More precise diagnostics of smart cast impossible #KT-7240 Fixed

idea/testData/checker/infos/SmartCasts.kt

@@ -231,8 +231,7 @@ class Mutable(var <info descr="This property has a backing field">x</info>: Stri
             return <error descr="[SMARTCAST_IMPOSSIBLE] Smart cast to 'kotlin.String' is impossible, because 'x' is a member variable that can be changed from another thread">x</error>
         }
         if (x != null) {
-            // It would be better to have smart cast impossible also here
-            return <error descr="[TYPE_MISMATCH] Type mismatch: inferred type is kotlin.String? but kotlin.String was expected">x</error>
+            return <error descr="[SMARTCAST_IMPOSSIBLE] Smart cast to 'kotlin.String' is impossible, because 'x' is a member variable that can be changed from another thread">x</error>
         }
         if (xx is String) {
             return <error descr="[SMARTCAST_IMPOSSIBLE] Smart cast to 'kotlin.String' is impossible, because 'xx' is a member value that has open or custom getter">xx</error>

idea/testData/refactoring/extractFunction/basic/extensionValUnderSmartCast.kt

@@ -1,14 +0,0 @@
-// SUGGESTED_NAMES: i, getKm
-// PARAM_TYPES: A
-// PARAM_DESCRIPTOR: val a: A defined in test
-class A {
-
-}
-
-val A.meters: Int? get() = 1
-
-fun test() {
-    val a = A()
-    if (a.meters == null) return
-    val km = <selection>a.meters / 10</selection>
-}

idea/testData/refactoring/extractFunction/basic/extensionValUnderSmartCast.kt.after

@@ -1,16 +0,0 @@
-// SUGGESTED_NAMES: i, getKm
-// PARAM_TYPES: A
-// PARAM_DESCRIPTOR: val a: A defined in test
-class A {
-
-}
-
-val A.meters: Int? get() = 1
-
-fun test() {
-    val a = A()
-    if (a.meters == null) return
-    val km = i(a)
-}
-
-private fun i(a: A) = a.meters / 10