diff --git a/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt b/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt
index 9d42f9305bf..24da511ad95 100644
--- a/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt
+++ b/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt
@@ -294,6 +294,8 @@ object DIAGNOSTICS_LIST : DiagnosticList() {
         val FINAL_UPPER_BOUND by warning<FirSourceElement, PsiElement> {
             parameter<ConeKotlinType>("type")
         }
+
+        val UPPER_BOUND_IS_EXTENSION_FUNCTION_TYPE by error<FirSourceElement, PsiElement>()
     }
 
     val REFLECTION by object : DiagnosticGroup("Reflection") {
diff --git a/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt b/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt
index cd5523b33ac..6bf8ea98baa 100644
--- a/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt
+++ b/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt
@@ -217,6 +217,7 @@ object FirErrors {
     val KCLASS_WITH_NULLABLE_TYPE_PARAMETER_IN_SIGNATURE by error1<FirSourceElement, KtNamedDeclaration, FirTypeParameterSymbol>(SourceElementPositioningStrategies.DECLARATION_NAME)
     val TYPE_PARAMETER_AS_REIFIED by error1<FirSourceElement, PsiElement, FirTypeParameterSymbol>()
     val FINAL_UPPER_BOUND by warning1<FirSourceElement, PsiElement, ConeKotlinType>()
+    val UPPER_BOUND_IS_EXTENSION_FUNCTION_TYPE by error0<FirSourceElement, PsiElement>()
 
     // Reflection
     val EXTENSION_IN_CLASS_REFERENCE_NOT_ALLOWED by error1<FirSourceElement, KtExpression, FirCallableDeclaration<*>>(SourceElementPositioningStrategies.REFERENCE_BY_QUALIFIED)
diff --git a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirTypeParameterBoundsChecker.kt b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirTypeParameterBoundsChecker.kt
index a01c981b086..5379b2de325 100644
--- a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirTypeParameterBoundsChecker.kt
+++ b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirTypeParameterBoundsChecker.kt
@@ -12,6 +12,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
 import org.jetbrains.kotlin.fir.analysis.diagnostics.reportOn
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.types.coneType
+import org.jetbrains.kotlin.fir.types.isExtensionFunctionType
 
 object FirTypeParameterBoundsChecker : FirTypeParameterChecker() {
 
@@ -25,6 +26,9 @@ object FirTypeParameterBoundsChecker : FirTypeParameterChecker() {
             if (!bound.coneType.canHaveSubtypes(context.session)) {
                 reporter.reportOn(bound.source, FirErrors.FINAL_UPPER_BOUND, bound.coneType, context)
             }
+            if (bound.isExtensionFunctionType(context.session)) {
+                reporter.reportOn(bound.source, FirErrors.UPPER_BOUND_IS_EXTENSION_FUNCTION_TYPE, context)
+            }
         }
     }
 }
diff --git a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt
index 33fe6e4df64..3618fe194d3 100644
--- a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt
+++ b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt
@@ -220,6 +220,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.UNSAFE_OPERATOR_C
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.UNSUPPORTED
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.UNSUPPORTED_FEATURE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.UNUSED_VARIABLE
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.UPPER_BOUND_IS_EXTENSION_FUNCTION_TYPE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.UPPER_BOUND_VIOLATED
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.USELESS_VARARG_ON_PARAMETER
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VALUE_CLASS_CANNOT_BE_CLONEABLE
@@ -447,6 +448,7 @@ class FirDefaultErrorMessages : DefaultErrorMessages.Extension {
                 "''{0}'' is a final type, and thus a value of the type parameter is predetermined",
                 RENDER_TYPE
             )
+            map.put(UPPER_BOUND_IS_EXTENSION_FUNCTION_TYPE, "Extension function type can not be used as an upper bound")
 
             // Reflection
             map.put(
diff --git a/compiler/testData/diagnostics/tests/typeParameters/extFunctionTypeAsUpperBound.fir.kt b/compiler/testData/diagnostics/tests/typeParameters/extFunctionTypeAsUpperBound.fir.kt
index e0683acd1be..e920d6cb9fa 100644
--- a/compiler/testData/diagnostics/tests/typeParameters/extFunctionTypeAsUpperBound.fir.kt
+++ b/compiler/testData/diagnostics/tests/typeParameters/extFunctionTypeAsUpperBound.fir.kt
@@ -1,7 +1,7 @@
-fun <T: Int.() -> String> foo() {}
+fun <T: <!UPPER_BOUND_IS_EXTENSION_FUNCTION_TYPE!>Int.() -> String<!>> foo() {}
 
-val <T: Int.() -> String> bar = fun (x: Int): String { return x.toString() }
+val <T: <!UPPER_BOUND_IS_EXTENSION_FUNCTION_TYPE!>Int.() -> String<!>> bar = fun (x: Int): String { return x.toString() }
 
-class A<T> where T : Double.(Int) -> Unit
+class A<T> where T : <!UPPER_BOUND_IS_EXTENSION_FUNCTION_TYPE!>Double.(Int) -> Unit<!>
 
-interface B<T, U : T.() -> Unit>
\ No newline at end of file
+interface B<T, U : <!UPPER_BOUND_IS_EXTENSION_FUNCTION_TYPE!>T.() -> Unit<!>>
diff --git a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt
index c3e469b43a3..e7bdac8c866 100644
--- a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt
+++ b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt
@@ -895,6 +895,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.UPPER_BOUND_IS_EXTENSION_FUNCTION_TYPE) { firDiagnostic ->
+        UpperBoundIsExtensionFunctionTypeImpl(
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
     add(FirErrors.EXTENSION_IN_CLASS_REFERENCE_NOT_ALLOWED) { firDiagnostic ->
         ExtensionInClassReferenceNotAllowedImpl(
             firSymbolBuilder.callableBuilder.buildCallableSymbol(firDiagnostic.a as FirCallableDeclaration),
diff --git a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt
index e934b41a532..ef7b20280aa 100644
--- a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt
+++ b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt
@@ -638,6 +638,10 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val type: KtType
     }
 
+    abstract class UpperBoundIsExtensionFunctionType : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = UpperBoundIsExtensionFunctionType::class
+    }
+
     abstract class ExtensionInClassReferenceNotAllowed : KtFirDiagnostic<KtExpression>() {
         override val diagnosticClass get() = ExtensionInClassReferenceNotAllowed::class
         abstract val referencedDeclaration: KtCallableSymbol
diff --git a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
index 4b3829c80bd..89080b1f917 100644
--- a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
+++ b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
@@ -1026,6 +1026,13 @@ internal class FinalUpperBoundImpl(
     override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
 }
 
+internal class UpperBoundIsExtensionFunctionTypeImpl(
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.UpperBoundIsExtensionFunctionType(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
 internal class ExtensionInClassReferenceNotAllowedImpl(
     override val referencedDeclaration: KtCallableSymbol,
     firDiagnostic: FirPsiDiagnostic<*>,