diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index 69380de83f722..ae25d9467e509 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -1972,10 +1972,12 @@ NOTE(objc_generic_extension_using_type_parameter_here,none,
      "generic parameter used here", ())
 NOTE(objc_generic_extension_using_type_parameter_try_objc,none,
      "add '@objc' to allow uses of 'self' within the function body", ())
+ERROR(unsupported_existential_extension,none,
+      "extension of existential type %0 is not supported", (Type))
 ERROR(invalid_nominal_extension,none,
       "extension of type %0 must be declared as an extension of %1",
       (Type, Type))
-NOTE(invalid_nominal_extension_rewrite,none,
+NOTE(invalid_extension_rewrite,none,
      "did you mean to extend %0 instead?", (Type))
 ERROR(synthesized_nominal_extension,none,
       "cannot extend synthesized type %0", (Type))
diff --git a/lib/Sema/TypeCheckDeclPrimary.cpp b/lib/Sema/TypeCheckDeclPrimary.cpp
index d476b7bec76e5..189b358be6bc6 100644
--- a/lib/Sema/TypeCheckDeclPrimary.cpp
+++ b/lib/Sema/TypeCheckDeclPrimary.cpp
@@ -3084,21 +3084,31 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       const bool wasAlreadyInvalid = ED->isInvalid();
       ED->setInvalid();
       if (!extType->hasError() && extType->getAnyNominal()) {
+        auto canExtType = extType->getCanonicalType();
+        if (auto existential = canExtType->getAs<ExistentialType>()) {
+          ED->diagnose(diag::unsupported_existential_extension, extType)
+              .highlight(ED->getExtendedTypeRepr()->getSourceRange());
+          ED->diagnose(diag::invalid_extension_rewrite,
+                       existential->getConstraintType())
+              .fixItReplace(ED->getExtendedTypeRepr()->getSourceRange(),
+                            existential->getConstraintType()->getString());
+          return;
+        }
+
         // If we've got here, then we have some kind of extension of a prima
-        // fascie non-nominal type.  This can come up when we're projecting
+        // facie non-nominal type.  This can come up when we're projecting
         // typealiases out of bound generic types.
         //
         // struct Array<T> { typealias Indices = Range<Int> }
         // extension Array.Indices.Bound {}
         //
         // Offer to rewrite it to the underlying nominal type.
-        auto canExtType = extType->getCanonicalType();
         if (canExtType.getPointer() != extType.getPointer()) {
           ED->diagnose(diag::invalid_nominal_extension, extType, canExtType)
-            .highlight(ED->getExtendedTypeRepr()->getSourceRange());
-          ED->diagnose(diag::invalid_nominal_extension_rewrite, canExtType)
-            .fixItReplace(ED->getExtendedTypeRepr()->getSourceRange(),
-                          canExtType->getString());
+              .highlight(ED->getExtendedTypeRepr()->getSourceRange());
+          ED->diagnose(diag::invalid_extension_rewrite, canExtType)
+              .fixItReplace(ED->getExtendedTypeRepr()->getSourceRange(),
+                            canExtType->getString());
           return;
         }
       }
diff --git a/test/decl/ext/extensions.swift b/test/decl/ext/extensions.swift
index c6ef3c72b488e..1ae4a4bc7e5da 100644
--- a/test/decl/ext/extensions.swift
+++ b/test/decl/ext/extensions.swift
@@ -349,7 +349,7 @@ extension Tree.LimbContent.Contents {
 
 extension Tree.BoughPayload.Contents {
   // expected-error@-1 {{extension of type 'Tree.BoughPayload.Contents' (aka 'Nest<Int>') must be declared as an extension of 'Nest<Int>'}}
-  // expected-note@-2 {{did you mean to extend 'Nest<Int>' instead?}}
+  // expected-note@-2 {{did you mean to extend 'Nest<Int>' instead?}} {{11-37=Nest<Int>}}
 }
 
 // https://github.com/apple/swift/issues/52866
@@ -367,3 +367,25 @@ protocol Rdar66943328 {
 }
 extension Rdar66943328 where Assoc == Int // expected-error {{expected '{' in extension}}
 #endif
+
+// Reject extension of existential type
+
+protocol P4 {}
+
+extension any P4 {
+// expected-error@-1 {{extension of existential type 'any P4' is not supported}}
+// expected-note@-2 {{did you mean to extend 'P4' instead?}} {{11-17=P4}}
+}
+
+typealias A4 = P4
+
+extension any A4 {
+// expected-error@-1 {{extension of existential type 'any A4' (aka 'any P4') is not supported}}
+// expected-note@-2 {{did you mean to extend 'P4' instead?}} {{11-17=P4}}
+}
+
+typealias B4 = any P4
+extension B4 {
+// expected-error@-1 {{extension of existential type 'B4' (aka 'any P4') is not supported}}
+// expected-note@-2 {{did you mean to extend 'P4' instead?}} {{11-13=P4}}
+}