diff --git a/lib/Sema/SemaExpr.cpp b/lib/Sema/SemaExpr.cpp
index 371a4e3..050386e 100644
--- a/lib/Sema/SemaExpr.cpp
+++ b/lib/Sema/SemaExpr.cpp
@@ -9404,7 +9404,11 @@
 
   Func->setReferenced();
 
-  if (Func->isUsed(false))
+  // Don't mark this function as used multiple times, unless it's a constexpr
+  // function which we need to instantiate.
+  if (Func->isUsed(false) &&
+      !(Func->isConstexpr() && !Func->getBody() &&
+        Func->isImplicitlyInstantiable()))
     return;
 
   if (!IsPotentiallyEvaluatedContext(*this))
@@ -9455,34 +9459,40 @@
   // class templates.
   if (Func->isImplicitlyInstantiable()) {
     bool AlreadyInstantiated = false;
+    SourceLocation PointOfInstantiation = Loc;
     if (FunctionTemplateSpecializationInfo *SpecInfo
                               = Func->getTemplateSpecializationInfo()) {
       if (SpecInfo->getPointOfInstantiation().isInvalid())
         SpecInfo->setPointOfInstantiation(Loc);
       else if (SpecInfo->getTemplateSpecializationKind()
-                 == TSK_ImplicitInstantiation)
+                 == TSK_ImplicitInstantiation) {
         AlreadyInstantiated = true;
+        PointOfInstantiation = SpecInfo->getPointOfInstantiation();
+      }
     } else if (MemberSpecializationInfo *MSInfo
                                 = Func->getMemberSpecializationInfo()) {
       if (MSInfo->getPointOfInstantiation().isInvalid())
         MSInfo->setPointOfInstantiation(Loc);
       else if (MSInfo->getTemplateSpecializationKind()
-                 == TSK_ImplicitInstantiation)
+                 == TSK_ImplicitInstantiation) {
         AlreadyInstantiated = true;
+        PointOfInstantiation = MSInfo->getPointOfInstantiation();
+      }
     }
 
-    if (!AlreadyInstantiated) {
+    if (!AlreadyInstantiated || Func->isConstexpr()) {
       if (isa<CXXRecordDecl>(Func->getDeclContext()) &&
           cast<CXXRecordDecl>(Func->getDeclContext())->isLocalClass())
-        PendingLocalImplicitInstantiations.push_back(std::make_pair(Func,
-                                                                    Loc));
-      else if (Func->getTemplateInstantiationPattern()->isConstexpr())
+        PendingLocalImplicitInstantiations.push_back(
+            std::make_pair(Func, PointOfInstantiation));
+      else if (Func->isConstexpr())
         // Do not defer instantiations of constexpr functions, to avoid the
         // expression evaluator needing to call back into Sema if it sees a
         // call to such a function.
-        InstantiateFunctionDefinition(Loc, Func);
+        InstantiateFunctionDefinition(PointOfInstantiation, Func);
       else {
-        PendingInstantiations.push_back(std::make_pair(Func, Loc));
+        PendingInstantiations.push_back(std::make_pair(Func,
+                                                       PointOfInstantiation));
         // Notify the consumer that a function was implicitly instantiated.
         Consumer.HandleCXXImplicitFunctionInstantiation(Func);
       }