Index: lib/Sema/SemaDeclCXX.cpp
===================================================================
--- lib/Sema/SemaDeclCXX.cpp
+++ lib/Sema/SemaDeclCXX.cpp
@@ -4747,6 +4747,15 @@
     }
 
     if (MD && ClassExported) {
+      // Instantiations of constexpr template member functions are eagerly
+      // emitted by MarkFunctionReferenced, and also emitted by
+      // InstantiateClassMembers. To avoid emitting the function twice, pass
+      // OdrUse = false to MarkFunctionReferenced and don't pass them to the
+      // consumer (PR21718).
+      bool OdrUse = !MD->isConstexpr() ||
+                    TSK != TSK_ExplicitInstantiationDefinition ||
+                    !MD->getInstantiatedFromMemberFunction();
+
       if (MD->isUserProvided()) {
         // Instantiate non-default class member functions ...
 
@@ -4756,7 +4765,7 @@
         if (TSK == TSK_ImplicitInstantiation && !ClassAttr->isInherited())
           continue;
 
-        S.MarkFunctionReferenced(Class->getLocation(), MD);
+        S.MarkFunctionReferenced(Class->getLocation(), MD, OdrUse);
 
         // The function will be passed to the consumer when its definition is
         // encountered.
@@ -4767,11 +4776,12 @@
         // defaulted methods, and the copy and move assignment operators. The
         // latter are exported even if they are trivial, because the address of
         // an operator can be taken and should compare equal accross libraries.
-        S.MarkFunctionReferenced(Class->getLocation(), MD);
+        S.MarkFunctionReferenced(Class->getLocation(), MD, OdrUse);
 
         // There is no later point when we will see the definition of this
         // function, so pass it to the consumer now.
-        S.Consumer.HandleTopLevelDecl(DeclGroupRef(MD));
+        if (OdrUse)
+          S.Consumer.HandleTopLevelDecl(DeclGroupRef(MD));
       }
     }
   }
Index: test/CodeGenCXX/dllexport.cpp
===================================================================
--- test/CodeGenCXX/dllexport.cpp
+++ test/CodeGenCXX/dllexport.cpp
@@ -615,6 +615,21 @@
 struct __declspec(dllexport) ExportedDerivedClass : NonExportedBaseClass {};
 // M32-DAG: weak_odr dllexport x86_thiscallcc void @"\01??1ExportedDerivedClass@@UAE@XZ"
 
+// Do not assert about generating code for constexpr functions twice during explicit instantiation (PR21718).
+template <typename T> struct ExplicitInstConstexprMembers {
+  // Copy assignment operator
+  // M32-DAG: define weak_odr dllexport x86_thiscallcc dereferenceable(1) %struct.ExplicitInstConstexprMembers* @"\01??4?$ExplicitInstConstexprMembers@X@@QAEAAU0@ABU0@@Z"
+
+  constexpr ExplicitInstConstexprMembers() {}
+  // M32-DAG: define weak_odr dllexport x86_thiscallcc %struct.ExplicitInstConstexprMembers* @"\01??0?$ExplicitInstConstexprMembers@X@@QAE@XZ"
+
+  ExplicitInstConstexprMembers(const ExplicitInstConstexprMembers&) = default;
+  // M32-DAG: define weak_odr dllexport x86_thiscallcc %struct.ExplicitInstConstexprMembers* @"\01??0?$ExplicitInstConstexprMembers@X@@QAE@ABU0@@Z"
+
+  constexpr int f() const { return 42; }
+  // M32-DAG: define weak_odr dllexport x86_thiscallcc i32 @"\01?f@?$ExplicitInstConstexprMembers@X@@QBEHXZ"
+};
+template struct __declspec(dllexport) ExplicitInstConstexprMembers<void>;
 
 //===----------------------------------------------------------------------===//
 // Classes with template base classes