diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -4840,7 +4840,8 @@
                                      /*Complain*/DefinitionRequired)) {
     if (DefinitionRequired)
       Function->setInvalidDecl();
-    else if (TSK == TSK_ExplicitInstantiationDefinition) {
+    else if (TSK == TSK_ExplicitInstantiationDefinition ||
+             (Function->isConstexpr() && !Recursive)) {
       // Try again at the end of the translation unit (at which point a
       // definition will be required).
       assert(!Recursive);
@@ -4855,7 +4856,7 @@
         Diag(PatternDecl->getLocation(), diag::note_forward_template_decl);
         if (getLangOpts().CPlusPlus11)
           Diag(PointOfInstantiation, diag::note_inst_declaration_hint)
-            << Function;
+              << Function;
       }
     }
 
diff --git a/clang/test/CodeGenCXX/constexpr-late-instantiation.cpp b/clang/test/CodeGenCXX/constexpr-late-instantiation.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenCXX/constexpr-late-instantiation.cpp
@@ -0,0 +1,17 @@
+// Make sure foo is instantiated and we don't get a link error
+// RUN: %clang_cc1 -S -emit-llvm %s -o- | FileCheck %s
+
+template <typename T>
+constexpr T foo(T a);
+
+// CHECK-LABEL: define {{.*}} @main
+int main() {
+  // CHECK: call {{.*}} @_Z3fooIiET_S0_
+  int k = foo<int>(5);
+}
+
+// CHECK-LABEL: define {{.*}} @_Z3fooIiET_S0_
+template <typename T>
+constexpr T foo(T a) {
+  return a;
+}
diff --git a/clang/test/SemaCXX/constexpr-late-instantiation.cpp b/clang/test/SemaCXX/constexpr-late-instantiation.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/constexpr-late-instantiation.cpp
@@ -0,0 +1,15 @@
+// Make sure foo is instantiated and we don't get a link error
+// RUN: not %clang_cc1 -S -emit-llvm %s -fsyntax-only 2>&1 | FileCheck %s
+
+template <typename T>
+constexpr T foo(T a);
+
+int main() {
+  int k = foo<int>(5);           // Ok
+  constexpr int j = foo<int>(5); // CHECK: error: constexpr variable 'j' must be initialized by a constant expression
+}
+
+template <typename T>
+constexpr T foo(T a) {
+  return a;
+}