Support single-underscore variable names partially

Currently only parameters of lambdas/function expressions
and destructuring entries are allowed

 #KT-3824 In Progress
 #KT-2783 In Progress

compiler/testData/diagnostics/tests/declarationChecks/destructuringDeclarations/ForLoopMissingLoopParameter.kt

@@ -6,9 +6,9 @@ fun useDeclaredVariables() {
 }
 
 fun checkersShouldRun() {
-    for ((@A a, <!UNDERSCORE_IS_RESERVED, UNUSED_VARIABLE!>_<!>)<!SYNTAX!><!>) {
+    for ((@A a, _)<!SYNTAX!><!>) {
 
     }
 }
 
-annotation class A
+annotation class A

compiler/testData/diagnostics/tests/declarationChecks/destructuringDeclarations/destructuringDeclarationAssignedUnresolved.kt

@@ -5,7 +5,7 @@ fun useDeclaredVariables() {
 }
 
 fun checkersShouldRun() {
-    val (@A a, <!UNDERSCORE_IS_RESERVED, UNUSED_VARIABLE!>_<!>) = <!UNRESOLVED_REFERENCE!>unresolved<!>
+    val (@A a, _) = <!UNRESOLVED_REFERENCE!>unresolved<!>
 }
 
-annotation class A
+annotation class A

compiler/testData/diagnostics/tests/declarationChecks/destructuringDeclarations/destructuringDeclarationMissingInitializer.kt

@@ -5,7 +5,7 @@ fun useDeclaredVariables() {
 }
 
 fun checkersShouldRun() {
-    <!INITIALIZER_REQUIRED_FOR_DESTRUCTURING_DECLARATION!>val (@A a, <!UNDERSCORE_IS_RESERVED, UNUSED_VARIABLE!>_<!>)<!>
+    <!INITIALIZER_REQUIRED_FOR_DESTRUCTURING_DECLARATION!>val (@A a, _)<!>
 }
 
-annotation class A
+annotation class A

compiler/testData/diagnostics/tests/declarationChecks/destructuringDeclarations/underscore.kt

@@ -0,0 +1,48 @@
+
+class A {
+    operator fun component1() = 1
+    operator fun component2() = ""
+}
+
+class C {
+    operator fun iterator(): Iterator<A> = null!!
+}
+
+fun test() {
+    for ((x, _) in C()) {
+        foo(x, <!UNRESOLVED_REFERENCE!>_<!>)
+    }
+
+    for ((_, y) in C()) {
+        foo(<!UNRESOLVED_REFERENCE!>_<!>, y)
+    }
+
+    for ((_, _) in C()) {
+        foo(<!UNRESOLVED_REFERENCE!>_<!>, <!UNRESOLVED_REFERENCE!>_<!>)
+    }
+
+    for ((_ : Int, _ : String) in C()) {
+        foo(<!UNRESOLVED_REFERENCE!>_<!>, <!UNRESOLVED_REFERENCE!>_<!>)
+    }
+
+    for ((_ : String, _ : Int) in <!COMPONENT_FUNCTION_RETURN_TYPE_MISMATCH, COMPONENT_FUNCTION_RETURN_TYPE_MISMATCH!>C()<!>) {
+        foo(<!UNRESOLVED_REFERENCE!>_<!>, <!UNRESOLVED_REFERENCE!>_<!>)
+    }
+
+    val (x, _) = A()
+    val (_, y) = A()
+
+    foo(x, y)
+    foo(x, <!UNRESOLVED_REFERENCE!>_<!>)
+    foo(<!UNRESOLVED_REFERENCE!>_<!>, y)
+
+    val (<!REDECLARATION!>`_`<!>, z) = A()
+
+    foo(_, z)
+
+    val (_, <!NAME_SHADOWING, REDECLARATION!>`_`<!>) = A()
+
+    foo(<!TYPE_MISMATCH!>_<!>, y)
+}
+
+fun foo(<!UNUSED_PARAMETER!>x<!>: Int, <!UNUSED_PARAMETER!>y<!>: String) {}

compiler/testData/diagnostics/tests/declarationChecks/destructuringDeclarations/underscore.txt

@@ -0,0 +1,21 @@
+package
+
+public fun foo(/*0*/ x: kotlin.Int, /*1*/ y: kotlin.String): kotlin.Unit
+public fun test(): kotlin.Unit
+
+public final class A {
+    public constructor A()
+    public final operator fun component1(): kotlin.Int
+    public final operator fun component2(): kotlin.String
+    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 C {
+    public constructor C()
+    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 final operator fun iterator(): kotlin.collections.Iterator<A>
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}