[modules] Merge variable template specializations.

Author: zygoloid

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);
     }
   }
 

clang/test/Modules/Inputs/merge-template-specializations/a.h

@@ -1 +1,3 @@
 template<unsigned> class SmallString {};
+
+template<int> int var_template = 0;

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>)) {}

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"

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);
+}