+@kotlin.Deprecated(value = "") @kotlin.data() public final class Pair {
+    public constructor Pair(/*0*/ x: kotlin.Int, /*1*/ y: kotlin.Int)
+    public final val x: kotlin.Int
+    public final val y: kotlin.Int
+    public final /*synthesized*/ fun component1(): kotlin.Int
+    public final /*synthesized*/ fun component2(): kotlin.Int
+    public final /*synthesized*/ fun copy(/*0*/ x: kotlin.Int = ..., /*1*/ y: kotlin.Int = ...): Pair
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class _</*0*/ ________> {
+    public constructor _</*0*/ ________>()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsIndexMismatch.kt

@@ -4,7 +4,7 @@
 // FILE: a.kt
 package p
 
-public interface B<T, _> {
+public interface B<T, Z> {
     public fun foo(a: T?)
 }
 
@@ -12,7 +12,7 @@ public interface B<T, _> {
 // FILE: b.kt
 package p
 
-public interface C<X, _> : B<X, _> {
+public interface C<X, Z> : B<X, Z> {
     override fun foo(a: X?)
 
 }
@@ -21,7 +21,7 @@ public interface C<X, _> : B<X, _> {
 // FILE: b.kt
 package p
 
-public interface B<_, T> {
+public interface B<Z, T> {
     public fun foo(a: T?)
 }
 

compiler/testData/diagnostics/tests/multimodule/duplicateMethod/classGenericsInParamsIndexMismatch.txt

@@ -3,7 +3,7 @@ package
 
 package p {
 
-    public interface B</*0*/ T, /*1*/ _> {
+    public interface B</*0*/ T, /*1*/ Z> {
         public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
         public abstract fun foo(/*0*/ a: T?): kotlin.Unit
         public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
@@ -17,11 +17,11 @@ package
 
 package p {
 
-    public interface B</*0*/ T, /*1*/ _> {
+    public interface B</*0*/ T, /*1*/ Z> {
         // -- Module: <m1> --
     }
 
-    public interface C</*0*/ X, /*1*/ _> : p.B<X, _> {
+    public interface C</*0*/ X, /*1*/ Z> : p.B<X, Z> {
         public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
         public abstract override /*1*/ fun foo(/*0*/ a: X?): kotlin.Unit
         public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
@@ -35,7 +35,7 @@ package
 
 package p {
 
-    public interface B</*0*/ _, /*1*/ T> {
+    public interface B</*0*/ Z, /*1*/ T> {
         public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
         public abstract fun foo(/*0*/ a: T?): kotlin.Unit
         public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
@@ -51,11 +51,11 @@ public fun </*0*/ Y, /*1*/ Z> test(/*0*/ b: p.B<Y, Z>?): kotlin.Unit
 
 package p {
 
-    public interface B</*0*/ _, /*1*/ T> {
+    public interface B</*0*/ Z, /*1*/ T> {
         // -- Module: <m3> --
     }
 
-    public interface C</*0*/ X, /*1*/ _> : p.B<X, _> {
+    public interface C</*0*/ X, /*1*/ Z> : p.B<X, Z> {
         // -- Module: <m2> --
     }
 }

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

@@ -541,6 +541,12 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
             doTest(fileName);
         }
 
+        @TestMetadata("Underscore.kt")
+        public void testUnderscore() throws Exception {
+            String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/Underscore.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("UnitByDefaultForFunctionTypes.kt")
         public void testUnitByDefaultForFunctionTypes() throws Exception {
             String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/UnitByDefaultForFunctionTypes.kt");