Fix unsound smartcast due to returns(null) in safecall

^KT-26382 Fixed

compiler/frontend/src/org/jetbrains/kotlin/contracts/EffectsExtractingVisitor.kt

@@ -19,12 +19,11 @@ package org.jetbrains.kotlin.contracts
 import org.jetbrains.kotlin.builtins.DefaultBuiltIns
 import org.jetbrains.kotlin.contracts.interpretation.ContractInterpretationDispatcher
 import org.jetbrains.kotlin.contracts.model.Computation
+import org.jetbrains.kotlin.contracts.model.ConditionalEffect
+import org.jetbrains.kotlin.contracts.model.ESEffect
 import org.jetbrains.kotlin.contracts.model.Functor
 import org.jetbrains.kotlin.contracts.model.functors.*
-import org.jetbrains.kotlin.contracts.model.structure.CallComputation
+import org.jetbrains.kotlin.contracts.model.structure.*
-import org.jetbrains.kotlin.contracts.model.structure.ESConstant
-import org.jetbrains.kotlin.contracts.model.structure.UNKNOWN_COMPUTATION
-import org.jetbrains.kotlin.contracts.model.structure.lift
 import org.jetbrains.kotlin.contracts.parsing.isEqualsDescriptor
 import org.jetbrains.kotlin.descriptors.FunctionDescriptor
 import org.jetbrains.kotlin.descriptors.ModuleDescriptor
@@ -114,6 +113,20 @@ class EffectsExtractingVisitor(
         )
     }
 
+    override fun visitSafeQualifiedExpression(expression: KtSafeQualifiedExpression, data: Unit?): Computation {
+        val computation = super.visitSafeQualifiedExpression(expression, data)
+        if (computation === UNKNOWN_COMPUTATION) return computation
+
+        // For safecall any clauses of form 'returns(null) -> ...' are incorrect, because safecall can return
+        // null bypassing function's contract, so we have to filter them out
+
+        fun ESEffect.containsReturnsNull(): Boolean =
+            this == ESReturns(ESConstant.NULL) || this is ConditionalEffect && this.simpleEffect.containsReturnsNull()
+
+        val effectsWithoutReturnsNull = computation.effects.filter { !it.containsReturnsNull() }
+        return CallComputation(computation.type, effectsWithoutReturnsNull)
+    }
+
     override fun visitBinaryExpression(expression: KtBinaryExpression, data: Unit): Computation {
         val left = extractOrGetCached(expression.left ?: return UNKNOWN_COMPUTATION)
         val right = extractOrGetCached(expression.right ?: return UNKNOWN_COMPUTATION)

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/safecallAndReturnsNull.kt

@@ -12,6 +12,6 @@ fun Any.nullWhenString(): Any? {
 
 fun test(x: Int?) {
     if (x?.nullWhenString() == null) {
-        <!DEBUG_INFO_SMARTCAST!>x<!>.length
+        x.<!UNRESOLVED_REFERENCE!>length<!>
     }
 }