Fix too aggressive data-flow clearing of loop-assigned vars

This is effectively a revert of
447c127036, which was an (incorrect) fix
for KT-22379.

The bug was that we've cleared data-flow info for assigned variables
*after* knowledge that loop condition if 'false' was applied (we can
claim that if loop has no jump-outs). Therefore, we broke smartcasts in
the innocent code like that:

  var x: Int? = null
  while (x == null) {
      x = readPotentiallyNullableInt()
  }
  // x should be non-null here

Commit reverts that "fix" for 1.3.0 and postpones deprecation until 1.4

KT-22379 Open
KT-27084 Fixed

compiler/frontend/src/org/jetbrains/kotlin/types/expressions/ControlStructureTypingVisitor.java

@@ -286,16 +286,9 @@ public class ControlStructureTypingVisitor extends ExpressionTypingVisitor {
             dataFlowInfo = dataFlowInfo.and(loopVisitor.clearDataFlowInfoForAssignedLocalVariables(bodyTypeInfo.getJumpFlowInfo(),
                                                                                                    components.languageVersionSettings));
         }
-
-        DataFlowInfo conservativeInfoAfterLoop =
-                components.languageVersionSettings.supportsFeature(LanguageFeature.SoundSmartcastFromLoopConditionForLoopAssignedVariables)
-                ? loopVisitor.clearDataFlowInfoForAssignedLocalVariables(dataFlowInfo, components.languageVersionSettings)
-                : dataFlowInfo;
-
-
         return components.dataFlowAnalyzer
                 .checkType(bodyTypeInfo.replaceType(components.builtIns.getUnitType()), expression, contextWithExpectedType)
-                .replaceDataFlowInfo(conservativeInfoAfterLoop);
+                .replaceDataFlowInfo(dataFlowInfo);
     }
 
     private boolean containsJumpOutOfLoop(@NotNull KtExpression expression, ExpressionTypingContext context) {
@@ -466,14 +459,9 @@ public class ControlStructureTypingVisitor extends ExpressionTypingVisitor {
             bodyTypeInfo = loopRangeInfo;
         }
 
-        DataFlowInfo conservativeInfoAfterLoop =
-                components.languageVersionSettings.supportsFeature(LanguageFeature.SoundSmartcastFromLoopConditionForLoopAssignedVariables)
-                ? loopVisitor.clearDataFlowInfoForAssignedLocalVariables(loopRangeInfo.getDataFlowInfo(), components.languageVersionSettings)
-                : loopRangeInfo.getDataFlowInfo();
-
         return components.dataFlowAnalyzer
                 .checkType(bodyTypeInfo.replaceType(components.builtIns.getUnitType()), expression, contextWithExpectedType)
-                .replaceDataFlowInfo(conservativeInfoAfterLoop);
+                .replaceDataFlowInfo(loopRangeInfo.getDataFlowInfo());
     }
 
     private VariableDescriptor createLoopParameterDescriptor(

compiler/testData/diagnostics/tests/smartCasts/loops/whileNullAssignToSomething.kt

@@ -5,5 +5,5 @@ fun main(args: Array<String>) {
         if (i == 10) result = "non null"
         else i++
     }
-    result<!UNSAFE_CALL!>.<!>length
+    <!DEBUG_INFO_SMARTCAST!>result<!>.length
 }

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -83,7 +83,6 @@ enum class LanguageFeature(
     NormalizeConstructorCalls(KOTLIN_1_3),
     StrictJavaNullabilityAssertions(KOTLIN_1_3, kind = BUG_FIX),
     SoundSmartcastForEnumEntries(KOTLIN_1_3, kind = BUG_FIX),
-    SoundSmartcastFromLoopConditionForLoopAssignedVariables(KOTLIN_1_3, kind = BUG_FIX),
     DslMarkerOnFunctionTypeReceiver(KOTLIN_1_4, kind = BUG_FIX),
     ProhibitErroneousExpressionsInAnnotationsWithUseSiteTargets(KOTLIN_1_3, kind = BUG_FIX),
     NewCapturedReceiverFieldNamingConvention(KOTLIN_1_3, kind = BUG_FIX),
@@ -94,6 +93,8 @@ enum class LanguageFeature(
     NoConstantValueAttributeForNonConstVals(KOTLIN_1_4, kind = BUG_FIX),
 
     ProperVisibilityForCompanionObjectInstanceField(sinceVersion = null, kind = BUG_FIX),
+    // Temporarily disabled, see KT-27084/KT-22379
+    SoundSmartcastFromLoopConditionForLoopAssignedVariables(sinceVersion = null, kind = BUG_FIX),
 
     ProperIeee754Comparisons(sinceVersion = null, defaultState = State.DISABLED, kind = BUG_FIX),