K2: Fix controversial inference results with !! operator

The problem was that inference starts working differently depending on
the presence of non-trivial bound (Any instead of Any?), see KT-69266
and KT-69326.

The fix for the issues above would lead to re-work of how we choose the
order in which we fix the variables.

But the cases from the issue were working in K1 correctly because
synthetic call for `!!` has a different signature (not having an upper
bound for the type parameter).

Thus, the solution is just repeating K1 behavior here, too.

^KT-69159 Fixed

Author: dzharkov

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirSyntheticCallGenerator.kt

@@ -471,14 +471,21 @@ class FirSyntheticCallGenerator(
 
     private fun generateSyntheticCheckNotNullFunction(): FirSimpleFunction {
         // Synthetic function signature:
-        //   fun <K : Any> checkNotNull(arg: K?): K
+        //   fun <K> checkNotNull(arg: K?): K & Any
         val functionSymbol = FirSyntheticFunctionSymbol(SyntheticCallableId.CHECK_NOT_NULL)
-        val (typeParameter, typeParameterTypeRef) = generateSyntheticSelectTypeParameter(functionSymbol, isNullableBound = false)
+        val (typeParameter, typeParameterTypeRef) = generateSyntheticSelectTypeParameter(functionSymbol, isNullableBound = true)
 
         return generateMemberFunction(
             functionSymbol,
             SyntheticCallableId.CHECK_NOT_NULL.callableName,
-            typeParameterTypeRef,
+            returnType = typeParameterTypeRef.withReplacedConeType(
+                typeParameterTypeRef.type.makeConeTypeDefinitelyNotNullOrNotNull(
+                    session.typeContext,
+                    // No checks are necessary because we're sure that the type parameter has default (nullable) upper bound.
+                    // At the same time, not having `avoidComprehensiveCheck = true` might lead to plugin initialization issues.
+                    avoidComprehensiveCheck = true,
+                )
+            ),
         ).apply {
             typeParameters += typeParameter
 

compiler/testData/diagnostics/tests/collectionLiterals/kt34515.fir.kt

@@ -1,3 +1,3 @@
 fun main() {
-    <!UNSUPPORTED!>[]<!><!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!><!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
+    <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!><!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!><!UNSUPPORTED!>[]<!><!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!><!>!!<!>
 }

compiler/testData/diagnostics/tests/inference/kt69159Simplified.fir.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // ISSUE: KT-69159
 
 interface MyExpression<F>

compiler/testData/diagnostics/tests/inference/kt69159Simplified.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // ISSUE: KT-69159
 interface MyExpression<F>
 

compiler/testData/diagnostics/tests/inference/kt69159WithComparable.fir.kt

@@ -10,9 +10,9 @@ fun <T : Comparable<T>, S : T> MyExpression<in S>.min2(): MyExpression<T?> = TOD
 fun <K : Any> checkNotNull(k: K?): K = TODO()
 
 fun foo(x: MyExpression<String>) {
-    getElement(x.min1())!!.<!UNRESOLVED_REFERENCE!>length<!>
+    getElement(x.min1())!!.length
     checkNotNull(getElement(x.min1())).<!UNRESOLVED_REFERENCE!>length<!>
 
-    getElement(x.min2())!!.<!UNRESOLVED_REFERENCE!>length<!>
+    getElement(x.min2())!!.length
     checkNotNull(getElement(x.min2())).<!UNRESOLVED_REFERENCE!>length<!>
 }

compiler/testData/diagnostics/tests/inference/kt69159WithComparable.kt

@@ -4,8 +4,8 @@ import java.lang.Exception
 fun <K> id(arg: K): K = arg
 
 fun test() {
-    id(<!UNRESOLVED_REFERENCE!>unresolved<!>)!!
-    <!UNRESOLVED_REFERENCE!>unresolved<!>!!<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
+    <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!><!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>id<!>(<!UNRESOLVED_REFERENCE!>unresolved<!>)!!<!>
+    <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!><!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!><!UNRESOLVED_REFERENCE!>unresolved<!>!!<!>!!<!>
     <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>try {
         <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>id<!>(<!UNRESOLVED_REFERENCE!>unresolved<!>)
     } catch (e: Exception) {