[clang][ASTImporter] Fix crash when import VarTemplateDecl in record (#67522)

[clang][ASTImporter] Fix crash when import `VarTemplateDecl` in record
static VarTemplateDecl in record isn't a definition, when imported
before, it will crash in `ASTContext::setTemplateOrSpecializationInfo`
due to setting specialization while it already exists. This patch skip
this specific case.

Co-authored-by: huqizhi <[email protected]>

Author: jcsxky

clang/lib/AST/ASTImporter.cpp

@@ -6242,6 +6242,9 @@ ExpectedDecl ASTNodeImporter::VisitVarTemplateDecl(VarTemplateDecl *D) {
           // FIXME Check for ODR error if the two definitions have
           // different initializers?
           return Importer.MapImported(D, FoundDef);
+        if (FoundTemplate->getDeclContext()->isRecord() &&
+            D->getDeclContext()->isRecord())
+          return Importer.MapImported(D, FoundTemplate);
 
         FoundByLookup = FoundTemplate;
         break;

clang/unittests/AST/ASTImporterTest.cpp

@@ -4988,6 +4988,37 @@ TEST_P(ASTImporterOptionSpecificTestBase,
   }
 }
 
+TEST_P(ImportFriendClasses, RecordVarTemplateDecl) {
+  Decl *ToTU = getToTuDecl(
+      R"(
+      template <class T>
+      class A {
+      public:
+        template <class U>
+        static constexpr bool X = true;
+      };
+      )",
+      Lang_CXX14);
+
+  auto *ToTUX = FirstDeclMatcher<VarTemplateDecl>().match(
+      ToTU, varTemplateDecl(hasName("X")));
+  Decl *FromTU = getTuDecl(
+      R"(
+      template <class T>
+      class A {
+      public:
+        template <class U>
+        static constexpr bool X = true;
+      };
+      )",
+      Lang_CXX14, "input1.cc");
+  auto *FromX = FirstDeclMatcher<VarTemplateDecl>().match(
+      FromTU, varTemplateDecl(hasName("X")));
+  auto *ToX = Import(FromX, Lang_CXX11);
+  EXPECT_TRUE(ToX);
+  EXPECT_EQ(ToTUX, ToX);
+}
+
 TEST_P(ASTImporterOptionSpecificTestBase, VarTemplateParameterDeclContext) {
   constexpr auto Code =
       R"(