diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -5969,27 +5969,19 @@
   return true;
 }
 
-static bool isCharType(QualType T) {
-  if (T.isNull())
+/// Returns whether S is a template specialization of std::Name with a single
+/// argument of type A.
+static bool isSpecializedAs(QualType S, llvm::StringRef Name, QualType A) {
+  if (S.isNull())
     return false;
 
-  return T->isSpecificBuiltinType(BuiltinType::Char_S) ||
-    T->isSpecificBuiltinType(BuiltinType::Char_U);
-}
-
-/// Returns whether a given type is a template specialization of a given name
-/// with a single argument of type char.
-static bool isCharSpecialization(QualType T, const char *Name) {
-  if (T.isNull())
-    return false;
-
-  const RecordType *RT = T->getAs<RecordType>();
+  const RecordType *RT = S->getAs<RecordType>();
   if (!RT)
     return false;
 
   const ClassTemplateSpecializationDecl *SD =
     dyn_cast<ClassTemplateSpecializationDecl>(RT->getDecl());
-  if (!SD)
+  if (!SD || !SD->getIdentifier()->isStr(Name))
     return false;
 
   if (!isStdNamespace(getEffectiveDeclContext(SD)))
@@ -5999,26 +5991,37 @@
   if (TemplateArgs.size() != 1)
     return false;
 
-  if (!isCharType(TemplateArgs[0].getAsType()))
+  if (TemplateArgs[0].getAsType() != A)
     return false;
 
-  return SD->getIdentifier()->getName() == Name;
+  return true;
 }
 
-template <std::size_t StrLen>
-static bool isStreamCharSpecialization(const ClassTemplateSpecializationDecl*SD,
-                                       const char (&Str)[StrLen]) {
-  if (!SD->getIdentifier()->isStr(Str))
+/// Returns whether SD is a template specialization std::Name<char,
+/// std::char_traits<char> [, std::allocator<char>]>
+/// HasAllocator controls whether the 3rd template argument is needed.
+static bool isStdCharSpecialization(const ClassTemplateSpecializationDecl *SD,
+                                    llvm::StringRef Name, bool HasAllocator) {
+  if (!SD->getIdentifier()->isStr(Name))
     return false;
 
   const TemplateArgumentList &TemplateArgs = SD->getTemplateArgs();
-  if (TemplateArgs.size() != 2)
+  if (TemplateArgs.size() != (HasAllocator ? 3 : 2))
     return false;
 
-  if (!isCharType(TemplateArgs[0].getAsType()))
+  QualType A = TemplateArgs[0].getAsType();
+  if (A.isNull())
+    return false;
+  // Plain 'char' is named Char_S or Char_U depending on the target ABI.
+  if (!A->isSpecificBuiltinType(BuiltinType::Char_S) &&
+      !A->isSpecificBuiltinType(BuiltinType::Char_U))
     return false;
 
-  if (!isCharSpecialization(TemplateArgs[1].getAsType(), "char_traits"))
+  if (!isSpecializedAs(TemplateArgs[1].getAsType(), "char_traits", A))
+    return false;
+
+  if (HasAllocator &&
+      !isSpecializedAs(TemplateArgs[2].getAsType(), "allocator", A))
     return false;
 
   return true;
@@ -6031,6 +6034,7 @@
       Out << "St";
       return true;
     }
+    return false;
   }
 
   if (const ClassTemplateDecl *TD = dyn_cast<ClassTemplateDecl>(ND)) {
@@ -6048,6 +6052,7 @@
       Out << "Sb";
       return true;
     }
+    return false;
   }
 
   if (const ClassTemplateSpecializationDecl *SD =
@@ -6058,46 +6063,34 @@
     //    <substitution> ::= Ss # ::std::basic_string<char,
     //                            ::std::char_traits<char>,
     //                            ::std::allocator<char> >
-    if (SD->getIdentifier()->isStr("basic_string")) {
-      const TemplateArgumentList &TemplateArgs = SD->getTemplateArgs();
-
-      if (TemplateArgs.size() != 3)
-        return false;
-
-      if (!isCharType(TemplateArgs[0].getAsType()))
-        return false;
-
-      if (!isCharSpecialization(TemplateArgs[1].getAsType(), "char_traits"))
-        return false;
-
-      if (!isCharSpecialization(TemplateArgs[2].getAsType(), "allocator"))
-        return false;
-
+    if (isStdCharSpecialization(SD, "basic_string", /*HasAllocator=*/true)) {
       Out << "Ss";
       return true;
     }
 
     //    <substitution> ::= Si # ::std::basic_istream<char,
     //                            ::std::char_traits<char> >
-    if (isStreamCharSpecialization(SD, "basic_istream")) {
+    if (isStdCharSpecialization(SD, "basic_istream", /*HasAllocator=*/false)) {
       Out << "Si";
       return true;
     }
 
     //    <substitution> ::= So # ::std::basic_ostream<char,
     //                            ::std::char_traits<char> >
-    if (isStreamCharSpecialization(SD, "basic_ostream")) {
+    if (isStdCharSpecialization(SD, "basic_ostream", /*HasAllocator=*/false)) {
       Out << "So";
       return true;
     }
 
     //    <substitution> ::= Sd # ::std::basic_iostream<char,
     //                            ::std::char_traits<char> >
-    if (isStreamCharSpecialization(SD, "basic_iostream")) {
+    if (isStdCharSpecialization(SD, "basic_iostream", /*HasAllocator=*/false)) {
       Out << "Sd";
       return true;
     }
+    return false;
   }
+
   return false;
 }