NI: Support forking inference with heuristics

Mostly, it only affects FIR

It partially allows to consider several variance of constraints like
A<Int> & A<T> <: A<X_var> that are mostly brought by smart casts

^KT-49542 Fixed
^KT-50489 Relates

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -1524,6 +1524,13 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.INFERENCE_UNSUCCESSFUL_FORK) { firDiagnostic ->
+        InferenceUnsuccessfulForkImpl(
+            firDiagnostic.a,
+            firDiagnostic as KtPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.OVERLOAD_RESOLUTION_AMBIGUITY) { firDiagnostic ->
         OverloadResolutionAmbiguityImpl(
             firDiagnostic.a.map { firBasedSymbol ->

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -1095,6 +1095,11 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = RedundantSpreadOperatorInNamedFormInFunction::class
     }
 
+    abstract class InferenceUnsuccessfulFork : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = InferenceUnsuccessfulFork::class
+        abstract val message: String
+    }
+
     abstract class OverloadResolutionAmbiguity : KtFirDiagnostic<PsiElement>() {
         override val diagnosticClass get() = OverloadResolutionAmbiguity::class
         abstract val candidates: List<KtSymbol>

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -1312,6 +1312,12 @@ internal class RedundantSpreadOperatorInNamedFormInFunctionImpl(
     override val token: ValidityToken,
 ) : KtFirDiagnostic.RedundantSpreadOperatorInNamedFormInFunction(), KtAbstractFirDiagnostic<KtExpression>
 
+internal class InferenceUnsuccessfulForkImpl(
+    override val message: String,
+    override val firDiagnostic: KtPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.InferenceUnsuccessfulFork(), KtAbstractFirDiagnostic<PsiElement>
+
 internal class OverloadResolutionAmbiguityImpl(
     override val candidates: List<KtSymbol>,
     override val firDiagnostic: KtPsiDiagnostic,

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosisCompilerTestFE10TestdataTestGenerated.java

@@ -13803,6 +13803,12 @@ public class DiagnosisCompilerTestFE10TestdataTestGenerated extends AbstractDiag
                 runTest("compiler/testData/diagnostics/tests/inference/selectOfLambdaWithExtensionEnabled.kt");
             }
 
+            @Test
+            @TestMetadata("smartCastFork.kt")
+            public void testSmartCastFork() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/inference/smartCastFork.kt");
+            }
+
             @Test
             @TestMetadata("specialCallsWithCallableReferences.kt")
             public void testSpecialCallsWithCallableReferences() throws Exception {
@@ -36583,6 +36589,12 @@ public class DiagnosisCompilerTestFE10TestdataTestGenerated extends AbstractDiag
                 runTest("compiler/testData/diagnostics/testsWithStdLib/inference/kt4975.kt");
             }
 
+            @Test
+            @TestMetadata("plusAssignOnArrayList.kt")
+            public void testPlusAssignOnArrayList() throws Exception {
+                runTest("compiler/testData/diagnostics/testsWithStdLib/inference/plusAssignOnArrayList.kt");
+            }
+
             @Test
             @TestMetadata("recursiveFlexibleAssertions.kt")
             public void testRecursiveFlexibleAssertions() throws Exception {