diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -177,6 +177,9 @@
   requires-expression. This fixes:
   (`#64138 <https://github.com/llvm/llvm-project/issues/64138>`_).
 
+- Update ``FunctionDeclBitfields.NumFunctionDeclBits``. This fixes:
+  (`#64171 <https://github.com/llvm/llvm-project/issues/64171>`_).
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.
diff --git a/clang/include/clang/AST/DeclBase.h b/clang/include/clang/AST/DeclBase.h
--- a/clang/include/clang/AST/DeclBase.h
+++ b/clang/include/clang/AST/DeclBase.h
@@ -1702,7 +1702,7 @@
   };
 
   /// Number of non-inherited bits in FunctionDeclBitfields.
-  enum { NumFunctionDeclBits = 30 };
+  enum { NumFunctionDeclBits = 31 };
 
   /// Stores the bits used by CXXConstructorDecl. If modified
   /// NumCXXConstructorDeclBits and the accessor
@@ -1714,12 +1714,12 @@
     /// For the bits in FunctionDeclBitfields.
     uint64_t : NumFunctionDeclBits;
 
-    /// 21 bits to fit in the remaining available space.
+    /// 20 bits to fit in the remaining available space.
     /// Note that this makes CXXConstructorDeclBitfields take
     /// exactly 64 bits and thus the width of NumCtorInitializers
     /// will need to be shrunk if some bit is added to NumDeclContextBitfields,
     /// NumFunctionDeclBitfields or CXXConstructorDeclBitfields.
-    uint64_t NumCtorInitializers : 18;
+    uint64_t NumCtorInitializers : 17;
     uint64_t IsInheritingConstructor : 1;
 
     /// Whether this constructor has a trail-allocated explicit specifier.
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -580,7 +580,7 @@
 }
 
 void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
-  static_assert(DeclContext::NumFunctionDeclBits == 30,
+  static_assert(DeclContext::NumFunctionDeclBits == 31,
                 "You need to update the serializer after you change the "
                 "FunctionDeclBits");
 
@@ -1495,7 +1495,7 @@
 }
 
 void ASTDeclWriter::VisitCXXConstructorDecl(CXXConstructorDecl *D) {
-  static_assert(DeclContext::NumCXXConstructorDeclBits == 21,
+  static_assert(DeclContext::NumCXXConstructorDeclBits == 20,
                 "You need to update the serializer after you change the "
                 "CXXConstructorDeclBits");
 
diff --git a/clang/unittests/AST/ASTImporterTest.cpp b/clang/unittests/AST/ASTImporterTest.cpp
--- a/clang/unittests/AST/ASTImporterTest.cpp
+++ b/clang/unittests/AST/ASTImporterTest.cpp
@@ -7832,6 +7832,47 @@
   CheckAST(ToTU, ToC);
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase,
+       ImportFunctionDeclBitShouldNotOverwriteCtorDeclBits) {
+  Decl *From, *To;
+  std::tie(From, To) = getImportedDecl(
+      R"s(
+        struct A {
+          A() : m() {}
+          int m;
+        };
+
+        A foo() { A a; return a; }
+        A bar() { return {}; }
+      )s",
+      Lang_CXX17,
+      R"s(
+        struct A {
+          A() : m() {}
+          int m;
+        };
+        A baz() { return {}; }
+      )s",
+      Lang_CXX17, "A");
+
+  auto HasCtorInit =
+      hasAnyConstructorInitializer(cxxCtorInitializer(isMemberInitializer()));
+  auto ImpMoveCtor =
+      cxxConstructorDecl(isMoveConstructor(), isImplicit(), HasCtorInit);
+
+  auto *FromImpMoveCtor = FirstDeclMatcher<CXXConstructorDecl>().match(
+      From, ImpMoveCtor);
+  auto *ToImpMoveCtor = FirstDeclMatcher<CXXConstructorDecl>().match(
+      To, ImpMoveCtor);
+
+  EXPECT_TRUE(FromImpMoveCtor->getNumCtorInitializers() == 1);
+  EXPECT_FALSE(FromImpMoveCtor->FriendConstraintRefersToEnclosingTemplate());
+
+  EXPECT_TRUE(ToImpMoveCtor->getNumCtorInitializers() == 1);
+  EXPECT_FALSE(ToImpMoveCtor->FriendConstraintRefersToEnclosingTemplate());
+  EXPECT_TRUE(*ToImpMoveCtor->init_begin());
+}
+
 AST_MATCHER_P(UsingShadowDecl, hasIntroducerDecl, internal::Matcher<NamedDecl>,
               InnerMatcher) {
   return InnerMatcher.matches(*Node.getIntroducer(), Finder, Builder);
diff --git a/clang/unittests/AST/DeclTest.cpp b/clang/unittests/AST/DeclTest.cpp
--- a/clang/unittests/AST/DeclTest.cpp
+++ b/clang/unittests/AST/DeclTest.cpp
@@ -353,6 +353,32 @@
   EXPECT_TRUE(getFooValue->isInlined());
 }
 
+TEST(Decl, FunctionDeclBitsShouldNotOverlapWithCXXConstructorDeclBits) {
+  llvm::Annotations Code(R"(
+    struct A {
+      A() : m() {}
+      int m;
+    };
+
+    A f() { return A(); }
+    )");
+
+  auto AST = tooling::buildASTFromCodeWithArgs(Code.code(), {"-std=c++14"});
+  ASTContext &Ctx = AST->getASTContext();
+
+  auto HasCtorInit =
+      hasAnyConstructorInitializer(cxxCtorInitializer(isMemberInitializer()));
+  auto ImpMoveCtor =
+      cxxConstructorDecl(isMoveConstructor(), isImplicit(), HasCtorInit)
+          .bind("MoveCtor");
+
+  auto *ToImpMoveCtor =
+      selectFirst<CXXConstructorDecl>("MoveCtor", match(ImpMoveCtor, Ctx));
+
+  EXPECT_TRUE(ToImpMoveCtor->getNumCtorInitializers() == 1);
+  EXPECT_FALSE(ToImpMoveCtor->FriendConstraintRefersToEnclosingTemplate());
+}
+
 TEST(Decl, NoProtoFunctionDeclAttributes) {
   llvm::Annotations Code(R"(
     void f();