fixup! [FIR] Ignore some `USELESS_IS_CHECK` in `when`s Still report `USELESS_IS_CHECK` in `when`s with only one branch.

diff --git a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirCastOperatorsChecker.kt b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirCastOperatorsChecker.kt
index 1285b83..fbc0291 100644
--- a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirCastOperatorsChecker.kt
+++ b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirCastOperatorsChecker.kt

@@ -126,7 +126,7 @@
                 expression.source, FirErrors.USELESS_IS_CHECK, expression.operation != FirOperation.IS, context,
             )
             Applicability.USELESS_IS_CHECK -> when {
-                !isLastBranchOfExhaustiveWhen(l, context) -> reportOn(
+                !isLastBranchOfExhaustiveWhen(l, r, context) -> reportOn(
                     expression.source, FirErrors.USELESS_IS_CHECK, expression.operation == FirOperation.IS, context,
                 )
             }
@@ -140,7 +140,7 @@
         }
     }
 
-    private fun isLastBranchOfExhaustiveWhen(l: ArgumentInfo, context: CheckerContext): Boolean {
+    private fun isLastBranchOfExhaustiveWhen(l: ArgumentInfo, r: ConeKotlinType, context: CheckerContext): Boolean {
         if (context.containingElements.size < 2) {
             return false
         }
@@ -151,6 +151,8 @@
                 && whenExpression.isExhaustive && whenBranch == whenExpression.branches.lastOrNull()
                 // Ensures it's not redundantly exhaustive
                 && !l.argument.resolvedType.isNothing
+                // Having an exhaustive `when` with only one branch is useless in general
+                && (whenExpression.branches.size > 1 || l.smartCastTypeInfo.type.equalTypes(r, context.session))
     }
 
     private fun getImpossibilityDiagnostic(l: TypeInfo, rType: ConeKotlinType, context: CheckerContext) = when {

diff --git a/compiler/testData/diagnostics/tests/sealed/ExhaustiveOnRoot.fir.kt b/compiler/testData/diagnostics/tests/sealed/ExhaustiveOnRoot.fir.kt
index d94a8d8..8ee0a9f 100644
--- a/compiler/testData/diagnostics/tests/sealed/ExhaustiveOnRoot.fir.kt
+++ b/compiler/testData/diagnostics/tests/sealed/ExhaustiveOnRoot.fir.kt

@@ -20,5 +20,5 @@
 
 fun test3(x: Expr): String =
         when (x) {
-            is Stmt -> "stmt"
+            <!USELESS_IS_CHECK!>is Stmt<!> -> "stmt"
         }
\ No newline at end of file

diff --git a/compiler/testData/diagnostics/tests/when/ExhaustiveSelftype.fir.kt b/compiler/testData/diagnostics/tests/when/ExhaustiveSelftype.fir.kt
index 2ab7331..47ccbb0 100644
--- a/compiler/testData/diagnostics/tests/when/ExhaustiveSelftype.fir.kt
+++ b/compiler/testData/diagnostics/tests/when/ExhaustiveSelftype.fir.kt

@@ -4,12 +4,12 @@
 fun testCharSequence(x: CharSequence): Int {
     var i = 0
 
-    i += when (x) { is Any -> 1 }
+    i += when (x) { <!USELESS_IS_CHECK!>is Any<!> -> 1 }
     i += when (x) { is CharSequence -> 1 }
     i += <!NO_ELSE_IN_WHEN!>when<!> (x) { is String -> 1 }
 
-    i += when (x) { is Any? -> 1 }
-    i += when (x) { is CharSequence? -> 1 }
+    i += when (x) { <!USELESS_IS_CHECK!>is Any?<!> -> 1 }
+    i += when (x) { <!USELESS_IS_CHECK!>is CharSequence?<!> -> 1 }
     i += <!NO_ELSE_IN_WHEN!>when<!> (x) { is String? -> 1 }
 
     return i
@@ -22,7 +22,7 @@
     i += <!NO_ELSE_IN_WHEN!>when<!> (x) { is CharSequence -> 1 }
     i += <!NO_ELSE_IN_WHEN!>when<!> (x) { is String -> 1 }
 
-    i += when (x) { is Any? -> 1 }
+    i += when (x) { <!USELESS_IS_CHECK!>is Any?<!> -> 1 }
     i += when (x) { is CharSequence? -> 1 }
     i += <!NO_ELSE_IN_WHEN!>when<!> (x) { is String? -> 1 }
 

diff --git a/compiler/testData/diagnostics/tests/when/nullableSubjectWithIsCheck.fir.kt b/compiler/testData/diagnostics/tests/when/nullableSubjectWithIsCheck.fir.kt
index 8954419..776d776 100644
--- a/compiler/testData/diagnostics/tests/when/nullableSubjectWithIsCheck.fir.kt
+++ b/compiler/testData/diagnostics/tests/when/nullableSubjectWithIsCheck.fir.kt

@@ -190,5 +190,5 @@
 }
 
 fun isNullable(a: Int?) = when (a) {
-    is Number? -> false
+    <!USELESS_IS_CHECK!>is Number?<!> -> false
 }