[Sema] Merge variable template specializations

Clang used to produce redefinition errors, see tests for examples.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D131258

clang/lib/Sema/SemaDecl.cpp

@@ -4739,6 +4739,7 @@ bool Sema::checkVarDeclRedefinition(VarDecl *Old, VarDecl *New) {
   if (!hasVisibleDefinition(Old) &&
       (New->getFormalLinkage() == InternalLinkage ||
        New->isInline() ||
+       isa<VarTemplateSpecializationDecl>(New) ||
        New->getDescribedVarTemplate() ||
        New->getNumTemplateParameterLists() ||
        New->getDeclContext()->isDependentContext())) {

clang/test/Modules/merge-var-template-spec-cxx-modules.cppm

@@ -0,0 +1,44 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/var_def.cppm -o %t/var_def.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface -fprebuilt-module-path=%t %t/reexport1.cppm -o %t/reexport1.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface -fprebuilt-module-path=%t %t/reexport2.cppm -o %t/reexport2.pcm
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t %t/use.cppm -fsyntax-only -verify
+
+//--- use.cppm
+import reexport1;
+import reexport2;
+
+auto foo = zero<Int>;
+auto bar = zero<int*>;
+auto baz = zero<int>;
+
+template <class T> constexpr T zero = 0; // expected-error-re {{declaration{{.*}}in the global module follows declaration in module var_def}}
+                                         // expected-note@* {{previous}}
+template <> constexpr Int zero<Int> = {0}; // expected-error-re {{declaration{{.*}}in the global module follows declaration in module var_def}}
+                                           // expected-note@* {{previous}}
+template <class T> constexpr T* zero<T*> = nullptr; // expected-error-re {{declaration{{.*}}in the global module follows declaration in module var_def}}
+                                                    // expected-note@* {{previous}}
+
+template <> constexpr int** zero<int**> = nullptr; // ok, new specialization.
+template <class T> constexpr T** zero<T**> = nullptr; // ok, new partial specilization.
+
+//--- var_def.cppm
+export module var_def;
+
+export template <class T> constexpr T zero = 0;
+export struct Int {
+    int value;
+};
+export template <> constexpr Int zero<Int> = {0};
+export template <class T> constexpr T* zero<T*> = nullptr;
+
+//--- reexport1.cppm
+export module reexport1;
+export import var_def;
+
+//--- reexport2.cppm
+export module reexport2;
+export import var_def;

clang/test/Modules/merge-var-template-spec.cpp

@@ -0,0 +1,70 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// We need '-fmodules-local-submodule-visibility' to properly test merging when building a module from multiple
+// headers inside the same TU. C++20 mode would imply this flag, but we need it to set it explicitly for C++14.
+//
+// RUN: %clang_cc1 -xc++ -std=c++14 -fmodules -fmodules-local-submodule-visibility -fmodule-name=library \
+// RUN:     -emit-module %t/modules.map \
+// RUN:     -o %t/module.pcm
+//
+//
+// RUN: %clang_cc1 -xc++ -std=c++14 -fmodules -fmodules-local-submodule-visibility -fmodule-file=%t/module.pcm  \
+// RUN:     -fmodule-map-file=%t/modules.map \
+// RUN:     -fsyntax-only -verify %t/use.cpp
+//
+//--- use.cpp
+
+#include "var1.h"
+#include "var2.h"
+
+auto foo = zero<Int>;
+auto bar = zero<int*>;
+auto baz = zero<int>;
+
+template <class T> constexpr T zero = 0; // expected-error {{redefinition}} expected-note@* {{previous}}
+template <> constexpr Int zero<Int> = {0}; // expected-error {{redefinition}} expected-note@* {{previous}}
+template <class T> constexpr T* zero<T*> = nullptr; // expected-error {{redefinition}} expected-note@* {{previous}}
+
+template <> constexpr int** zero<int**> = nullptr; // ok, new specialization.
+template <class T> constexpr T** zero<T**> = nullptr; // ok, new partial specilization.
+
+//--- modules.map
+module "library" {
+	export *
+	module "var1" {
+		export *
+		header "var1.h"
+	}
+	module "var2" {
+		export *
+		header "var2.h"
+	}
+}
+
+//--- var1.h
+#ifndef VAR1_H
+#define VAR1_H
+
+template <class T> constexpr T zero = 0;
+struct Int {
+    int value;
+};
+template <> constexpr int zero<Int> = {0};
+template <class T> constexpr T* zero<T*> = nullptr;
+
+#endif // VAR1_H
+
+//--- var2.h
+#ifndef VAR2_H
+#define VAR2_H
+
+template <class T> constexpr T zero = 0;
+struct Int {
+    int value;
+};
+template <> constexpr int zero<Int> = {0};
+template <class T> constexpr T* zero<T*> = nullptr;
+
+#endif // VAR2_H