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/functionLiterals/destructuringInLambdas/underscore.kt

@@ -0,0 +1,62 @@
+// !CHECK_TYPE
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+data class A(val x: Int, val y: String)
+data class B(val u: Double, val w: Short)
+
+fun foo(block: (A) -> Unit) { }
+
+fun bar() {
+    foo { (_, b) ->
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+        b checkType { _<String>() }
+    }
+
+    foo { (a, _) ->
+        a checkType { _<Int>() }
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+    }
+
+    foo { (_, _) ->
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+    }
+
+    foo { (_: Int, b: String) ->
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+        b checkType { _<String>() }
+    }
+
+    foo { (a: Int, _: String) ->
+        a checkType { _<Int>() }
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+    }
+
+    foo { (_: Int, _: String) ->
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+    }
+
+    foo { (_, _): A ->
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+    }
+
+    foo { (`_`, _) ->
+        _ checkType { _<Int>() }
+    }
+
+    foo { (_, `_`) ->
+        _ checkType { _<String>() }
+    }
+
+    foo { (<!REDECLARATION!>`_`<!>, <!REDECLARATION!>`_`<!>) ->
+        _ checkType { _<String>() }
+    }
+
+    foo { (<!COMPONENT_FUNCTION_RETURN_TYPE_MISMATCH!>_: String<!>, b) ->
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+        b checkType { _<String>() }
+    }
+
+    foo { <!EXPECTED_PARAMETER_TYPE_MISMATCH!>(_, b): B<!> ->
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+        b checkType { _<Short>() }
+    }
+}

compiler/testData/diagnostics/tests/functionLiterals/destructuringInLambdas/underscore.txt

@@ -0,0 +1,28 @@
+package
+
+public fun bar(): kotlin.Unit
+public fun foo(/*0*/ block: (A) -> kotlin.Unit): kotlin.Unit
+
+public final data class A {
+    public constructor A(/*0*/ x: kotlin.Int, /*1*/ y: kotlin.String)
+    public final val x: kotlin.Int
+    public final val y: kotlin.String
+    public final operator /*synthesized*/ fun component1(): kotlin.Int
+    public final operator /*synthesized*/ fun component2(): kotlin.String
+    public final /*synthesized*/ fun copy(/*0*/ x: kotlin.Int = ..., /*1*/ y: kotlin.String = ...): 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
+}
+
+public final data class B {
+    public constructor B(/*0*/ u: kotlin.Double, /*1*/ w: kotlin.Short)
+    public final val u: kotlin.Double
+    public final val w: kotlin.Short
+    public final operator /*synthesized*/ fun component1(): kotlin.Double
+    public final operator /*synthesized*/ fun component2(): kotlin.Short
+    public final /*synthesized*/ fun copy(/*0*/ u: kotlin.Double = ..., /*1*/ w: kotlin.Short = ...): B
+    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/testData/diagnostics/tests/functionLiterals/underscopeParameters.kt

@@ -0,0 +1,49 @@
+// !CHECK_TYPE
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+fun foo(block: (Int, String) -> Unit) { }
+fun foobar(block: (Double) -> Unit) { }
+
+fun bar() {
+    foo { _, b ->
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+        b checkType { _<String>() }
+    }
+
+    foo { a, _ ->
+        a checkType { _<Int>() }
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+    }
+
+    foo { _, _ ->
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+    }
+
+    foo { _: Int, b: String ->
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+        b checkType { _<String>() }
+    }
+
+    foo { a: Int, _: String ->
+        a checkType { _<Int>() }
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+    }
+
+    foo { _: Int, _: String ->
+        <!UNRESOLVED_REFERENCE!>_<!>.<!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>hashCode<!>()
+    }
+
+    foo { `_`, _ ->
+        _ checkType { _<Int>() }
+    }
+
+    foo { _, `_` ->
+        _ checkType { _<String>() }
+    }
+
+    foo { <!REDECLARATION, REDECLARATION!>`_`<!>, <!REDECLARATION, REDECLARATION!>`_`<!> ->
+        _ checkType { _<String>() }
+    }
+
+    foo(fun(x: Int, _: String) {})
+}

compiler/testData/diagnostics/tests/functionLiterals/underscopeParameters.txt

@@ -0,0 +1,5 @@
+package
+
+public fun bar(): kotlin.Unit
+public fun foo(/*0*/ block: (kotlin.Int, kotlin.String) -> kotlin.Unit): kotlin.Unit
+public fun foobar(/*0*/ block: (kotlin.Double) -> kotlin.Unit): kotlin.Unit