Index: llvm/cmake/modules/TableGen.cmake
===================================================================
--- llvm/cmake/modules/TableGen.cmake
+++ llvm/cmake/modules/TableGen.cmake
@@ -80,6 +80,10 @@
     set(tblgen_change_flag "--write-if-changed")
   endif()
 
+  if (LLVM_ENABLE_WARNINGS)
+    list(APPEND LLVM_TABLEGEN_FLAGS "-warn-on-unused-template-args")
+  endif()
+
   # We need both _TABLEGEN_TARGET and _TABLEGEN_EXE in the  DEPENDS list
   # (both the target and the file) to have .inc files rebuilt on
   # a tablegen change, as cmake does not propagate file-level dependencies
Index: llvm/include/llvm/TableGen/Record.h
===================================================================
--- llvm/include/llvm/TableGen/Record.h
+++ llvm/include/llvm/TableGen/Record.h
@@ -1414,6 +1414,7 @@
   SMLoc Loc; // Source location of definition of name.
   PointerIntPair<RecTy *, 2, FieldKind> TyAndKind;
   Init *Value;
+  bool IsUsed = false;
 
 public:
   RecordVal(Init *N, RecTy *T, FieldKind K);
@@ -1458,6 +1459,11 @@
   /// Set the value and source location of the field.
   bool setValue(Init *V, SMLoc NewLoc);
 
+  /// Whether this value is used. Useful for reporting warnings, for example
+  /// when a template argument is unused.
+  void setUsed(bool Used) { IsUsed = Used; }
+  bool isUsed() const { return IsUsed; }
+
   void dump() const;
 
   /// Print the value to an output stream, possibly with a semicolon.
@@ -1632,6 +1638,7 @@
   }
 
   void checkRecordAssertions();
+  void checkUnusedTemplateArgs();
 
   bool isSubClassOf(const Record *R) const {
     for (const auto &SCPair : SuperClasses)
Index: llvm/lib/TableGen/Main.cpp
===================================================================
--- llvm/lib/TableGen/Main.cpp
+++ llvm/lib/TableGen/Main.cpp
@@ -55,6 +55,10 @@
 static cl::opt<bool>
 TimePhases("time-phases", cl::desc("Time phases of parser and backend"));
 
+static cl::opt<bool> WarnOnUnusedTemplateArgs(
+    "warn-on-unused-template-args",
+    cl::desc("Emit a warning on unused template arguments."));
+
 static int reportError(const char *ProgName, Twine Msg) {
   errs() << ProgName << ": " << Msg;
   errs().flush();
@@ -107,7 +111,7 @@
   // it later.
   SrcMgr.setIncludeDirs(IncludeDirs);
 
-  TGParser Parser(SrcMgr, MacroNames, Records);
+  TGParser Parser(SrcMgr, MacroNames, Records, WarnOnUnusedTemplateArgs);
 
   if (Parser.ParseFile())
     return 1;
Index: llvm/lib/TableGen/Record.cpp
===================================================================
--- llvm/lib/TableGen/Record.cpp
+++ llvm/lib/TableGen/Record.cpp
@@ -2660,6 +2660,17 @@
   }
 }
 
+// Report a warning if the record has unused template arguments.
+void Record::checkUnusedTemplateArgs() {
+  ArrayRef<Init *> TArgs = getTemplateArgs();
+  for (unsigned I = 0, E = TArgs.size(); I != E; ++I) {
+    RecordVal *Arg = getValue(TArgs[I]);
+    if (!Arg->isUsed())
+      PrintWarning(Arg->getLoc(),
+                   "unused template argument: " + Twine(Arg->getName()));
+  }
+}
+
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 LLVM_DUMP_METHOD void RecordKeeper::dump() const { errs() << *this; }
 #endif
Index: llvm/lib/TableGen/TGParser.h
===================================================================
--- llvm/lib/TableGen/TGParser.h
+++ llvm/lib/TableGen/TGParser.h
@@ -160,10 +160,13 @@
                       // exist.
   };
 
+  bool WarnOnUnusedTemplateArgs = false;
+
 public:
-  TGParser(SourceMgr &SM, ArrayRef<std::string> Macros,
-           RecordKeeper &records)
-    : Lex(SM, Macros), CurMultiClass(nullptr), Records(records) {}
+  TGParser(SourceMgr &SM, ArrayRef<std::string> Macros, RecordKeeper &records,
+           const bool WarnOnUnusedTemplateArgs = false)
+      : Lex(SM, Macros), CurMultiClass(nullptr), Records(records),
+        WarnOnUnusedTemplateArgs(WarnOnUnusedTemplateArgs) {}
 
   /// ParseFile - Main entrypoint for parsing a tblgen file.  These parser
   /// routines return true on error, or false on success.
Index: llvm/lib/TableGen/TGParser.cpp
===================================================================
--- llvm/lib/TableGen/TGParser.cpp
+++ llvm/lib/TableGen/TGParser.cpp
@@ -874,8 +874,9 @@
 
     Record *TemplateRec = CurMultiClass ? &CurMultiClass->Rec : CurRec;
     if (TemplateRec->isTemplateArg(TemplateArgName)) {
-      const RecordVal *RV = TemplateRec->getValue(TemplateArgName);
+      RecordVal *RV = TemplateRec->getValue(TemplateArgName);
       assert(RV && "Template arg doesn't exist??");
+      RV->setUsed(true);
       return VarInit::get(TemplateArgName, RV->getType());
     } else if (Name->getValue() == "NAME") {
       return VarInit::get(TemplateArgName, StringRecTy::get());
@@ -3346,7 +3347,12 @@
     if (ParseTemplateArgList(CurRec))
       return true;
 
-  return ParseObjectBody(CurRec);
+  if (ParseObjectBody(CurRec))
+    return true;
+
+  if (WarnOnUnusedTemplateArgs)
+    CurRec->checkUnusedTemplateArgs();
+  return false;
 }
 
 /// ParseLetList - Parse a non-empty list of assignment expressions into a list
@@ -3541,6 +3547,9 @@
     PopLocalScope(MulticlassScope);
   }
 
+  if (WarnOnUnusedTemplateArgs)
+    CurMultiClass->Rec.checkUnusedTemplateArgs();
+
   CurMultiClass = nullptr;
   return false;
 }
Index: llvm/test/TableGen/warn-unused-template-arg.td
===================================================================
--- /dev/null
+++ llvm/test/TableGen/warn-unused-template-arg.td
@@ -0,0 +1,23 @@
+// RUN: llvm-tblgen --warn-on-unused-template-args %s 2>&1 | FileCheck %s
+
+class UnusedClassArg<int foo> {}
+
+// CHECK: warning: unused template argument: UnusedClassArg:foo
+// CHECK-NEXT: class UnusedClassArg<int foo> {}
+// CHECK-NEXT:                          ^
+
+multiclass UnusedMultiClassArg<int foo> {
+  def bar;
+}
+
+defm : UnusedMultiClassArg<1>;
+
+// CHECK: warning: unused template argument: UnusedMultiClassArg::foo
+// CHECK-NEXT: multiclass UnusedMultiClassArg<int foo> {
+// CHECK-NEXT:                                    ^
+
+class NoWarning<int b> {
+  int a = b;
+}
+
+// CHECK-NOT: warning: unused template argument: NoWarning:b