From 63814be4fa23ef9b36cf2d14388c7f6ba591d3b6 Mon Sep 17 00:00:00 2001
From: Richard Smith <richard@metafoo.co.uk>
Date: Tue, 19 Apr 2022 14:40:52 -0700
Subject: [PATCH] [modules] Merge variable template specializations.

---
 clang/lib/Serialization/ASTReaderDecl.cpp            | 12 ++++++++----
 .../Inputs/merge-template-specializations/a.h        |  2 ++
 .../Inputs/merge-template-specializations/b.h        |  2 ++
 .../Inputs/merge-template-specializations/c.h        |  3 +++
 .../test/Modules/merge-template-specializations.cpp  |  4 ++++
 5 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 43aacdeda998..9e0c3d558323 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -2382,13 +2382,17 @@ ASTDeclReader::VisitVarTemplateSpecializationDeclImpl(
   if (writtenAsCanonicalDecl) {
     auto *CanonPattern = readDeclAs<VarTemplateDecl>();
     if (D->isCanonicalDecl()) { // It's kept in the folding set.
-      // FIXME: If it's already present, merge it.
+      VarTemplateSpecializationDecl *CanonSpec;
       if (auto *Partial = dyn_cast<VarTemplatePartialSpecializationDecl>(D)) {
-        CanonPattern->getCommonPtr()->PartialSpecializations
-            .GetOrInsertNode(Partial);
+        CanonSpec = CanonPattern->getCommonPtr()
+                        ->PartialSpecializations.GetOrInsertNode(Partial);
       } else {
-        CanonPattern->getCommonPtr()->Specializations.GetOrInsertNode(D);
+        CanonSpec =
+            CanonPattern->getCommonPtr()->Specializations.GetOrInsertNode(D);
       }
+      // If we already have a matching specialization, merge it.
+      if (CanonSpec != D)
+        mergeRedeclarable<VarDecl>(D, CanonSpec, Redecl);
     }
   }
 
diff --git a/clang/test/Modules/Inputs/merge-template-specializations/a.h b/clang/test/Modules/Inputs/merge-template-specializations/a.h
index fb05647c0aa9..00426af2b1d5 100644
--- a/clang/test/Modules/Inputs/merge-template-specializations/a.h
+++ b/clang/test/Modules/Inputs/merge-template-specializations/a.h
@@ -1 +1,3 @@
 template<unsigned> class SmallString {};
+
+template<int> int var_template = 0;
diff --git a/clang/test/Modules/Inputs/merge-template-specializations/b.h b/clang/test/Modules/Inputs/merge-template-specializations/b.h
index 96ce2bb1d8d6..89d0e36d6c96 100644
--- a/clang/test/Modules/Inputs/merge-template-specializations/b.h
+++ b/clang/test/Modules/Inputs/merge-template-specializations/b.h
@@ -1,2 +1,4 @@
 #include "a.h"
 void f(SmallString<256>&);
+
+template<typename T> void use_var_template(decltype(T() + var_template<0>)) {}
diff --git a/clang/test/Modules/Inputs/merge-template-specializations/c.h b/clang/test/Modules/Inputs/merge-template-specializations/c.h
index 100463a2f7f2..44fe4925d7c2 100644
--- a/clang/test/Modules/Inputs/merge-template-specializations/c.h
+++ b/clang/test/Modules/Inputs/merge-template-specializations/c.h
@@ -1,3 +1,6 @@
 #include "a.h"
 struct X { SmallString<256> ss; };
+
+template<typename T> void use_var_template(decltype(T() + var_template<0>));
+
 #include "b.h"
diff --git a/clang/test/Modules/merge-template-specializations.cpp b/clang/test/Modules/merge-template-specializations.cpp
index 25db93fbbb20..1f8e68a2e925 100644
--- a/clang/test/Modules/merge-template-specializations.cpp
+++ b/clang/test/Modules/merge-template-specializations.cpp
@@ -3,3 +3,7 @@
 // expected-no-diagnostics
 #include "c.h"
 X x;
+
+void test_var_template() {
+  use_var_template<int>(0);
+}