[clang] [clang][ASTImporter] Fix import of variable template redeclarations. (PR #72841)

[Balázs Kéri via cfe-commits]

================
@@ -5050,6 +5050,59 @@ TEST_P(ImportFriendClasses, RecordVarTemplateDecl) {
   EXPECT_EQ(ToTUX, ToX);
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase, VarTemplateDeclConflict) {
+  getToTuDecl(
+      R"(
+      template <class U>
+      constexpr int X = 1;
+      )",
+      Lang_CXX14);
+
+  Decl *FromTU = getTuDecl(
+      R"(
+      template <class U>
+      constexpr int X = 2;
+      )",
+      Lang_CXX14, "input1.cc");
+  auto *FromX = FirstDeclMatcher<VarTemplateDecl>().match(
+      FromTU, varTemplateDecl(hasName("X")));
+  auto *ToX = Import(FromX, Lang_CXX11);
+  // FIXME: This import should fail.
+  EXPECT_TRUE(ToX);
----------------
balazske wrote:

This import should not fail (it is no error to compile code with a variable template that is declared as extern and has a non-extern definition, and the "extern" part could be in an include file), and the same does not fail with non-template variables. This case should be checked and the structural equivalence of variable templates is missing now, but that change could go into a new pull request.

https://github.com/llvm/llvm-project/pull/72841