Front-end fix: result type corrected for increment / decrement expressions. Smart cast allowed on a postfix increment result. See KT-7561.

Now result type is receiver type for postfix increment, or increment result type for prefix increment.
A set of relevant tests.

compiler/testData/diagnostics/tests/smartCasts/varnotnull/postfixNotnullClassIncrement.kt

@@ -0,0 +1,13 @@
+class MyClass
+
+// In principle it is not correct, MyClass? is not a subtype of MyClass
+fun MyClass.inc(): MyClass? { return null }
+
+public fun box() : MyClass? {
+    var i : MyClass? 
+    i = MyClass()
+    // type of j can be inferred as MyClass()
+    var j = <!DEBUG_INFO_SMARTCAST!>i<!>++
+    <!DEBUG_INFO_SMARTCAST!>j<!>.hashCode()
+    return i
+}

compiler/testData/diagnostics/tests/smartCasts/varnotnull/postfixNotnullClassIncrement.txt

@@ -0,0 +1,11 @@
+package
+
+public fun box(): MyClass?
+internal fun MyClass.inc(): MyClass?
+
+internal final class MyClass {
+    public constructor MyClass()
+    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/smartCasts/varnotnull/postfixNullableClassIncrement.kt

@@ -0,0 +1,12 @@
+class MyClass
+
+// Correct at compile time but wrong at run-time
+fun MyClass?.inc(): MyClass? { return null }
+
+public fun box() : MyClass? {
+    var i : MyClass? 
+    i = MyClass()
+    var j = i++
+    <!DEBUG_INFO_SMARTCAST!>j<!>.hashCode()
+    return i
+}

compiler/testData/diagnostics/tests/smartCasts/varnotnull/postfixNullableClassIncrement.txt

@@ -0,0 +1,11 @@
+package
+
+public fun box(): MyClass?
+internal fun MyClass?.inc(): MyClass?
+
+internal final class MyClass {
+    public constructor MyClass()
+    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/smartCasts/varnotnull/postfixNullableIncrement.kt

@@ -0,0 +1,8 @@
+fun Int?.inc(): Int? { return this }
+
+public fun box(arg: Int?) : Int? {
+    var i : Int? = arg
+    var j = i++
+    j<!UNSAFE_CALL!>.<!>toInt()
+    return i
+}

compiler/testData/diagnostics/tests/smartCasts/varnotnull/postfixNullableIncrement.txt

@@ -0,0 +1,4 @@
+package
+
+public fun box(/*0*/ arg: kotlin.Int?): kotlin.Int?
+internal fun kotlin.Int?.inc(): kotlin.Int?

compiler/testData/diagnostics/tests/smartCasts/varnotnull/prefixNotnullClassIncrement.kt

@@ -0,0 +1,13 @@
+class MyClass
+
+// In principle it is not correct, MyClass? is not a subtype of MyClass
+fun MyClass.inc(): MyClass? { return null }
+
+public fun box() {
+    var i : MyClass? 
+    i = MyClass()
+    // Type of j should be inferred as MyClass?
+    var j = ++<!DEBUG_INFO_SMARTCAST!>i<!>
+    // j is null so call is unsafe
+    j<!UNSAFE_CALL!>.<!>hashCode()
+}

compiler/testData/diagnostics/tests/smartCasts/varnotnull/prefixNotnullClassIncrement.txt

@@ -0,0 +1,11 @@
+package
+
+public fun box(): kotlin.Unit
+internal fun MyClass.inc(): MyClass?
+
+internal final class MyClass {
+    public constructor MyClass()
+    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/smartCasts/varnotnull/prefixNullableClassIncrement.kt

@@ -0,0 +1,13 @@
+class MyClass
+
+// Correct at compile time but wrong at run-time
+fun MyClass?.inc(): MyClass? { return null }
+
+public fun box() {
+    var i : MyClass? 
+    i = MyClass()
+    // type of j should be MyClass?
+    var j = ++i
+    // j is null so call should be unsafe
+    j<!UNSAFE_CALL!>.<!>hashCode()
+}

compiler/testData/diagnostics/tests/smartCasts/varnotnull/prefixNullableClassIncrement.txt

@@ -0,0 +1,11 @@
+package
+
+public fun box(): kotlin.Unit
+internal fun MyClass?.inc(): MyClass?
+
+internal final class MyClass {
+    public constructor MyClass()
+    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/smartCasts/varnotnull/prefixNullableIncrement.kt

@@ -0,0 +1,8 @@
+fun Int?.inc(): Int? { return this }
+
+public fun box(arg: Int?) : Int? {
+    var i = arg
+    var j = ++i
+    j<!UNSAFE_CALL!>.<!>toInt()
+    return ++j
+}

compiler/testData/diagnostics/tests/smartCasts/varnotnull/prefixNullableIncrement.txt

@@ -0,0 +1,4 @@
+package
+
+public fun box(/*0*/ arg: kotlin.Int?): kotlin.Int?
+internal fun kotlin.Int?.inc(): kotlin.Int?