diff --git a/include/swift/AST/ASTBridging.h b/include/swift/AST/ASTBridging.h
index 54220772a2462..feef92608003d 100644
--- a/include/swift/AST/ASTBridging.h
+++ b/include/swift/AST/ASTBridging.h
@@ -215,10 +215,23 @@ BridgedDeclNameLoc BridgedDeclNameLoc_createParsed(
     BridgedSourceLoc cLParenLoc, BridgedArrayRef cLabelLocs,
     BridgedSourceLoc cRParenLoc);
 
+SWIFT_NAME("BridgedDeclNameLoc.createParsed(_:moduleSelectorLoc:baseNameLoc:"
+           "lParenLoc:argumentLabelLocs:rParenLoc:)")
+BridgedDeclNameLoc BridgedDeclNameLoc_createParsed(
+    BridgedASTContext cContext, BridgedSourceLoc cModuleSelectorLoc,
+    BridgedSourceLoc cBaseNameLoc, BridgedSourceLoc cLParenLoc,
+    BridgedArrayRef cLabelLocs, BridgedSourceLoc cRParenLoc);
+
 SWIFT_NAME("BridgedDeclNameLoc.createParsed(_:)")
 BridgedDeclNameLoc
 BridgedDeclNameLoc_createParsed(BridgedSourceLoc cBaseNameLoc);
 
+SWIFT_NAME("BridgedDeclNameLoc.createParsed(_:moduleSelectorLoc:baseNameLoc:)")
+BridgedDeclNameLoc
+BridgedDeclNameLoc_createParsed(
+    BridgedASTContext cContext, BridgedSourceLoc cModuleSelectorLoc,
+    BridgedSourceLoc cBaseNameLoc);
+
 //===----------------------------------------------------------------------===//
 // MARK: ASTContext
 //===----------------------------------------------------------------------===//
diff --git a/include/swift/AST/DeclNameLoc.h b/include/swift/AST/DeclNameLoc.h
index a4a65ef50be99..494a4ae721faf 100644
--- a/include/swift/AST/DeclNameLoc.h
+++ b/include/swift/AST/DeclNameLoc.h
@@ -20,6 +20,8 @@
 #include "swift/Basic/LLVM.h"
 #include "swift/Basic/SourceLoc.h"
 
+#include "llvm/ADT/ArrayRef.h"
+
 class BridgedDeclNameLoc;
 
 namespace swift {
@@ -71,6 +73,10 @@ class DeclNameLoc {
   explicit DeclNameLoc(SourceLoc baseNameLoc)
       : DeclNameLoc(baseNameLoc.getOpaquePointerValue(), 0) {}
 
+  explicit DeclNameLoc(ASTContext &ctx, SourceLoc moduleSelectorLoc,
+                       SourceLoc baseNameLoc)
+    : DeclNameLoc(baseNameLoc) { }
+
   /// Create declaration name location information for a compound
   /// name.
   DeclNameLoc(ASTContext &ctx, SourceLoc baseNameLoc,
@@ -78,6 +84,13 @@ class DeclNameLoc {
               ArrayRef<SourceLoc> argumentLabelLocs,
               SourceLoc rParenLoc);
 
+  DeclNameLoc(ASTContext &ctx, SourceLoc moduleSelectorLoc,
+              SourceLoc baseNameLoc,
+              SourceLoc lParenLoc,
+              ArrayRef<SourceLoc> argumentLabelLocs,
+              SourceLoc rParenLoc)
+    : DeclNameLoc(ctx, baseNameLoc, lParenLoc, argumentLabelLocs, rParenLoc) { }
+
   /// Whether the location information is valid.
   bool isValid() const { return getBaseNameLoc().isValid(); }
 
@@ -111,6 +124,10 @@ class DeclNameLoc {
     return getSourceLocs()[FirstArgumentLabelIndex + index];
   }
 
+  SourceLoc getModuleSelectorLoc() const {
+    return SourceLoc();
+  }
+
   SourceLoc getStartLoc() const {
     return getBaseNameLoc();
   }
diff --git a/include/swift/AST/Identifier.h b/include/swift/AST/Identifier.h
index 036685873da73..7268aff1d1fbb 100644
--- a/include/swift/AST/Identifier.h
+++ b/include/swift/AST/Identifier.h
@@ -693,65 +693,73 @@ class DeclNameRef {
   void *getOpaqueValue() const { return FullName.getOpaqueValue(); }
   static DeclNameRef getFromOpaqueValue(void *p);
 
+  explicit DeclNameRef(ASTContext &C, Identifier moduleSelector,
+                       DeclName fullName)
+    : FullName(fullName) { }
+
+  explicit DeclNameRef(ASTContext &C, Identifier moduleSelector,
+                       DeclBaseName baseName, ArrayRef<Identifier> argLabels)
+    : FullName(C, baseName, argLabels) { }
+
   explicit DeclNameRef(DeclName FullName)
     : FullName(FullName) { }
 
-  explicit DeclNameRef(DeclBaseName BaseName)
-    : FullName(BaseName) { }
+  bool hasModuleSelector() const {
+    return false;
+  }
 
-  explicit DeclNameRef(Identifier BaseName)
-    : FullName(BaseName) { }
+  Identifier getModuleSelector() const {
+    return Identifier();
+  }
 
   /// The name of the declaration being referenced.
   DeclName getFullName() const {
     return FullName;
   }
 
-  DeclName &getFullName() {
-    return FullName;
-  }
-
   /// The base name of the declaration being referenced.
   DeclBaseName getBaseName() const {
-    return FullName.getBaseName();
+    return getFullName().getBaseName();
   }
 
   Identifier getBaseIdentifier() const {
-    return FullName.getBaseIdentifier();
+    return getFullName().getBaseIdentifier();
   }
 
   ArrayRef<Identifier> getArgumentNames() const {
-    return FullName.getArgumentNames();
+    return getFullName().getArgumentNames();
   }
 
   bool isSimpleName() const {
-    return FullName.isSimpleName();
+    return getFullName().isSimpleName();
   }
 
   bool isSimpleName(DeclBaseName name) const {
-    return FullName.isSimpleName(name);
+    return getFullName().isSimpleName(name);
   }
 
   bool isSimpleName(StringRef name) const {
-    return FullName.isSimpleName(name);
+    return getFullName().isSimpleName(name);
   }
 
   bool isSpecial() const {
-    return FullName.isSpecial();
+    return getFullName().isSpecial();
   }
 
   bool isOperator() const {
-    return FullName.isOperator();
+    return getFullName().isOperator();
   }
 
-  bool mustAlwaysBeEscaped() const { return FullName.mustAlwaysBeEscaped(); }
+  bool mustAlwaysBeEscaped() const {
+    return getFullName().mustAlwaysBeEscaped();
+  }
 
   bool isCompoundName() const {
-    return FullName.isCompoundName();
+    return getFullName().isCompoundName();
   }
 
   explicit operator bool() const {
-    return (bool)FullName;
+    return (bool)getFullName();
   }
 
   /// Compare two declaration names, producing -1 if \c *this comes before
@@ -791,7 +799,7 @@ class DeclNameRef {
     return lhs.compare(rhs) >= 0;
   }
 
-  DeclNameRef withoutArgumentLabels() const;
+  DeclNameRef withoutArgumentLabels(ASTContext &C) const;
   DeclNameRef withArgumentLabels(ASTContext &C,
                                  ArrayRef<Identifier> argumentNames) const;
 
@@ -824,16 +832,15 @@ inline DeclNameRef DeclNameRef::getFromOpaqueValue(void *p) {
   return DeclNameRef(DeclName::getFromOpaqueValue(p));
 }
 
-inline DeclNameRef DeclNameRef::withoutArgumentLabels() const {
-  return DeclNameRef(getBaseName());
+inline DeclNameRef DeclNameRef::withoutArgumentLabels(ASTContext &C) const {
+  return DeclNameRef(C, getModuleSelector(), getBaseName());
 }
 
 inline DeclNameRef DeclNameRef::withArgumentLabels(
     ASTContext &C, ArrayRef<Identifier> argumentNames) const {
-  return DeclNameRef(DeclName(C, getBaseName(), argumentNames));
+  return DeclNameRef(C, getModuleSelector(), getBaseName(), argumentNames);
 }
 
-
 inline DeclNameRef DeclNameRef::createSubscript() {
   return DeclNameRef(DeclBaseName::createSubscript());
 }
diff --git a/include/swift/Basic/Features.def b/include/swift/Basic/Features.def
index 1b46be47bb81a..2c82cf382c628 100644
--- a/include/swift/Basic/Features.def
+++ b/include/swift/Basic/Features.def
@@ -508,6 +508,9 @@ EXPERIMENTAL_FEATURE(CDecl, false)
 /// Allow use of `@extensible` on public enums
 SUPPRESSIBLE_EXPERIMENTAL_FEATURE(ExtensibleAttribute, false)
 
+/// Allow use of `Module::name` syntax
+EXPERIMENTAL_FEATURE(ModuleSelector, false)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE
diff --git a/lib/AST/Bridging/DeclBridging.cpp b/lib/AST/Bridging/DeclBridging.cpp
index bbf6b89917a0e..f9d5691c0d4c3 100644
--- a/lib/AST/Bridging/DeclBridging.cpp
+++ b/lib/AST/Bridging/DeclBridging.cpp
@@ -70,13 +70,23 @@ BridgedDeclNameLoc BridgedDeclNameLoc_createParsed(
     BridgedASTContext cContext, BridgedSourceLoc cBaseNameLoc,
     BridgedSourceLoc cLParenLoc, BridgedArrayRef cLabelLocs,
     BridgedSourceLoc cRParenLoc) {
+  return BridgedDeclNameLoc_createParsed(
+       cContext, BridgedSourceLoc(), cBaseNameLoc, cLParenLoc, cLabelLocs,
+       cRParenLoc);
+}
+
+BridgedDeclNameLoc BridgedDeclNameLoc_createParsed(
+    BridgedASTContext cContext, BridgedSourceLoc cModuleSelectorLoc,
+    BridgedSourceLoc cBaseNameLoc, BridgedSourceLoc cLParenLoc,
+    BridgedArrayRef cLabelLocs, BridgedSourceLoc cRParenLoc) {
 
   ASTContext &context = cContext.unbridged();
   SmallVector<SourceLoc, 4> labelLocs;
   for (auto &cLabelLoc : cLabelLocs.unbridged<BridgedSourceLoc>())
     labelLocs.push_back(cLabelLoc.unbridged());
 
-  return DeclNameLoc(context, cBaseNameLoc.unbridged(), cLParenLoc.unbridged(),
+  return DeclNameLoc(context, cModuleSelectorLoc.unbridged(),
+                     cBaseNameLoc.unbridged(), cLParenLoc.unbridged(),
                      labelLocs, cRParenLoc.unbridged());
 }
 
@@ -85,6 +95,14 @@ BridgedDeclNameLoc_createParsed(BridgedSourceLoc cBaseNameLoc) {
   return DeclNameLoc(cBaseNameLoc.unbridged());
 }
 
+BridgedDeclNameLoc
+BridgedDeclNameLoc_createParsed(
+    BridgedASTContext cContext, BridgedSourceLoc cModuleSelectorLoc,
+    BridgedSourceLoc cBaseNameLoc) {
+  return DeclNameLoc(cContext.unbridged(), cModuleSelectorLoc.unbridged(),
+                     cBaseNameLoc.unbridged());
+}
+
 //===----------------------------------------------------------------------===//
 // MARK: Decls
 //===----------------------------------------------------------------------===//
diff --git a/lib/AST/FeatureSet.cpp b/lib/AST/FeatureSet.cpp
index ec5f8e7d7f947..cd36c913d8db1 100644
--- a/lib/AST/FeatureSet.cpp
+++ b/lib/AST/FeatureSet.cpp
@@ -126,6 +126,9 @@ UNINTERESTING_FEATURE(MacrosOnImports)
 UNINTERESTING_FEATURE(NonisolatedNonsendingByDefault)
 UNINTERESTING_FEATURE(KeyPathWithMethodMembers)
 
+// TODO: Return true for inlinable function bodies with module selectors in them
+UNINTERESTING_FEATURE(ModuleSelector)
+
 static bool usesFeatureNonescapableTypes(Decl *decl) {
   auto containsNonEscapable =
       [](SmallVectorImpl<InverseRequirement> &inverseReqs) {
diff --git a/lib/AST/Identifier.cpp b/lib/AST/Identifier.cpp
index 90bf6e612f69c..0e32186ebdbee 100644
--- a/lib/AST/Identifier.cpp
+++ b/lib/AST/Identifier.cpp
@@ -55,6 +55,8 @@ void swift::simple_display(llvm::raw_ostream &out, DeclName name) {
 }
 
 raw_ostream &llvm::operator<<(raw_ostream &OS, DeclNameRef I) {
+  if (I.hasModuleSelector())
+    OS << I.getModuleSelector() << "::";
   OS << I.getFullName();
   return OS;
 }
@@ -208,17 +210,27 @@ void DeclNameRef::dump() const {
 }
 
 StringRef DeclNameRef::getString(llvm::SmallVectorImpl<char> &scratch,
-                             bool skipEmptyArgumentNames) const {
-  return FullName.getString(scratch, skipEmptyArgumentNames);
+                                 bool skipEmptyArgumentNames) const {
+  {
+    llvm::raw_svector_ostream out(scratch);
+    print(out, skipEmptyArgumentNames);
+  }
+
+  return StringRef(scratch.data(), scratch.size());
 }
 
-llvm::raw_ostream &DeclNameRef::print(llvm::raw_ostream &os,
-                                  bool skipEmptyArgumentNames) const {
-  return FullName.print(os, skipEmptyArgumentNames);
+llvm::raw_ostream &
+DeclNameRef::print(llvm::raw_ostream &os,
+                   bool skipEmptyArgumentNames) const {
+  if (hasModuleSelector())
+    os << getModuleSelector() << "::";
+  return getFullName().print(os, skipEmptyArgumentNames);
 }
 
 llvm::raw_ostream &DeclNameRef::printPretty(llvm::raw_ostream &os) const {
-  return FullName.printPretty(os);
+  if (hasModuleSelector())
+    os << getModuleSelector() << "::";
+  return getFullName().printPretty(os);
 }
 
 ObjCSelector::ObjCSelector(ASTContext &ctx, unsigned numArgs,
diff --git a/lib/Sema/CSDiagnostics.cpp b/lib/Sema/CSDiagnostics.cpp
index acc0f261b3d7c..821a3168b4f5f 100644
--- a/lib/Sema/CSDiagnostics.cpp
+++ b/lib/Sema/CSDiagnostics.cpp
@@ -4449,8 +4449,9 @@ bool MissingMemberFailure::diagnoseAsError() {
       auto &cs = getConstraintSystem();
 
       auto result = cs.performMemberLookup(
-          ConstraintKind::ValueMember, getName().withoutArgumentLabels(),
-          metatypeTy, FunctionRefInfo::doubleBaseNameApply(), getLocator(),
+          ConstraintKind::ValueMember,
+          getName().withoutArgumentLabels(getASTContext()), metatypeTy,
+          FunctionRefInfo::doubleBaseNameApply(), getLocator(),
           /*includeInaccessibleMembers=*/true);
 
       // If there are no `init` members at all produce a tailored
diff --git a/lib/Sema/TypeCheckAttr.cpp b/lib/Sema/TypeCheckAttr.cpp
index 7425d88b7ff5f..b3682af95c029 100644
--- a/lib/Sema/TypeCheckAttr.cpp
+++ b/lib/Sema/TypeCheckAttr.cpp
@@ -8389,9 +8389,10 @@ ValueDecl *RenamedDeclRequest::evaluate(Evaluator &evaluator,
   if (attr->Rename.empty())
     return nullptr;
 
+  auto &ctx = attached->getASTContext();
   auto attachedContext = attached->getDeclContext();
   auto parsedName = parseDeclName(attr->Rename);
-  auto nameRef = parsedName.formDeclNameRef(attached->getASTContext());
+  auto nameRef = parsedName.formDeclNameRef(ctx);
 
   // Handle types separately
   if (isa<NominalTypeDecl>(attached)) {
@@ -8400,7 +8401,7 @@ ValueDecl *RenamedDeclRequest::evaluate(Evaluator &evaluator,
 
     SmallVector<ValueDecl *, 1> lookupResults;
     attachedContext->lookupQualified(attachedContext->getParentModule(),
-                                     nameRef.withoutArgumentLabels(),
+                                     nameRef.withoutArgumentLabels(ctx),
                                      attr->getLocation(), NL_OnlyTypes,
                                      lookupResults);
     if (lookupResults.size() == 1)
diff --git a/lib/Sema/TypeCheckPattern.cpp b/lib/Sema/TypeCheckPattern.cpp
index 7d32bc32ecdf8..87e0a39c13651 100644
--- a/lib/Sema/TypeCheckPattern.cpp
+++ b/lib/Sema/TypeCheckPattern.cpp
@@ -138,7 +138,7 @@ static EnumElementDecl *
 lookupUnqualifiedEnumMemberElement(DeclContext *DC, DeclNameRef name,
                                    SourceLoc UseLoc) {
   // FIXME: We should probably pay attention to argument labels someday.
-  name = name.withoutArgumentLabels();
+  name = name.withoutArgumentLabels(DC->getASTContext());
 
   auto lookup =
       TypeChecker::lookupUnqualified(DC, name, UseLoc,
@@ -153,7 +153,7 @@ static LookupResult lookupMembers(DeclContext *DC, Type ty, DeclNameRef name,
     return LookupResult();
 
   // FIXME: We should probably pay attention to argument labels someday.
-  name = name.withoutArgumentLabels();
+  name = name.withoutArgumentLabels(DC->getASTContext());
 
   // Look up the case inside the enum.
   // FIXME: We should be able to tell if this is a private lookup.
diff --git a/lib/Sema/TypeCheckProtocol.cpp b/lib/Sema/TypeCheckProtocol.cpp
index 8619559ea1bda..29bfb2f3f23a6 100644
--- a/lib/Sema/TypeCheckProtocol.cpp
+++ b/lib/Sema/TypeCheckProtocol.cpp
@@ -1489,7 +1489,7 @@ swift::lookupValueWitnesses(DeclContext *DC, ValueDecl *req, bool *ignoringNames
   lookupValueWitnessesViaImplementsAttr(DC, req, witnesses);
 
   auto reqName = req->createNameRef();
-  auto reqBaseName = reqName.withoutArgumentLabels();
+  auto reqBaseName = reqName.withoutArgumentLabels(DC->getASTContext());
 
   // An operator function is the only kind of witness that requires global
   // lookup. However, because global lookup doesn't enter local contexts,
diff --git a/lib/Serialization/DeclTypeRecordNodes.def b/lib/Serialization/DeclTypeRecordNodes.def
index 3ffd7b345a76f..0ee58b5d3e668 100644
--- a/lib/Serialization/DeclTypeRecordNodes.def
+++ b/lib/Serialization/DeclTypeRecordNodes.def
@@ -213,12 +213,12 @@ OTHER(DERIVATIVE_FUNCTION_CONFIGURATION, 154)
 
 OTHER(ERROR_FLAG, 155)
 OTHER(ABI_ONLY_COUNTERPART, 156)
+OTHER(DECL_NAME_REF, 157)
 
 TRAILING_INFO(CONDITIONAL_SUBSTITUTION)
 TRAILING_INFO(CONDITIONAL_SUBSTITUTION_COND)
 
 OTHER(LIFETIME_DEPENDENCE, 160)
-
 TRAILING_INFO(INHERITED_PROTOCOLS)
 
 #ifndef DECL_ATTR
diff --git a/lib/Serialization/Deserialization.cpp b/lib/Serialization/Deserialization.cpp
index e00c099d3f225..aa60b8802c223 100644
--- a/lib/Serialization/Deserialization.cpp
+++ b/lib/Serialization/Deserialization.cpp
@@ -3498,6 +3498,58 @@ class DeclDeserializer {
   /// offsets in \c customAttrOffsets.
   llvm::Error deserializeCustomAttrs();
 
+  DeclNameRef deserializeDeclNameRefIfPresent() {
+    using namespace decls_block;
+
+    SmallVector<uint64_t, 64> scratch;
+    StringRef blobData;
+
+    BCOffsetRAII restoreOffset(MF.DeclTypeCursor);
+    llvm::BitstreamEntry entry =
+        MF.fatalIfUnexpected(MF.DeclTypeCursor.advance());
+
+    unsigned recordID = MF.fatalIfUnexpected(
+        MF.DeclTypeCursor.readRecord(entry.ID, scratch, &blobData));
+
+    if (recordID != DECL_NAME_REF)
+      // This is normal--it just means there isn't a DeclNameRef here.
+      return { DeclNameRef() };
+
+    bool isCompoundName;
+    bool hasModuleSelector;
+    ArrayRef<uint64_t> rawPieceIDs;
+
+    DeclNameRefLayout::readRecord(scratch, isCompoundName, hasModuleSelector,
+                                  rawPieceIDs);
+    restoreOffset.cancel();
+
+    Identifier moduleSelector;
+    DeclBaseName baseName;
+
+    unsigned restIndex = 0;
+
+    ASSERT(rawPieceIDs.size() > 0);
+    if (hasModuleSelector) {
+      moduleSelector = MF.getIdentifier(rawPieceIDs[restIndex]);
+      restIndex++;
+    }
+
+    ASSERT(rawPieceIDs.size() > restIndex);
+    baseName = MF.getDeclBaseName(rawPieceIDs[restIndex]);
+    restIndex++;
+
+    if (isCompoundName) {
+      SmallVector<Identifier, 8> argLabels;
+      for (auto rawArgLabel : rawPieceIDs.drop_front(restIndex))
+        argLabels.push_back(MF.getIdentifier(rawArgLabel));
+
+      return DeclNameRef(ctx, moduleSelector, baseName, argLabels);
+    }
+
+    ASSERT(rawPieceIDs.size() == restIndex);
+    return DeclNameRef(ctx, moduleSelector, baseName);
+  }
+
   Expected<Decl *> getDeclCheckedImpl(
     llvm::function_ref<bool(DeclAttributes)> matchAttributes = nullptr);
 
@@ -6141,56 +6193,32 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
         unsigned specializationKindVal;
         GenericSignatureID specializedSigID;
 
-        ArrayRef<uint64_t> rawPieceIDs;
-        uint64_t numArgs;
+        ArrayRef<uint64_t> rawTrailingIDs;
         uint64_t numSPIGroups;
         uint64_t numAvailabilityAttrs;
-        uint64_t numTypeErasedParams;
         DeclID targetFunID;
 
         serialization::decls_block::SpecializeDeclAttrLayout::readRecord(
             scratch, exported, specializationKindVal, specializedSigID,
-            targetFunID, numArgs, numSPIGroups, numAvailabilityAttrs,
-            numTypeErasedParams, rawPieceIDs);
+            targetFunID, numSPIGroups, numAvailabilityAttrs, rawTrailingIDs);
 
-        assert(rawPieceIDs.size() == numArgs + numSPIGroups + numTypeErasedParams ||
-               rawPieceIDs.size() == (numArgs - 1 + numSPIGroups + numTypeErasedParams));
         specializationKind = specializationKindVal
                                  ? SpecializeAttr::SpecializationKind::Partial
                                  : SpecializeAttr::SpecializationKind::Full;
-        // The 'target' parameter.
-        DeclNameRef replacedFunctionName;
-        if (numArgs) {
-          bool numArgumentLabels = (numArgs == 1) ? 0 : numArgs - 2;
-          auto baseName = MF.getDeclBaseName(rawPieceIDs[0]);
-          SmallVector<Identifier, 4> pieces;
-          if (numArgumentLabels) {
-            for (auto pieceID : rawPieceIDs.slice(1, numArgumentLabels))
-              pieces.push_back(MF.getIdentifier(pieceID));
-          }
-          replacedFunctionName = (numArgs == 1)
-                                     ? DeclNameRef({baseName}) // simple name
-                                     : DeclNameRef({ctx, baseName, pieces});
-        }
 
+        auto specializedSig = MF.getGenericSignature(specializedSigID);
+
+        // Take `numSPIGroups` trailing identifiers for the SPI groups.
         SmallVector<Identifier, 4> spis;
-        if (numSPIGroups) {
-          auto numTargetFunctionPiecesToSkip =
-              (rawPieceIDs.size() == numArgs + numSPIGroups + numTypeErasedParams) ? numArgs
-                                                             : numArgs - 1;
-          for (auto id : rawPieceIDs.slice(numTargetFunctionPiecesToSkip))
+          for (auto id : rawTrailingIDs.take_front(numSPIGroups))
             spis.push_back(MF.getIdentifier(id));
-        }
 
+        // Take the rest for type-erased parameters.
         SmallVector<Type, 4> typeErasedParams;
-        if (numTypeErasedParams) {
-          auto numTargetFunctionPiecesToSkip =
-              (rawPieceIDs.size() == numArgs + numSPIGroups + numTypeErasedParams) ? numArgs + numSPIGroups
-                                                             : numArgs - 1 + numSPIGroups;
-          for (auto id : rawPieceIDs.slice(numTargetFunctionPiecesToSkip))
-            typeErasedParams.push_back(MF.getType(id));
-        }
+        for (auto id : rawTrailingIDs.drop_front(numSPIGroups))
+          typeErasedParams.push_back(MF.getType(id));
 
+        // Read availability attrs.
         SmallVector<AvailableAttr *, 4> availabilityAttrs;
         while (numAvailabilityAttrs) {
           // Prepare to read the next record.
@@ -6220,10 +6248,12 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
           --numAvailabilityAttrs;
         }
 
-        auto specializedSig = MF.getGenericSignature(specializedSigID);
+        // Read target function DeclNameRef, if present.
+        DeclNameRef targetFunName = deserializeDeclNameRefIfPresent();
+
         Attr = SpecializeAttr::create(ctx, exported != 0, specializationKind,
                                       spis, availabilityAttrs, typeErasedParams,
-                                      specializedSig, replacedFunctionName, &MF,
+                                      specializedSig, targetFunName, &MF,
                                       targetFunID);
         break;
       }
@@ -6254,21 +6284,15 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
 
       case decls_block::DynamicReplacement_DECL_ATTR: {
         bool isImplicit;
-        uint64_t numArgs;
-        ArrayRef<uint64_t> rawPieceIDs;
         DeclID replacedFunID;
         serialization::decls_block::DynamicReplacementDeclAttrLayout::
-            readRecord(scratch, isImplicit, replacedFunID, numArgs, rawPieceIDs);
+            readRecord(scratch, isImplicit, replacedFunID);
 
-        auto baseName = MF.getDeclBaseName(rawPieceIDs[0]);
-        SmallVector<Identifier, 4> pieces;
-        for (auto pieceID : rawPieceIDs.slice(1))
-          pieces.push_back(MF.getIdentifier(pieceID));
+        DeclNameRef replacedFunName = deserializeDeclNameRefIfPresent();
 
-        assert(numArgs != 0);
         assert(!isImplicit && "Need to update for implicit");
-        Attr = DynamicReplacementAttr::create(
-            ctx, DeclNameRef({ ctx, baseName, pieces }), &MF, replacedFunID);
+        Attr = DynamicReplacementAttr::create(ctx, replacedFunName, &MF,
+                                              replacedFunID);
         break;
       }
 
@@ -6339,7 +6363,6 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
 
       case decls_block::Derivative_DECL_ATTR: {
         bool isImplicit;
-        uint64_t origNameId;
         bool hasAccessorKind;
         uint64_t rawAccessorKind;
         DeclID origDeclId;
@@ -6347,7 +6370,7 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
         ArrayRef<uint64_t> parameters;
 
         serialization::decls_block::DerivativeDeclAttrLayout::readRecord(
-            scratch, isImplicit, origNameId, hasAccessorKind, rawAccessorKind,
+            scratch, isImplicit, hasAccessorKind, rawAccessorKind,
             origDeclId, rawDerivativeKind, parameters);
 
         std::optional<AccessorKind> accessorKind = std::nullopt;
@@ -6358,8 +6381,6 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
           accessorKind = *maybeAccessorKind;
         }
 
-        DeclNameRefWithLoc origName{DeclNameRef(MF.getDeclBaseName(origNameId)),
-                                    DeclNameLoc(), accessorKind};
         auto derivativeKind =
             getActualAutoDiffDerivativeFunctionKind(rawDerivativeKind);
         if (!derivativeKind)
@@ -6369,9 +6390,14 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
           parametersBitVector[i] = parameters[i];
         auto *indices = IndexSubset::get(ctx, parametersBitVector);
 
+        auto origName = deserializeDeclNameRefIfPresent();
+        DeclNameRefWithLoc origNameWithLoc{origName, DeclNameLoc(),
+                                           accessorKind};
+
         auto *derivativeAttr =
             DerivativeAttr::create(ctx, isImplicit, SourceLoc(), SourceRange(),
-                                   /*baseType*/ nullptr, origName, indices);
+                                   /*baseType*/ nullptr, origNameWithLoc,
+                                   indices);
         derivativeAttr->setOriginalFunctionResolver(&MF, origDeclId);
         derivativeAttr->setDerivativeKind(*derivativeKind);
         Attr = derivativeAttr;
@@ -6380,20 +6406,21 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
 
       case decls_block::Transpose_DECL_ATTR: {
         bool isImplicit;
-        uint64_t origNameId;
         DeclID origDeclId;
         ArrayRef<uint64_t> parameters;
 
         serialization::decls_block::TransposeDeclAttrLayout::readRecord(
-            scratch, isImplicit, origNameId, origDeclId, parameters);
+            scratch, isImplicit, origDeclId, parameters);
 
-        DeclNameRefWithLoc origName{DeclNameRef(MF.getDeclBaseName(origNameId)),
-                                    DeclNameLoc(), std::nullopt};
         auto *origDecl = cast<AbstractFunctionDecl>(MF.getDecl(origDeclId));
         llvm::SmallBitVector parametersBitVector(parameters.size());
         for (unsigned i : indices(parameters))
           parametersBitVector[i] = parameters[i];
         auto *indices = IndexSubset::get(ctx, parametersBitVector);
+
+        auto origNameRef = deserializeDeclNameRefIfPresent();
+        DeclNameRefWithLoc origName{origNameRef, DeclNameLoc(), std::nullopt};
+
         auto *transposeAttr =
             TransposeAttr::create(ctx, isImplicit, SourceLoc(), SourceRange(),
                                   /*baseTypeRepr*/ nullptr, origName, indices);
diff --git a/lib/Serialization/ModuleFormat.h b/lib/Serialization/ModuleFormat.h
index 8ee427566de8e..d6043236fb4bc 100644
--- a/lib/Serialization/ModuleFormat.h
+++ b/lib/Serialization/ModuleFormat.h
@@ -58,7 +58,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 949; // vector_base_addr instruction
+const uint16_t SWIFTMODULE_VERSION_MINOR = 950; // DeclNameRef module selectors
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///
@@ -1258,6 +1258,15 @@ namespace decls_block {
     DeclIDField // API decl
   >;
 
+  /// A field containing the pieces of a \c DeclNameRef and the information
+  /// needed to reconstruct it.
+  using DeclNameRefLayout = BCRecordLayout<
+    DECL_NAME_REF,
+    BCFixed<1>, // isCompoundName
+    BCFixed<1>, // hasModuleSelector
+    BCArray<IdentifierIDField> // module selector, base name, arg labels
+  >;
+
   /// A placeholder for invalid types
   TYPE_LAYOUT(ErrorTypeLayout,
     ERROR_TYPE,
@@ -2444,11 +2453,9 @@ namespace decls_block {
       BCFixed<1>,              // specialization kind
       GenericSignatureIDField, // specialized signature
       DeclIDField,             // target function
-      BCVBR<4>, // # of arguments (+1) or 1 if simple decl name, 0 if no target
       BCVBR<4>, // # of SPI groups
       BCVBR<4>, // # of availability attributes
-      BCVBR<4>, // # of type erased parameters
-      BCArray<IdentifierIDField> // target function pieces, spi groups, type erased params
+      BCArray<IdentifierIDField> // spi groups, type erased params
       >;
 
   using StorageRestrictionsDeclAttrLayout = BCRecordLayout<
@@ -2468,7 +2475,6 @@ namespace decls_block {
   using DerivativeDeclAttrLayout = BCRecordLayout<
     Derivative_DECL_ATTR,
     BCFixed<1>, // Implicit flag.
-    IdentifierIDField, // Original name.
     BCFixed<1>, // Has original accessor kind?
     AccessorKindField, // Original accessor kind.
     DeclIDField, // Original function declaration.
@@ -2479,7 +2485,6 @@ namespace decls_block {
   using TransposeDeclAttrLayout = BCRecordLayout<
     Transpose_DECL_ATTR,
     BCFixed<1>, // Implicit flag.
-    IdentifierIDField, // Original name.
     DeclIDField, // Original function declaration.
     BCArray<BCFixed<1>> // Transposed parameter indices' bitvector.
   >;
@@ -2494,9 +2499,7 @@ namespace decls_block {
   using DynamicReplacementDeclAttrLayout = BCRecordLayout<
     DynamicReplacement_DECL_ATTR,
     BCFixed<1>, // implicit flag
-    DeclIDField, // replaced function
-    BCVBR<4>,   // # of arguments (+1) or zero if no name
-    BCArray<IdentifierIDField>
+    DeclIDField // replaced function
   >;
 
   using TypeEraserDeclAttrLayout = BCRecordLayout<
diff --git a/lib/Serialization/Serialization.cpp b/lib/Serialization/Serialization.cpp
index adc4742aeb497..23e2af0c9b868 100644
--- a/lib/Serialization/Serialization.cpp
+++ b/lib/Serialization/Serialization.cpp
@@ -3204,49 +3204,34 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
     case DeclAttrKind::Specialize: {
       auto abbrCode = S.DeclTypeAbbrCodes[SpecializeDeclAttrLayout::Code];
       auto attr = cast<SpecializeAttr>(DA);
-      auto targetFun = attr->getTargetFunctionName();
       auto *afd = cast<AbstractFunctionDecl>(D);
       auto *targetFunDecl = attr->getTargetFunctionDecl(afd);
 
       SmallVector<IdentifierID, 4> pieces;
 
-      // encodes whether this a a simple or compound name by adding one.
-      size_t numArgs = 0;
-      if (targetFun) {
-        pieces.push_back(S.addDeclBaseNameRef(targetFun.getBaseName()));
-        for (auto argName : targetFun.getArgumentNames())
-          pieces.push_back(S.addDeclBaseNameRef(argName));
-        if (targetFun.isSimpleName()) {
-          assert(pieces.size() == 1);
-          numArgs = 1;
-        } else
-          numArgs = pieces.size() + 1;
-      }
-
+      // SPI groups
+      auto numSPIGroups = attr->getSPIGroups().size();
       for (auto spi : attr->getSPIGroups()) {
         assert(!spi.empty() && "Empty SPI name");
         pieces.push_back(S.addDeclBaseNameRef(spi));
       }
 
+      // Type-erased params
       for (auto ty : attr->getTypeErasedParams()) {
         pieces.push_back(S.addTypeRef(ty));
       }
 
-      auto numSPIGroups = attr->getSPIGroups().size();
-      auto numTypeErasedParams = attr->getTypeErasedParams().size();
-      assert(pieces.size() == numArgs + numSPIGroups + numTypeErasedParams ||
-             pieces.size() == (numArgs - 1 + numSPIGroups + numTypeErasedParams));
       auto numAvailabilityAttrs = attr->getAvailableAttrs().size();
       SpecializeDeclAttrLayout::emitRecord(
           S.Out, S.ScratchRecord, abbrCode, (unsigned)attr->isExported(),
           (unsigned)attr->getSpecializationKind(),
           S.addGenericSignatureRef(attr->getSpecializedSignature(afd)),
-          S.addDeclRef(targetFunDecl), numArgs, numSPIGroups,
-          numAvailabilityAttrs, numTypeErasedParams,
+          S.addDeclRef(targetFunDecl), numSPIGroups, numAvailabilityAttrs,
           pieces);
       for (auto availAttr : attr->getAvailableAttrs()) {
         writeDeclAttribute(D, availAttr);
       }
+      writeDeclNameRefIfNeeded(attr->getTargetFunctionName());
       return;
     }
 
@@ -3278,16 +3263,13 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
       auto abbrCode =
           S.DeclTypeAbbrCodes[DynamicReplacementDeclAttrLayout::Code];
       auto theAttr = cast<DynamicReplacementAttr>(DA);
-      auto replacedFun = theAttr->getReplacedFunctionName();
-      SmallVector<IdentifierID, 4> pieces;
-      pieces.push_back(S.addDeclBaseNameRef(replacedFun.getBaseName()));
-      for (auto argName : replacedFun.getArgumentNames())
-        pieces.push_back(S.addDeclBaseNameRef(argName));
+
       auto *afd = cast<ValueDecl>(D)->getDynamicallyReplacedDecl();
       assert(afd && "Missing replaced decl!");
       DynamicReplacementDeclAttrLayout::emitRecord(
           S.Out, S.ScratchRecord, abbrCode, false, /*implicit flag*/
-          S.addDeclRef(afd), pieces.size(), pieces);
+          S.addDeclRef(afd));
+      writeDeclNameRefIfNeeded(theAttr->getReplacedFunctionName());
       return;
     }
 
@@ -3360,8 +3342,7 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
              "`@derivative` attribute should have original declaration set "
              "during construction or parsing");
       auto origDeclNameRef = attr->getOriginalFunctionName();
-      auto origName = origDeclNameRef.Name.getBaseName();
-      IdentifierID origNameId = S.addDeclBaseNameRef(origName);
+
       DeclID origDeclID = S.addDeclRef(attr->getOriginalFunction(ctx));
       auto derivativeKind =
           getRawStableAutoDiffDerivativeFunctionKind(attr->getDerivativeKind());
@@ -3375,9 +3356,10 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
       for (unsigned i : range(parameterIndices->getCapacity()))
         paramIndicesVector.push_back(parameterIndices->contains(i));
       DerivativeDeclAttrLayout::emitRecord(
-          S.Out, S.ScratchRecord, abbrCode, attr->isImplicit(), origNameId,
+          S.Out, S.ScratchRecord, abbrCode, attr->isImplicit(),
           origAccessorKind.has_value(), rawAccessorKind, origDeclID,
           derivativeKind, paramIndicesVector);
+      writeDeclNameRefIfNeeded(origDeclNameRef.Name);
       return;
     }
 
@@ -3387,8 +3369,7 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
       assert(attr->getOriginalFunction() &&
              "`@transpose` attribute should have original declaration set "
              "during construction or parsing");
-      auto origName = attr->getOriginalFunctionName().Name.getBaseName();
-      IdentifierID origNameId = S.addDeclBaseNameRef(origName);
+
       DeclID origDeclID = S.addDeclRef(attr->getOriginalFunction());
       auto *parameterIndices = attr->getParameterIndices();
       assert(parameterIndices && "Parameter indices must be resolved");
@@ -3396,8 +3377,9 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
       for (unsigned i : range(parameterIndices->getCapacity()))
         paramIndicesVector.push_back(parameterIndices->contains(i));
       TransposeDeclAttrLayout::emitRecord(
-          S.Out, S.ScratchRecord, abbrCode, attr->isImplicit(), origNameId,
-          origDeclID, paramIndicesVector);
+          S.Out, S.ScratchRecord, abbrCode, attr->isImplicit(), origDeclID,
+          paramIndicesVector);
+      writeDeclNameRefIfNeeded(attr->getOriginalFunctionName().Name);
       return;
     }
 
@@ -3621,6 +3603,32 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
     }
   }
 
+  void writeDeclNameRefIfNeeded(DeclNameRef name) {
+    using namespace decls_block;
+
+    // DeclNameRefs are always optional and write nothing when absent.
+    if (!name)
+      return;
+
+    bool isCompoundName = name.isCompoundName();
+    bool hasModuleSelector = name.hasModuleSelector();
+    SmallVector<IdentifierID, 8> rawPieceIDs;
+
+    if (hasModuleSelector)
+      rawPieceIDs.push_back(S.addDeclBaseNameRef(name.getModuleSelector()));
+
+    rawPieceIDs.push_back(S.addDeclBaseNameRef(name.getBaseName()));
+
+    if (isCompoundName)
+      for (auto argName : name.getArgumentNames())
+        rawPieceIDs.push_back(S.addDeclBaseNameRef(argName));
+
+    auto abbrCode = S.DeclTypeAbbrCodes[DeclNameRefLayout::Code];
+    DeclNameRefLayout::emitRecord(S.Out, S.ScratchRecord, abbrCode,
+                                  isCompoundName, hasModuleSelector,
+                                  rawPieceIDs);
+  }
+  
   size_t addConformances(const IterableDeclContext *declContext,
                          ConformanceLookupKind lookupKind,
                          SmallVectorImpl<uint64_t> &data) {
@@ -6337,6 +6345,7 @@ void Serializer::writeAllDeclsAndTypes() {
   registerDeclTypeAbbr<ErrorFlagLayout>();
   registerDeclTypeAbbr<ErrorTypeLayout>();
   registerDeclTypeAbbr<ABIOnlyCounterpartLayout>();
+  registerDeclTypeAbbr<DeclNameRefLayout>();
 
   registerDeclTypeAbbr<ClangTypeLayout>();
 
diff --git a/test/AutoDiff/Serialization/derivative_attr.swift b/test/AutoDiff/Serialization/derivative_attr.swift
index c41c0a36d1a50..2063f0283423e 100644
--- a/test/AutoDiff/Serialization/derivative_attr.swift
+++ b/test/AutoDiff/Serialization/derivative_attr.swift
@@ -141,7 +141,7 @@ extension S {
     self
   }
 
-  // CHECK: @derivative(of: subscript, wrt: self)
+  // CHECK: @derivative(of: subscript(_:), wrt: self)
   @derivative(of: subscript(_:), wrt: self)
   func derivativeSubscript<T: Differentiable>(x: T) -> (value: S, differential: (S) -> S) {
     (self, { $0 })
diff --git a/test/AutoDiff/Serialization/transpose_attr.swift b/test/AutoDiff/Serialization/transpose_attr.swift
index 9f864862cda95..d969da11c3b8c 100644
--- a/test/AutoDiff/Serialization/transpose_attr.swift
+++ b/test/AutoDiff/Serialization/transpose_attr.swift
@@ -89,7 +89,7 @@ extension S {
 extension S {
   subscript<T: Differentiable>(x: T) -> Self { self }
 
-  // CHECK: @transpose(of: subscript, wrt: self)
+  // CHECK: @transpose(of: subscript(_:), wrt: self)
   @transpose(of: subscript(_:), wrt: self)
   static func transposeSubscript<T: Differentiable>(x: T, t: Self) -> Self {
     t
diff --git a/test/NameLookup/module_selector.swift b/test/NameLookup/module_selector.swift
new file mode 100644
index 0000000000000..0efc97316ec2c
--- /dev/null
+++ b/test/NameLookup/module_selector.swift
@@ -0,0 +1,3 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature ModuleSelector
+
+// REQUIRES: swift_feature_ModuleSelector