Add SingleUnderscoreForParameterName language feature

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

@@ -206,7 +206,7 @@ public class ModifiersChecker {
             for (KtDestructuringDeclarationEntry multiEntry: multiDeclaration.getEntries()) {
                 annotationChecker.check(multiEntry, trace, null);
                 ModifierCheckerCore.INSTANCE.check(multiEntry, trace, null, languageVersionSettings);
-                UnderscoreChecker.INSTANCE.checkNamed(multiEntry, trace, /* allowSingleUnderscore = */ true);
+                UnderscoreChecker.INSTANCE.checkNamed(multiEntry, trace, languageVersionSettings, /* allowSingleUnderscore = */ true);
             }
         }
 

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

@@ -17,6 +17,8 @@
 package org.jetbrains.kotlin.resolve.checkers
 
 import com.intellij.psi.PsiElement
+import org.jetbrains.kotlin.config.LanguageFeature
+import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.descriptors.DeclarationDescriptor
 import org.jetbrains.kotlin.descriptors.VariableDescriptor
 import org.jetbrains.kotlin.descriptors.impl.FunctionExpressionDescriptor
@@ -25,39 +27,57 @@ import org.jetbrains.kotlin.diagnostics.Errors
 import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.resolve.BindingContext
 
-object UnderscoreChecker : SimpleDeclarationChecker {
+object UnderscoreChecker : DeclarationChecker {
 
     @JvmOverloads
-    fun checkIdentifier(identifier: PsiElement?, diagnosticHolder: DiagnosticSink, allowSingleUnderscore: Boolean = false) {
+    fun checkIdentifier(
+            identifier: PsiElement?,
+            diagnosticHolder: DiagnosticSink,
+            languageVersionSettings: LanguageVersionSettings,
+            allowSingleUnderscore: Boolean = false
+    ) {
         if (identifier == null || identifier.text.isEmpty()) return
-        if (identifier.text.all { it == '_' } && (!allowSingleUnderscore || identifier.text.length != 1)) {
+        val isValidSingleUnderscore = allowSingleUnderscore && identifier.text == "_"
+        if (!isValidSingleUnderscore && identifier.text.all { it == '_' }) {
             diagnosticHolder.report(Errors.UNDERSCORE_IS_RESERVED.on(identifier))
         }
+        else if (isValidSingleUnderscore && !languageVersionSettings.supportsFeature(LanguageFeature.SingleUnderscoreForParameterName)) {
+            diagnosticHolder.report(Errors.UNSUPPORTED_FEATURE.on(identifier, LanguageFeature.SingleUnderscoreForParameterName))
+        }
     }
 
     @JvmOverloads
-    fun checkNamed(declaration: KtNamedDeclaration, diagnosticHolder: DiagnosticSink, allowSingleUnderscore: Boolean = false) {
-        checkIdentifier(declaration.nameIdentifier, diagnosticHolder, allowSingleUnderscore)
+    fun checkNamed(
+            declaration: KtNamedDeclaration,
+            diagnosticHolder: DiagnosticSink,
+            languageVersionSettings: LanguageVersionSettings,
+            allowSingleUnderscore: Boolean = false
+    ) {
+        checkIdentifier(declaration.nameIdentifier, diagnosticHolder, languageVersionSettings, allowSingleUnderscore)
     }
 
     override fun check(
             declaration: KtDeclaration,
             descriptor: DeclarationDescriptor,
             diagnosticHolder: DiagnosticSink,
-            bindingContext: BindingContext
+            bindingContext: BindingContext,
+            languageVersionSettings: LanguageVersionSettings
     ) {
         if (declaration is KtProperty && descriptor !is VariableDescriptor) return
         if (declaration is KtCallableDeclaration) {
             for (parameter in declaration.valueParameters) {
-                checkNamed(parameter, diagnosticHolder, allowSingleUnderscore = descriptor is FunctionExpressionDescriptor)
+                checkNamed(
+                        parameter, diagnosticHolder, languageVersionSettings,
+                        allowSingleUnderscore = descriptor is FunctionExpressionDescriptor
+                )
             }
         }
         if (declaration is KtTypeParameterListOwner) {
             for (typeParameter in declaration.typeParameters) {
-                checkNamed(typeParameter, diagnosticHolder)
+                checkNamed(typeParameter, diagnosticHolder, languageVersionSettings)
             }
         }
         if (declaration !is KtNamedDeclaration) return
-        checkNamed(declaration, diagnosticHolder)
+        checkNamed(declaration, diagnosticHolder, languageVersionSettings)
     }
 }

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

@@ -870,7 +870,7 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
         KtSimpleNameExpression labelExpression = expression.getTargetLabel();
         if (labelExpression != null) {
             PsiElement labelIdentifier = labelExpression.getIdentifier();
-            UnderscoreChecker.INSTANCE.checkIdentifier(labelIdentifier, context.trace);
+            UnderscoreChecker.INSTANCE.checkIdentifier(labelIdentifier, context.trace, components.languageVersionSettings);
         }
         KtExpression baseExpression = expression.getBaseExpression();
         if (baseExpression == null) return TypeInfoFactoryKt.noTypeInfo(context);

compiler/frontend/src/org/jetbrains/kotlin/types/expressions/FunctionsTypingVisitor.kt

@@ -146,7 +146,7 @@ internal class FunctionsTypingVisitor(facade: ExpressionTypingInternals) : Expre
         val functionDescriptor = createFunctionLiteralDescriptor(expression, context)
         expression.valueParameters.forEach {
             components.identifierChecker.checkDeclaration(it, context.trace)
-            UnderscoreChecker.checkNamed(it, context.trace, allowSingleUnderscore = true)
+            UnderscoreChecker.checkNamed(it, context.trace, components.languageVersionSettings, allowSingleUnderscore = true)
         }
         val safeReturnType = computeReturnType(expression, context, functionDescriptor, functionTypeExpected)
         functionDescriptor.setReturnType(safeReturnType)

compiler/testData/diagnostics/tests/SingleUnderscoreUnsupported.kt

@@ -0,0 +1,27 @@
+// !LANGUAGE: -SingleUnderscoreForParameterName
+
+data class A(val x: Int, val y: Int)
+
+fun foo(a: Array<A>) {
+    val (<!UNSUPPORTED_FEATURE!>_<!>, y) = A(1, 2)
+    y.hashCode()
+
+    val q1: (Int, String) -> Unit = {
+        <!UNSUPPORTED_FEATURE!>_<!>, s -> s.hashCode()
+    }
+    q1(1, "")
+
+    val q2: (Int, String) -> Unit = fun(<!UNSUPPORTED_FEATURE!>_<!>: Int, s: String) {
+        s.hashCode()
+    }
+    q2(1, "")
+
+    val q3: (A) -> Unit = {
+        (<!UNSUPPORTED_FEATURE!>_<!>, y) -> y.hashCode()
+    }
+    q3(A(2, 3))
+
+    for ((<!UNSUPPORTED_FEATURE!>_<!>, z) in a) {
+        z.hashCode()
+    }
+}

compiler/testData/diagnostics/tests/SingleUnderscoreUnsupported.txt

@@ -0,0 +1,15 @@
+package
+
+public fun foo(/*0*/ a: kotlin.Array<A>): kotlin.Unit
+
+public final data class A {
+    public constructor A(/*0*/ x: kotlin.Int, /*1*/ y: kotlin.Int)
+    public final val x: kotlin.Int
+    public final val y: kotlin.Int
+    public final operator /*synthesized*/ fun component1(): kotlin.Int
+    public final operator /*synthesized*/ fun component2(): kotlin.Int
+    public final /*synthesized*/ fun copy(/*0*/ x: kotlin.Int = ..., /*1*/ y: kotlin.Int = ...): A
+    public open override /*1*/ /*synthesized*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*synthesized*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*synthesized*/ fun toString(): kotlin.String
+}

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

@@ -655,6 +655,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
             doTest(fileName);
         }
 
+        @TestMetadata("SingleUnderscoreUnsupported.kt")
+        public void testSingleUnderscoreUnsupported() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/SingleUnderscoreUnsupported.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("StarsInFunctionCalls.kt")
         public void testStarsInFunctionCalls() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/StarsInFunctionCalls.kt");

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

@@ -29,6 +29,7 @@ enum class LanguageFeature(val sinceVersion: LanguageVersion) {
     DataClassInheritance(KOTLIN_1_1),
     InlineProperties(KOTLIN_1_1),
     DestructuringLambdaParameters(KOTLIN_1_1),
+    SingleUnderscoreForParameterName(KOTLIN_1_1)
     ;
 
     val presentableText: String