PatternMatchingTypingVisitor:

rewrite type inference for 'when' using special constructs.
This fixes several type inference issues for 'when':
KT-9929, KT-9972, KT-10439, KT-10463
along with some other diagnostics-related issues.

compiler/testData/diagnostics/tests/smartCasts/elvisExprNotNull.kt

@@ -1,11 +1,10 @@
 fun foo(s: Any?): String {
     val t = when {
         // To resolve: String U Nothing? = String?
-        s is String -> s
+        s is String -> <!DEBUG_INFO_SMARTCAST!>s<!>
         else -> null
     } ?: ""
-    // Ideally we should have smart cast to String here
-    return <!TYPE_MISMATCH!>t<!>
+    return t
 }
 
 fun bar(s: Any?): String {
@@ -24,10 +23,10 @@ fun bar(s: Any?): String {
 
 fun baz(s: String?, r: String?): String {
     val t = r ?: when {
-        s != null -> s
+        s != null -> <!DEBUG_INFO_SMARTCAST!>s<!>
         else -> ""
     }
-    return <!DEBUG_INFO_SMARTCAST!>t<!>
+    return t
 }
 
 fun withNull(s: String?): String {

compiler/testData/diagnostics/tests/smartCasts/ifWhenExprNonNull.kt

@@ -6,8 +6,8 @@ fun baz(s: String?): String {
         val u: String? = null
         when (u) {
             null -> ""
-            else -> u
+            else -> <!DEBUG_INFO_SMARTCAST!>u<!>
         }
     }
-    return <!DEBUG_INFO_SMARTCAST!>t<!>
+    return t
 }

compiler/testData/diagnostics/tests/smartCasts/smartCastOnElvis.kt

@@ -1,15 +1,15 @@
 fun foo(s: String) = s.length
 
 fun baz(s: String?, r: String?): Int {
-    return foo(<!DEBUG_INFO_SMARTCAST!>r ?: when {
-        s != null -> s
+    return foo(r ?: when {
+        s != null -> <!DEBUG_INFO_SMARTCAST!>s<!>
         else -> ""
-    }<!>)
+    })
 }
 
 fun bar(s: String?, r: String?): Int {
-    return <!DEBUG_INFO_SMARTCAST!>(r ?: when {
-        s != null -> s
+    return (r ?: when {
+        s != null -> <!DEBUG_INFO_SMARTCAST!>s<!>
         else -> ""
-    })<!>.length
+    }).length
 }

compiler/testData/diagnostics/tests/smartCasts/smartCastOnWhen.kt

@@ -1,10 +1,10 @@
 // Smart casts on complex expressions
 fun baz(s: String?): Int {
     if (s == null) return 0
-    return <!DEBUG_INFO_SMARTCAST!>when(<!DEBUG_INFO_SMARTCAST!>s<!>) {
-        "abc" -> s
+    return when(<!DEBUG_INFO_SMARTCAST!>s<!>) {
+        "abc" -> <!DEBUG_INFO_SMARTCAST!>s<!>
         else -> "xyz"
-    }<!>.length
+    }.length
 }
 
 var ss: String? = null

compiler/testData/diagnostics/tests/smartCasts/whenExprNonNull.kt

@@ -3,16 +3,16 @@ fun baz(s: String?): String {
     // if explicit type String is given for t, problem disappears
     val t = when(<!DEBUG_INFO_SMARTCAST!>s<!>) {
         // !! is detected as unnecessary here
-        "abc" -> s
+        "abc" -> <!DEBUG_INFO_SMARTCAST!>s<!>
         else -> "xyz"
     }
-    return <!DEBUG_INFO_SMARTCAST!>t<!>
+    return t
 }
 
 fun foo(s: String?): String {
     val t = when {
-        s != null -> s
+        s != null -> <!DEBUG_INFO_SMARTCAST!>s<!>
         else -> ""
     }
-    return <!DEBUG_INFO_SMARTCAST!>t<!>
+    return t
 }