From c081bf4098c729afacf20303cc9df62bf13c3362 Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton@redhat.com>
Date: Sat, 18 Jun 2022 13:48:41 +0200
Subject: [PATCH] [clang] Enforce instantiation of constexpr template functions
 during non-constexpr evaluation

Otherwise these functions are not instantiated and we end up with an undefined
symbol.

Fix #55560

Differential Revision: https://reviews.llvm.org/D128119
---
 clang/lib/Sema/SemaTemplateInstantiateDecl.cpp  |  5 +++--
 .../CodeGenCXX/constexpr-late-instantiation.cpp | 17 +++++++++++++++++
 .../SemaCXX/constexpr-late-instantiation.cpp    | 15 +++++++++++++++
 3 files changed, 35 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/CodeGenCXX/constexpr-late-instantiation.cpp
 create mode 100644 clang/test/SemaCXX/constexpr-late-instantiation.cpp

diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 467372c71496..293782822e83 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -4826,7 +4826,8 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
                                      /*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);
@@ -4841,7 +4842,7 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
         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
index 000000000000..1c8eef73f2dd
--- /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 -triple %itanium_abi_triple %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: }
+
+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
index 000000000000..ec8e071217c1
--- /dev/null
+++ b/clang/test/SemaCXX/constexpr-late-instantiation.cpp
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 %s -fsyntax-only -verify
+
+template <typename T>
+constexpr T foo(T a);   // expected-note {{declared here}}
+
+int main() {
+  int k = foo<int>(5);  // Ok
+  constexpr int j =     // expected-error {{constexpr variable 'j' must be initialized by a constant expression}}
+          foo<int>(5);  // expected-note {{undefined function 'foo<int>' cannot be used in a constant expression}}
+}
+
+template <typename T>
+constexpr T foo(T a) {
+  return a;
+}
-- 
2.38.1