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

core/compiler.common/src/org/jetbrains/kotlin/types/AbstractTypeChecker.kt

@@ -62,6 +62,34 @@ open class TypeCheckerState(
         isFromNullabilityConstraint: Boolean = false
     ): Boolean? = null
 
+    // Handling cases like A<Int> & A<T> <: A<F_var>
+    // There are two possible solutions for F_var (Int and T) and both of them may work well or not with other constrains
+    // Effectively, we need to fork constraint system to two copies: one with F_var=Int and the other with F_var=T
+    // and then maintain them both until we find some contradiction with one of the versions.
+    //
+    // But that might lead to the exponential size of CS, thus we use the following heuristics:
+    // we accumulate forks data until the last stage of the candidate resolution and then try to apply back then
+    // until some of the constrains set has no contradiction.
+    //
+    // `atForkPoint` works trivially in non-inference context and for FE1.0: it just run basic subtyping mechanism for each subTypeArguments
+    // component until the first success
+    open fun runForkingPoint(block: ForkPointContext.() -> Unit): Boolean = with(ForkPointContext.Default()) {
+        block()
+        result
+    }
+
+    interface ForkPointContext {
+        fun fork(block: () -> Boolean)
+
+        class Default : ForkPointContext {
+            var result: Boolean = false
+            override fun fork(block: () -> Boolean) {
+                if (result) return
+                result = block()
+            }
+        }
+    }
+
     enum class LowerCapturedTypePolicy {
         CHECK_ONLY_LOWER,
         CHECK_SUBTYPE_AND_LOWER,
@@ -362,8 +390,11 @@ object AbstractTypeChecker {
 
                 if (!anyNonOutParameter && state.isSubtypeForSameConstructor(newArguments, superType)) return true
 
-                // TODO: rethink this; now components order in intersection type affects semantic due to run subtyping (which can add constraints) only until the first successful candidate
-                return supertypesWithSameConstructor.any { state.isSubtypeForSameConstructor(it.asArgumentList(), superType) }
+                return state.runForkingPoint {
+                    for (subTypeArguments in supertypesWithSameConstructor) {
+                        fork { state.isSubtypeForSameConstructor(subTypeArguments.asArgumentList(), superType) }
+                    }
+                }
             }
         }
     }

core/util.runtime/src/org/jetbrains/kotlin/utils/addToStdlib.kt

@@ -235,3 +235,5 @@ inline fun <T, U, K, V> List<T>.flatGroupBy(
     }
     return result
 }
+
+fun <E> MutableList<E>.popLast(): E = removeAt(lastIndex)