diff --git a/docs/ABI/Mangling.rst b/docs/ABI/Mangling.rst
index 95accf67aa5..9754ceb5883 100644
--- a/docs/ABI/Mangling.rst
+++ b/docs/ABI/Mangling.rst
@@ -789,6 +789,7 @@ Some kinds need arguments, which precede ``Tf``.
   ARG-SPEC-KIND ::= 'c'                      // Consumes n 'type' arguments which are closed over types in argument order
                                              // and one 'identifier' argument which is the closure symbol name
   ARG-SPEC-KIND ::= 'p' CONST-PROP           // Constant propagated argument
+  ARG-SPEC-KIND ::= 'e' 'D'? 'G'? 'X'?       // Generic argument, with optional dead, owned=>guaranteed or exploded-specifier
   ARG-SPEC-KIND ::= 'd' 'G'? 'X'?            // Dead argument, with optional owned=>guaranteed or exploded-specifier
   ARG-SPEC-KIND ::= 'g' 'X'?                 // Owned => Guaranteed,, with optional exploded-specifier
   ARG-SPEC-KIND ::= 'x'                      // Exploded
diff --git a/include/swift/Demangling/Demangle.h b/include/swift/Demangling/Demangle.h
index 11868c69b5f..41a3f953bea 100644
--- a/include/swift/Demangling/Demangle.h
+++ b/include/swift/Demangling/Demangle.h
@@ -95,6 +95,7 @@ enum class FunctionSigSpecializationParamKind : unsigned {
   OwnedToGuaranteed = 1 << 7,
   SROA = 1 << 8,
   GuaranteedToOwned = 1 << 9,
+  ExistentialToGeneric = 1 << 10,
 };
 
 /// The pass that caused the specialization to occur. We use this to make sure
diff --git a/include/swift/SILOptimizer/Utils/SpecializationMangler.h b/include/swift/SILOptimizer/Utils/SpecializationMangler.h
index 1d21522ed99..f78fd3af560 100644
--- a/include/swift/SILOptimizer/Utils/SpecializationMangler.h
+++ b/include/swift/SILOptimizer/Utils/SpecializationMangler.h
@@ -128,6 +128,7 @@ class FunctionSignatureSpecializationMangler : public SpecializationMangler {
     OwnedToGuaranteed=64,
     SROA=128,
     GuaranteedToOwned=256,
+    ExistentialToGeneric=512,
     First_OptionSetEntry=32, LastOptionSetEntry=32768,
   };
 
@@ -151,6 +152,7 @@ public:
   void setArgumentDead(unsigned OrigArgIdx);
   void setArgumentOwnedToGuaranteed(unsigned OrigArgIdx);
   void setArgumentGuaranteedToOwned(unsigned OrigArgIdx);
+  void setArgumentExistentialToGeneric(unsigned OrigArgIdx);
   void setArgumentSROA(unsigned OrigArgIdx);
   void setArgumentBoxToValue(unsigned OrigArgIdx);
   void setArgumentBoxToStack(unsigned OrigArgIdx);
diff --git a/lib/Demangling/Demangler.cpp b/lib/Demangling/Demangler.cpp
index df85e5fbe4a..2e69e307e91 100644
--- a/lib/Demangling/Demangler.cpp
+++ b/lib/Demangling/Demangler.cpp
@@ -1982,6 +1982,24 @@ NodePointer Demangler::demangleFuncSpecParam(Node::IndexType ParamIdx) {
           return nullptr;
       }
     }
+    case 'e': {
+      unsigned Value =
+          unsigned(FunctionSigSpecializationParamKind::ExistentialToGeneric);
+      if (nextIf('D'))
+        Value |= unsigned(FunctionSigSpecializationParamKind::Dead);
+      if (nextIf('G'))
+        Value |=
+            unsigned(FunctionSigSpecializationParamKind::OwnedToGuaranteed);
+      if (nextIf('O'))
+        Value |=
+            unsigned(FunctionSigSpecializationParamKind::GuaranteedToOwned);
+      if (nextIf('X'))
+        Value |= unsigned(FunctionSigSpecializationParamKind::SROA);
+      return addChild(
+          Param,
+          createNode(Node::Kind::FunctionSignatureSpecializationParamKind,
+                     Value));
+    }
     case 'd': {
       unsigned Value = unsigned(FunctionSigSpecializationParamKind::Dead);
       if (nextIf('G'))
diff --git a/lib/Demangling/NodePrinter.cpp b/lib/Demangling/NodePrinter.cpp
index 39e3eaf282b..98e81beebff 100644
--- a/lib/Demangling/NodePrinter.cpp
+++ b/lib/Demangling/NodePrinter.cpp
@@ -816,7 +816,9 @@ unsigned NodePrinter::printFunctionSigSpecializationParam(NodePointer Node,
       ((V & unsigned(FunctionSigSpecializationParamKind::OwnedToGuaranteed)) ||
        (V & unsigned(FunctionSigSpecializationParamKind::GuaranteedToOwned)) ||
        (V & unsigned(FunctionSigSpecializationParamKind::SROA)) ||
-       (V & unsigned(FunctionSigSpecializationParamKind::Dead))) &&
+       (V & unsigned(FunctionSigSpecializationParamKind::Dead))||
+       (V & unsigned(
+                FunctionSigSpecializationParamKind::ExistentialToGeneric))) &&
       "Invalid OptionSet");
   print(Node->getChild(Idx++));
   return Idx;
@@ -1215,11 +1217,18 @@ NodePointer NodePrinter::print(NodePointer Node, bool asPrefixContext) {
     uint64_t raw = Node->getIndex();
 
     bool printedOptionSet = false;
+    if (raw &
+        uint64_t(FunctionSigSpecializationParamKind::ExistentialToGeneric)) {
+      printedOptionSet = true;
+      Printer << "Existential To Protocol Constrained Generic";
+    }
+
     if (raw & uint64_t(FunctionSigSpecializationParamKind::Dead)) {
+      if (printedOptionSet)
+        Printer << " and ";
       printedOptionSet = true;
       Printer << "Dead";
     }
-
     if (raw & uint64_t(FunctionSigSpecializationParamKind::OwnedToGuaranteed)) {
       if (printedOptionSet)
         Printer << " and ";
@@ -1269,6 +1278,7 @@ NodePointer NodePrinter::print(NodePointer Node, bool asPrefixContext) {
     case FunctionSigSpecializationParamKind::ClosureProp:
       Printer << "Closure Propagated";
       return nullptr;
+    case FunctionSigSpecializationParamKind::ExistentialToGeneric:
     case FunctionSigSpecializationParamKind::Dead:
     case FunctionSigSpecializationParamKind::OwnedToGuaranteed:
     case FunctionSigSpecializationParamKind::GuaranteedToOwned:
diff --git a/lib/Demangling/Remangler.cpp b/lib/Demangling/Remangler.cpp
index 2505ff5f52d..6639c7c6212 100644
--- a/lib/Demangling/Remangler.cpp
+++ b/lib/Demangling/Remangler.cpp
@@ -1019,7 +1019,20 @@ void Remangler::mangleFunctionSignatureSpecializationParam(Node *node) {
       Buffer << 'x';
       return;
     default:
-      if (kindValue & unsigned(FunctionSigSpecializationParamKind::Dead)) {
+      if (kindValue &
+          unsigned(
+              FunctionSigSpecializationParamKind::ExistentialToGeneric)) {
+        Buffer << 'e';
+        if (kindValue & unsigned(FunctionSigSpecializationParamKind::Dead))
+          Buffer << 'D';
+        if (kindValue &
+            unsigned(FunctionSigSpecializationParamKind::OwnedToGuaranteed))
+          Buffer << 'G';
+        if (kindValue &
+            unsigned(FunctionSigSpecializationParamKind::GuaranteedToOwned))
+          Buffer << 'O';
+      } else if (kindValue &
+                 unsigned(FunctionSigSpecializationParamKind::Dead)) {
         Buffer << 'd';
         if (kindValue &
             unsigned(FunctionSigSpecializationParamKind::OwnedToGuaranteed))
diff --git a/lib/SILOptimizer/Utils/SpecializationMangler.cpp b/lib/SILOptimizer/Utils/SpecializationMangler.cpp
index cc25ef16fa8..38fd628db62 100644
--- a/lib/SILOptimizer/Utils/SpecializationMangler.cpp
+++ b/lib/SILOptimizer/Utils/SpecializationMangler.cpp
@@ -162,6 +162,12 @@ void FunctionSignatureSpecializationMangler::setArgumentGuaranteedToOwned(
       ArgumentModifierIntBase(ArgumentModifier::GuaranteedToOwned);
 }
 
+void FunctionSignatureSpecializationMangler::setArgumentExistentialToGeneric(
+    unsigned OrigArgIdx) {
+  OrigArgs[OrigArgIdx].first |=
+      ArgumentModifierIntBase(ArgumentModifier::ExistentialToGeneric);
+}
+
 void FunctionSignatureSpecializationMangler::setArgumentBoxToValue(
     unsigned OrigArgIdx) {
   OrigArgs[OrigArgIdx].first =
@@ -287,6 +293,11 @@ void FunctionSignatureSpecializationMangler::mangleArgument(
   }
 
   bool hasSomeMod = false;
+  if (ArgMod & ArgumentModifierIntBase(ArgumentModifier::ExistentialToGeneric)) {
+    ArgOpBuffer << 'e';
+    hasSomeMod = true;
+  }
+
   if (ArgMod & ArgumentModifierIntBase(ArgumentModifier::Dead)) {
     ArgOpBuffer << 'd';
     hasSomeMod = true;