Index: docs/LangRef.rst
===================================================================
--- docs/LangRef.rst
+++ docs/LangRef.rst
@@ -6591,6 +6591,9 @@
    - The calling conventions of the caller and callee must match.
    - All ABI-impacting function attributes, such as sret, byval, inreg,
      returned, and inalloca, must match.
+   - The callee must be varargs iff the caller is varargs. Bitcasting a
+     non-varargs function to the appropriate varargs type is legal so
+     long as the non-varargs prefixes obey the other rules.
 
    Tail call optimization for calls marked ``tail`` is guaranteed to occur if
    the following conditions are met:
Index: lib/AsmParser/LLParser.h
===================================================================
--- lib/AsmParser/LLParser.h
+++ lib/AsmParser/LLParser.h
@@ -372,7 +372,9 @@
         : Loc(loc), V(v), Attrs(attrs) {}
     };
     bool ParseParameterList(SmallVectorImpl<ParamInfo> &ArgList,
-                            PerFunctionState &PFS);
+                            PerFunctionState &PFS,
+                            bool IsMustTailCall = false,
+                            bool InVarArgsFunc = false);
 
     // Constant Parsing.
     bool ParseValID(ValID &ID, PerFunctionState *PFS = nullptr);
Index: lib/AsmParser/LLParser.cpp
===================================================================
--- lib/AsmParser/LLParser.cpp
+++ lib/AsmParser/LLParser.cpp
@@ -1819,7 +1819,8 @@
 ///  Arg
 ///    ::= Type OptionalAttributes Value OptionalAttributes
 bool LLParser::ParseParameterList(SmallVectorImpl<ParamInfo> &ArgList,
-                                  PerFunctionState &PFS) {
+                                  PerFunctionState &PFS, bool IsMustTailCall,
+                                  bool InVarArgsFunc) {
   if (ParseToken(lltok::lparen, "expected '(' in call"))
     return true;
 
@@ -1830,6 +1831,17 @@
         ParseToken(lltok::comma, "expected ',' in argument list"))
       return true;
 
+    // Parse an ellipsis if this is a musttail call in a variadic function.
+    if (Lex.getKind() == lltok::dotdotdot) {
+      const char *Msg = "unexpected ellipsis in argument list for ";
+      if (!IsMustTailCall)
+        return TokError(Twine(Msg) + "non-musttail call");
+      if (!InVarArgsFunc)
+        return TokError(Twine(Msg) + "musttail call in non-varargs function");
+      Lex.Lex();  // Lex the '...', it is purely for readability.
+      return ParseToken(lltok::rparen, "expected ')' at end of argument list");
+    }
+
     // Parse the argument.
     LocTy ArgLoc;
     Type *ArgTy = nullptr;
@@ -1846,6 +1858,10 @@
                                                              ArgAttrs)));
   }
 
+  if (IsMustTailCall && InVarArgsFunc)
+    return TokError("expected '...' at end of argument list for musttail call "
+                    "in varargs function");
+
   Lex.Lex();  // Lex the ')'.
   return false;
 }
@@ -4214,7 +4230,8 @@
       ParseOptionalReturnAttrs(RetAttrs) ||
       ParseType(RetType, RetTypeLoc, true /*void allowed*/) ||
       ParseValID(CalleeID) ||
-      ParseParameterList(ArgList, PFS) ||
+      ParseParameterList(ArgList, PFS, TCK == CallInst::TCK_MustTail,
+                         PFS.getFunction().isVarArg()) ||
       ParseFnAttributeValuePairs(FnAttrs, FwdRefAttrGrps, false,
                                  BuiltinLoc))
     return true;
Index: lib/IR/AsmWriter.cpp
===================================================================
--- lib/IR/AsmWriter.cpp
+++ lib/IR/AsmWriter.cpp
@@ -2174,6 +2174,14 @@
         Out << ", ";
       writeParamOperand(CI->getArgOperand(op), PAL, op + 1);
     }
+
+    // Emit an ellipsis if this is a musttail call in a vararg function.  This
+    // is only to aid readability, musttail calls forward varargs by default.
+    if (CI->isMustTailCall() && CI->getParent() &&
+        CI->getParent()->getParent() &&
+        CI->getParent()->getParent()->isVarArg())
+      Out << ", ...";
+
     Out << ')';
     if (PAL.hasAttributes(AttributeSet::FunctionIndex))
       Out << " #" << Machine.getAttributeGroupSlot(PAL.getFnAttributes());
Index: test/Assembler/musttail-invalid-1.ll
===================================================================
--- /dev/null
+++ test/Assembler/musttail-invalid-1.ll
@@ -0,0 +1,14 @@
+; RUN: not llvm-as < %s -o /dev/null 2>&1 | FileCheck %s
+
+; Check the error message on using ", ..." when we can't actually forward
+; varargs.
+
+%struct.A = type { i32 }
+
+declare i8* @f(i8*, ...)
+
+define i8* @f_thunk(i8* %this) {
+  %rv = musttail call i8* (i8*, ...)* @f(i8* %this, ...)
+; CHECK: error: unexpected ellipsis in argument list for musttail call in non-varargs function
+  ret i8* %rv
+}
Index: test/Assembler/musttail-invalid-2.ll
===================================================================
--- /dev/null
+++ test/Assembler/musttail-invalid-2.ll
@@ -0,0 +1,13 @@
+; RUN: not llvm-as < %s -o /dev/null 2>&1 | FileCheck %s
+
+; Check the error message on skipping ", ..." at the end of a musttail call argument list.
+
+%struct.A = type { i32 }
+
+declare i8* @f(i8*, ...)
+
+define i8* @f_thunk(i8* %this, ...) {
+  %rv = musttail call i8* (i8*, ...)* @f(i8* %this)
+; CHECK: error: expected '...' at end of argument list for musttail call in varargs function
+  ret i8* %rv
+}
Index: test/Assembler/musttail.ll
===================================================================
--- /dev/null
+++ test/Assembler/musttail.ll
@@ -0,0 +1,14 @@
+; RUN: llvm-as < %s | llvm-dis | FileCheck %s
+
+; Check that the ellipsis round trips.
+
+%struct.A = type { i32 }
+
+declare i8* @f(i8*, ...)
+
+define i8* @f_thunk(i8* %this, ...) {
+  %rv = musttail call i8* (i8*, ...)* @f(i8* %this, ...)
+  ret i8* %rv
+}
+; CHECK-LABEL: define i8* @f_thunk(i8* %this, ...)
+; CHECK: %rv = musttail call i8* (i8*, ...)* @f(i8* %this, ...)
Index: test/Transforms/Inline/inline-musttail-varargs.ll
===================================================================
--- /dev/null
+++ test/Transforms/Inline/inline-musttail-varargs.ll
@@ -0,0 +1,22 @@
+; RUN: opt < %s -inline -instcombine -S | FileCheck %s
+
+; We can't inline this thunk yet, but one day we will be able to.  And when we
+; do, this test case will be ready.
+
+declare void @ext_method(i8*, i32)
+
+define linkonce_odr void @thunk(i8* %this, ...) {
+  %this_adj = getelementptr i8* %this, i32 4
+  musttail call void (i8*, ...)* bitcast (void (i8*, i32)* @ext_method to void (i8*, ...)*)(i8* %this_adj, ...)
+  ret void
+}
+
+define void @thunk_caller(i8* %p) {
+  call void (i8*, ...)* @thunk(i8* %p, i32 42)
+  ret void
+}
+; CHECK-LABEL: define void @thunk_caller(i8* %p)
+; CHECK: call void (i8*, ...)* @thunk(i8* %p, i32 42)
+
+; FIXME: Inline the thunk. This should be significantly easier than inlining
+; general varargs functions.