Elvis / if / when now infer error type in case of ErrorType + Nothing #KT-6665 Fixed

(cherry picked from commit 9a50a0b)

compiler/testData/diagnostics/tests/controlFlowAnalysis/unresolvedReference.kt

@@ -0,0 +1,48 @@
+// See KT-6665: unresolved reference (v.bar) should not produce "unreachable code" after it
+fun foo(): Int {
+    val v = 1
+    val <!UNUSED_VARIABLE!>c<!> = v.<!UNRESOLVED_REFERENCE!>bar<!> ?: return 0
+    return 42
+}
+
+fun foo2(): Int {
+    val v = 1
+    val c = if (true) v.<!UNRESOLVED_REFERENCE!>bar<!> else return 3
+    val <!UNUSED_VARIABLE!>b<!> = <!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>c<!>
+    return 42
+}
+
+fun foo3(): Int {
+    val v = 1
+    val c = when {
+        true -> v.<!UNRESOLVED_REFERENCE!>bar<!>
+        else -> return 3
+    }
+    val <!UNUSED_VARIABLE!>b<!> = <!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>c<!>
+    return 42
+}
+
+// Type + ErrorType should give Type, unless Type is Nothing
+
+fun bar(): Int {
+    val v = 1
+    val c = v.<!UNRESOLVED_REFERENCE!>bar<!> ?: 42
+    return c
+}
+
+fun bar2(): Int {
+    val v = 1
+    val c = if (true) v.<!UNRESOLVED_REFERENCE!>bar<!> else 3
+    val b = c
+    return b
+}
+
+fun bar3(): Int {
+    val v = 1
+    val c = when {
+        true -> v.<!UNRESOLVED_REFERENCE!>bar<!>
+        else -> 3
+    }
+    val b = c
+    return b
+}