diff --git a/include/swift/AST/ProtocolConformanceRef.h b/include/swift/AST/ProtocolConformanceRef.h
index 7972750ab93..1368cff87bf 100644
--- a/include/swift/AST/ProtocolConformanceRef.h
+++ b/include/swift/AST/ProtocolConformanceRef.h
@@ -20,7 +20,6 @@
 #include "llvm/ADT/PointerUnion.h"
 #include "swift/AST/TypeAlignments.h"
 #include "swift/AST/Type.h"
-#include "swift/AST/ProtocolConformance.h"
 
 namespace llvm {
   class raw_ostream;
@@ -28,6 +27,8 @@ namespace llvm {
 
 namespace swift {
 
+class ProtocolConformance;
+
 /// A ProtocolConformanceRef is a handle to a protocol conformance which
 /// may be either concrete or abstract.
 ///
diff --git a/lib/AST/ASTContext.cpp b/lib/AST/ASTContext.cpp
index 70f5135a6c7..104506289c6 100644
--- a/lib/AST/ASTContext.cpp
+++ b/lib/AST/ASTContext.cpp
@@ -28,6 +28,7 @@
 #include "swift/AST/LazyResolver.h"
 #include "swift/AST/ModuleLoader.h"
 #include "swift/AST/NameLookup.h"
+#include "swift/AST/ProtocolConformance.h"
 #include "swift/AST/RawComment.h"
 #include "swift/AST/SILLayout.h"
 #include "swift/AST/TypeCheckerDebugConsumer.h"
@@ -3606,6 +3607,36 @@ static NominalTypeDecl *findUnderlyingTypeInModule(ASTContext &ctx,
   return nullptr;
 }
 
+bool ForeignRepresentationInfo::isRepresentableAsOptional() const {
+  switch (getKind()) {
+  case ForeignRepresentableKind::None:
+    llvm_unreachable("this type is not representable");
+
+  case ForeignRepresentableKind::Trivial:
+    return Storage.getPointer() != 0;
+
+  case ForeignRepresentableKind::Bridged: {
+    auto KPK_ObjectiveCBridgeable = KnownProtocolKind::ObjectiveCBridgeable;
+    ProtocolDecl *proto = getConformance()->getProtocol();
+    assert(proto->isSpecificProtocol(KPK_ObjectiveCBridgeable) &&
+           "unknown protocol; does it support optional?");
+    (void)proto;
+    (void)KPK_ObjectiveCBridgeable;
+
+    return true;
+  }
+
+  case ForeignRepresentableKind::BridgedError:
+    return true;
+
+  case ForeignRepresentableKind::Object:
+  case ForeignRepresentableKind::StaticBridged:
+    llvm_unreachable("unexpected kind in ForeignRepresentableCacheEntry");
+  }
+
+  llvm_unreachable("Unhandled ForeignRepresentableKind in switch.");
+}
+
 ForeignRepresentationInfo
 ASTContext::getForeignRepresentationInfo(NominalTypeDecl *nominal,
                                          ForeignLanguage language,
diff --git a/lib/AST/ForeignRepresentationInfo.h b/lib/AST/ForeignRepresentationInfo.h
index 7dd06292446..05fdb94719d 100644
--- a/lib/AST/ForeignRepresentationInfo.h
+++ b/lib/AST/ForeignRepresentationInfo.h
@@ -13,7 +13,6 @@
 #ifndef SWIFT_FOREIGNREPRESENTATIONINFO_H
 #define SWIFT_FOREIGNREPRESENTATIONINFO_H
 
-#include "swift/AST/ProtocolConformance.h"
 #include "swift/AST/Type.h"
 #include "swift/Basic/LLVM.h"
 #include "llvm/ADT/PointerEmbeddedInt.h"
@@ -21,6 +20,8 @@
 
 namespace swift {
 
+class ProtocolConformance;
+
 class ForeignRepresentationInfo {
   using PayloadTy =
     llvm::PointerEmbeddedInt<uintptr_t, sizeof(uintptr_t) * CHAR_BIT - 3>;
@@ -112,35 +113,7 @@ public:
   }
 
   /// Returns true if the optional version of this type is also representable.
-  bool isRepresentableAsOptional() const {
-    switch (getKind()) {
-    case ForeignRepresentableKind::None:
-      llvm_unreachable("this type is not representable");
-
-    case ForeignRepresentableKind::Trivial:
-      return Storage.getPointer() != 0;
-
-    case ForeignRepresentableKind::Bridged: {
-      auto KPK_ObjectiveCBridgeable = KnownProtocolKind::ObjectiveCBridgeable;
-      ProtocolDecl *proto = getConformance()->getProtocol();
-      assert(proto->isSpecificProtocol(KPK_ObjectiveCBridgeable) &&
-             "unknown protocol; does it support optional?");
-      (void)proto;
-      (void)KPK_ObjectiveCBridgeable;
-
-      return true;
-    }
-
-    case ForeignRepresentableKind::BridgedError:
-      return true;
-
-    case ForeignRepresentableKind::Object:
-    case ForeignRepresentableKind::StaticBridged:
-      llvm_unreachable("unexpected kind in ForeignRepresentableCacheEntry");
-    }
-
-    llvm_unreachable("Unhandled ForeignRepresentableKind in switch.");
-  }
+  bool isRepresentableAsOptional() const;
 };
 
 } // end namespace swift
diff --git a/lib/AST/Type.cpp b/lib/AST/Type.cpp
index 659ad44d56c..e80102800a2 100644
--- a/lib/AST/Type.cpp
+++ b/lib/AST/Type.cpp
@@ -23,6 +23,7 @@
 #include "swift/AST/GenericEnvironment.h"
 #include "swift/AST/LazyResolver.h"
 #include "swift/AST/Module.h"
+#include "swift/AST/ProtocolConformance.h"
 #include "swift/AST/SubstitutionMap.h"
 #include "swift/AST/TypeLoc.h"
 #include "llvm/ADT/APFloat.h"
diff --git a/lib/Sema/CSApply.cpp b/lib/Sema/CSApply.cpp
index de33fabf1ef..069203a6630 100644
--- a/lib/Sema/CSApply.cpp
+++ b/lib/Sema/CSApply.cpp
@@ -19,6 +19,7 @@
 #include "ConstraintSystem.h"
 #include "swift/AST/ASTVisitor.h"
 #include "swift/AST/ASTWalker.h"
+#include "swift/AST/ProtocolConformance.h"
 #include "swift/AST/SubstitutionMap.h"
 #include "swift/Basic/StringExtras.h"
 #include "llvm/ADT/APFloat.h"
diff --git a/lib/Sema/CSRanking.cpp b/lib/Sema/CSRanking.cpp
index 02cc31d708a..d437e0e0c11 100644
--- a/lib/Sema/CSRanking.cpp
+++ b/lib/Sema/CSRanking.cpp
@@ -15,6 +15,7 @@
 //
 //===----------------------------------------------------------------------===//
 #include "ConstraintSystem.h"
+#include "swift/AST/ProtocolConformance.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Support/Compiler.h"
 
diff --git a/lib/Sema/CodeSynthesis.cpp b/lib/Sema/CodeSynthesis.cpp
index 2756738ce78..dc02e5e5f34 100644
--- a/lib/Sema/CodeSynthesis.cpp
+++ b/lib/Sema/CodeSynthesis.cpp
@@ -23,6 +23,7 @@
 #include "swift/AST/Expr.h"
 #include "swift/AST/GenericEnvironment.h"
 #include "swift/AST/ParameterList.h"
+#include "swift/AST/ProtocolConformance.h"
 #include "swift/Basic/Defer.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringExtras.h"
diff --git a/lib/Sema/ConstraintSystem.h b/lib/Sema/ConstraintSystem.h
index ef5f3860fc1..fadbf85bf38 100644
--- a/lib/Sema/ConstraintSystem.h
+++ b/lib/Sema/ConstraintSystem.h
@@ -29,7 +29,6 @@
 #include "swift/AST/ASTVisitor.h"
 #include "swift/AST/ASTWalker.h"
 #include "swift/AST/NameLookup.h"
-#include "swift/AST/ProtocolConformance.h"
 #include "swift/AST/Types.h"
 #include "swift/AST/TypeCheckerDebugConsumer.h"
 #include "llvm/ADT/ilist.h"
diff --git a/lib/Sema/TypeCheckDecl.cpp b/lib/Sema/TypeCheckDecl.cpp
index d324496ef62..46518d4211e 100644
--- a/lib/Sema/TypeCheckDecl.cpp
+++ b/lib/Sema/TypeCheckDecl.cpp
@@ -30,6 +30,7 @@
 #include "swift/AST/GenericSignatureBuilder.h"
 #include "swift/AST/NameLookup.h"
 #include "swift/AST/PrettyStackTrace.h"
+#include "swift/AST/ProtocolConformance.h"
 #include "swift/AST/ReferencedNameTracker.h"
 #include "swift/AST/TypeWalker.h"
 #include "swift/Basic/Statistic.h"
diff --git a/lib/Sema/TypeCheckProtocol.cpp b/lib/Sema/TypeCheckProtocol.cpp
index ddf4d8a720c..9896f438e38 100644
--- a/lib/Sema/TypeCheckProtocol.cpp
+++ b/lib/Sema/TypeCheckProtocol.cpp
@@ -28,6 +28,7 @@
 #include "swift/AST/GenericEnvironment.h"
 #include "swift/AST/GenericSignature.h"
 #include "swift/AST/NameLookup.h"
+#include "swift/AST/ProtocolConformance.h"
 #include "swift/AST/ReferencedNameTracker.h"
 #include "swift/AST/TypeMatcher.h"
 #include "swift/AST/TypeWalker.h"