if (whenExpression != null) {
                    val subject = whenExpression.subjectExpression ?: return with(analysisSession) { builtinTypes.BOOLEAN }
                    val kotlinType = analysisContext.analyze(subject).getType(subject)
                    return kotlinType?.toKtNonErrorType(analysisContext)
                }
            }

  }

  @Test
  @TestMetadata("when_subject.kt")
  public void testWhen_subject() {
    runTest("analysis/analysis-api/testData/components/expressionInfoProvider/isUsedAsExpression/when_subject.kt");
  }

  @Test
  @TestMetadata("when_subject_with_branches.kt")
  public void testWhen_subject_with_branches() {

        val whenExpression = (condition.parent as? KtWhenEntry)?.parent as? KtWhenExpression ?: return null
        val subject = whenExpression.subjectExpression ?: return with(analysisSession) { builtinTypes.BOOLEAN }
        return getKtExpressionNonErrorType(subject)
    }

    private fun getKtExpressionNonErrorType(expression: KtExpression): KtType? =
        getKtExpressionType(expression)?.nonErrorTypeOrNull()

            // Parentheses are ignored for this analysis.
            is KtParenthesizedExpression ->
                doesParentUseChild(parent.parent, parent)

            // When expressions use the subject expression _unless_ the first branch in the
            // when is an `else`.
            is KtWhenExpression ->
                parent.subjectExpression == child && parent.entries.firstOrNull()?.isElse == false

) : KtAbstractFirDiagnostic<PsiElement>(firDiagnostic, token), KtFirDiagnostic.TypeVarianceConflictInExpandedType

internal class SmartcastImpossibleImpl(
    override val desiredType: KtType,
    override val subject: KtExpression,
    override val description: String,
    override val isCastToNotNull: Boolean,
    firDiagnostic: KtPsiDiagnostic,
    token: KtLifetimeToken,

        val desiredType: KtType
        val subject: KtExpression
        val description: String
        val isCastToNotNull: Boolean
    }

    interface SmartcastImpossibleOnImplicitInvokeReceiver : KtFirDiagnostic<KtExpression> {
        override val diagnosticClass get() = SmartcastImpossibleOnImplicitInvokeReceiver::class
        val desiredType: KtType
        val subject: KtExpression
        val description: String

        ElseMisplacedInWhenImpl(
            firDiagnostic as KtPsiDiagnostic,
            token,
        )
    }
    add(FirErrors.ILLEGAL_DECLARATION_IN_WHEN_SUBJECT) { firDiagnostic ->
        IllegalDeclarationInWhenSubjectImpl(
            firDiagnostic.a,
            firDiagnostic as KtPsiDiagnostic,
            token,
        )
    }

- [`KT-56942`](https://youtrack.jetbrains.com/issue/KT-56942) K2: False-negative NO_ELSE_IN_WHEN if subject is flexible type
- [`KT-56687`](https://youtrack.jetbrains.com/issue/KT-56687) Unexpected behaviour with enum entries when using outdated stdlib