diff --git a/llvm/include/llvm/Demangle/ItaniumDemangle.h b/llvm/include/llvm/Demangle/ItaniumDemangle.h
--- a/llvm/include/llvm/Demangle/ItaniumDemangle.h
+++ b/llvm/include/llvm/Demangle/ItaniumDemangle.h
@@ -1583,7 +1583,8 @@
     if (isInstantiation()) {
       // The instantiations are typedefs that drop the "basic_" prefix.
       assert(SV.startsWith("basic_"));
-      SV = SV.dropFront(sizeof("basic_") - 1);
+      assert(sizeof("basic_") - 1 <= SV.size());
+      SV = SV.substr(sizeof("basic_") - 1);
     }
     return SV;
   }
@@ -1838,7 +1839,8 @@
       OB += "0";
     } else if (Offset[0] == 'n') {
       OB += "-";
-      OB += Offset.dropFront();
+      assert(1 <= Offset.size());
+      OB += Offset.substr(1);
     } else {
       OB += Offset;
     }
@@ -2263,9 +2265,10 @@
     Ty->print(OB);
     OB.printClose();
 
-    if (Integer[0] == 'n')
-      OB << "-" << Integer.dropFront(1);
-    else
+    if (Integer[0] == 'n') {
+      assert(1 <= Integer.size());
+      OB << "-" << Integer.substr(1);
+    } else
       OB << Integer;
   }
 };
@@ -2289,7 +2292,8 @@
 
     if (Value[0] == 'n') {
       OB += '-';
-      OB += Value.dropFront(1);
+      assert(1 <= Value.size());
+      OB += Value.substr(1);
     } else
       OB += Value;
 
@@ -2632,7 +2636,8 @@
       if (Kind < Unnameable) {
         assert(Res.startsWith("operator") &&
                "operator name does not start with 'operator'");
-        Res = Res.dropFront(sizeof("operator") - 1);
+        assert(sizeof("operator") - 1 <= Res.size());
+        Res = Res.substr(sizeof("operator") - 1);
         Res.consumeFront(' ');
       }
       return Res;
@@ -3706,7 +3711,8 @@
 
     // extension            ::= U <objc-name> <objc-type>  # objc-type<identifier>
     if (Qual.startsWith("objcproto")) {
-      StringView ProtoSourceName = Qual.dropFront(std::strlen("objcproto"));
+      assert(std::strlen("objcproto") <= Qual.size());
+      StringView ProtoSourceName = Qual.substr(std::strlen("objcproto"));
       StringView Proto;
       {
         ScopedOverride<const char *> SaveFirst(First, ProtoSourceName.begin()),
diff --git a/llvm/include/llvm/Demangle/StringView.h b/llvm/include/llvm/Demangle/StringView.h
--- a/llvm/include/llvm/Demangle/StringView.h
+++ b/llvm/include/llvm/Demangle/StringView.h
@@ -55,12 +55,6 @@
     return npos;
   }
 
-  StringView dropFront(size_t N = 1) const {
-    if (N >= size())
-      N = size();
-    return StringView(First + N, Last);
-  }
-
   StringView dropBack(size_t N = 1) const {
     if (N >= size())
       N = size();
@@ -85,14 +79,16 @@
   bool consumeFront(char C) {
     if (!startsWith(C))
       return false;
-    *this = dropFront(1);
+    assert(1 <= size());
+    *this = substr(1);
     return true;
   }
 
   bool consumeFront(StringView S) {
     if (!startsWith(S))
       return false;
-    *this = dropFront(S.size());
+    assert(S.size() <= size());
+    *this = substr(S.size());
     return true;
   }
 
diff --git a/llvm/lib/Demangle/MicrosoftDemangle.cpp b/llvm/lib/Demangle/MicrosoftDemangle.cpp
--- a/llvm/lib/Demangle/MicrosoftDemangle.cpp
+++ b/llvm/lib/Demangle/MicrosoftDemangle.cpp
@@ -168,11 +168,13 @@
   // ambiguity is also why single digit encoded numbers use 0-9 rather than A-J.
   if (Candidate[0] < 'B' || Candidate[0] > 'P')
     return false;
-  Candidate = Candidate.dropFront();
+  assert(1 <= Candidate.size());
+  Candidate = Candidate.substr(1);
   while (!Candidate.empty()) {
     if (Candidate[0] < 'A' || Candidate[0] > 'P')
       return false;
-    Candidate = Candidate.dropFront();
+    assert(1 <= Candidate.size());
+    Candidate = Candidate.substr(1);
   }
 
   return true;
@@ -486,7 +488,8 @@
 IdentifierNode *
 Demangler::demangleFunctionIdentifierCode(StringView &MangledName) {
   assert(MangledName.startsWith('?'));
-  MangledName = MangledName.dropFront();
+  assert(1 <= MangledName.size());
+  MangledName = MangledName.substr(1);
   if (MangledName.empty()) {
     Error = true;
     return nullptr;
@@ -765,7 +768,8 @@
     return nullptr;
   }
   const char *Start = MangledName.begin();
-  MangledName = MangledName.dropFront(MD5Last + 1);
+  assert(MD5Last + 1 <= MangledName.size());
+  MangledName = MangledName.substr(MD5Last + 1);
 
   // There are two additional special cases for MD5 names:
   // 1. For complete object locators where the object name is long enough
@@ -902,7 +906,8 @@
 
   if (startsWithDigit(MangledName)) {
     uint64_t Ret = MangledName[0] - '0' + 1;
-    MangledName = MangledName.dropFront(1);
+    assert(1 <= MangledName.size());
+    MangledName = MangledName.substr(1);
     return {Ret, IsNegative};
   }
 
@@ -910,7 +915,8 @@
   for (size_t i = 0; i < MangledName.size(); ++i) {
     char C = MangledName[i];
     if (C == '@') {
-      MangledName = MangledName.dropFront(i + 1);
+      assert(i + 1 <= MangledName.size());
+      MangledName = MangledName.substr(i + 1);
       return {Ret, IsNegative};
     }
     if ('A' <= C && C <= 'P') {
@@ -965,7 +971,8 @@
     return nullptr;
   }
 
-  MangledName = MangledName.dropFront();
+  assert(1 <= MangledName.size());
+  MangledName = MangledName.substr(1);
   return Backrefs.Names[I];
 }
 
@@ -1036,7 +1043,8 @@
   if (!MangledName.startsWith('?'))
     return MangledName.popFront();
 
-  MangledName = MangledName.dropFront();
+  assert(1 <= MangledName.size());
+  MangledName = MangledName.substr(1);
   if (MangledName.empty())
     goto CharLiteralError;
 
@@ -1050,14 +1058,16 @@
     // Don't append the null terminator.
     uint8_t C1 = rebasedHexDigitToNumber(Nibbles[0]);
     uint8_t C2 = rebasedHexDigitToNumber(Nibbles[1]);
-    MangledName = MangledName.dropFront(2);
+    assert(2 <= MangledName.size());
+    MangledName = MangledName.substr(2);
     return (C1 << 4) | C2;
   }
 
   if (startsWithDigit(MangledName)) {
     const char *Lookup = ",/\\:. \n\t'-";
     char C = Lookup[MangledName[0] - '0'];
-    MangledName = MangledName.dropFront();
+    assert(1 <= MangledName.size());
+    MangledName = MangledName.substr(1);
     return C;
   }
 
@@ -1067,7 +1077,8 @@
                        '\xEF', '\xF0', '\xF1', '\xF2', '\xF3', '\xF4', '\xF5',
                        '\xF6', '\xF7', '\xF8', '\xF9', '\xFA'};
     char C = Lookup[MangledName[0] - 'a'];
-    MangledName = MangledName.dropFront();
+    assert(1 <= MangledName.size());
+    MangledName = MangledName.substr(1);
     return C;
   }
 
@@ -1077,7 +1088,8 @@
                        '\xCF', '\xD0', '\xD1', '\xD2', '\xD3', '\xD4', '\xD5',
                        '\xD6', '\xD7', '\xD8', '\xD9', '\xDA'};
     char C = Lookup[MangledName[0] - 'A'];
-    MangledName = MangledName.dropFront();
+    assert(1 <= MangledName.size());
+    MangledName = MangledName.substr(1);
     return C;
   }
 
@@ -1310,7 +1322,8 @@
   if (CrcEndPos == StringView::npos)
     goto StringLiteralError;
   CRC = MangledName.substr(0, CrcEndPos);
-  MangledName = MangledName.dropFront(CrcEndPos + 1);
+  assert(CrcEndPos + 1 <= MangledName.size());
+  MangledName = MangledName.substr(CrcEndPos + 1);
   if (MangledName.empty())
     goto StringLiteralError;
 
@@ -1391,7 +1404,8 @@
     if (i == 0)
       break;
     S = MangledName.substr(0, i);
-    MangledName = MangledName.dropFront(i + 1);
+    assert(i + 1 <= MangledName.size());
+    MangledName = MangledName.substr(i + 1);
 
     if (Memorize)
       memorizeString(S);
@@ -1416,6 +1430,7 @@
   }
   StringView NamespaceKey = MangledName.substr(0, EndPos);
   memorizeString(NamespaceKey);
+  assert(EndPos + 1 <= MangledName.size());
   MangledName = MangledName.substr(EndPos + 1);
   return Node;
 }
@@ -2124,7 +2139,8 @@
         Error = true;
         return nullptr;
       }
-      MangledName = MangledName.dropFront();
+      assert(1 <= MangledName.size());
+      MangledName = MangledName.substr(1);
 
       *Current = Arena.alloc<NodeList>();
       (*Current)->N = Backrefs.FunctionParams[N];
@@ -2206,7 +2222,8 @@
       TP.N = TPRN = Arena.alloc<TemplateParameterReferenceNode>();
       TPRN->IsMemberPointer = true;
 
-      MangledName = MangledName.dropFront();
+      assert(1 <= MangledName.size());
+      MangledName = MangledName.substr(1);
       // 1 - single inheritance       <name>
       // H - multiple inheritance     <name> <number>
       // I - virtual inheritance      <name> <number> <number>
@@ -2252,7 +2269,8 @@
       TP.N = TPRN = Arena.alloc<TemplateParameterReferenceNode>();
 
       // Data member pointer.
-      MangledName = MangledName.dropFront();
+      assert(1 <= MangledName.size());
+      MangledName = MangledName.substr(1);
       char InheritanceSpecifier = MangledName.popFront();
 
       switch (InheritanceSpecifier) {