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

In some cases variable templates (specially if static member of record)
were not correctly imported and an assertion "Missing call to
MapImported?" could happen.

Author: balazske

clang/lib/AST/ASTImporter.cpp

@@ -6280,17 +6280,21 @@ ExpectedDecl ASTNodeImporter::VisitVarTemplateDecl(VarTemplateDecl *D) {
                                               D->getTemplatedDecl()))
         continue;
       if (IsStructuralMatch(D, FoundTemplate)) {
-        // The Decl in the "From" context has a definition, but in the
-        // "To" context we already have a definition.
+        // FIXME Check for ODR error if the two definitions have
+        // different initializers?
         VarTemplateDecl *FoundDef = getTemplateDefinition(FoundTemplate);
-        if (D->isThisDeclarationADefinition() && FoundDef)
-          // 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);
-
+        if (D->getDeclContext()->isRecord()) {
+          assert(FoundTemplate->getDeclContext()->isRecord() &&
+                 "Member variable template imported as non-member, "
+                 "inconsistent imported AST?");
+          if (FoundDef)
+            return Importer.MapImported(D, FoundDef);
+          if (!D->isThisDeclarationADefinition())
+            return Importer.MapImported(D, FoundTemplate);
+        } else {
+          if (FoundDef && D->isThisDeclarationADefinition())
+            return Importer.MapImported(D, FoundDef);
+        }
         FoundByLookup = FoundTemplate;
         break;
       }

clang/unittests/AST/ASTImporterTest.cpp

@@ -5219,6 +5219,59 @@ 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);
+}
+
+TEST_P(ASTImporterOptionSpecificTestBase, VarTemplateStaticDefinition) {
+  Decl *ToTU = getToTuDecl(
+      R"(
+      struct A {
+        template <class U>
+        static int X;
+      };
+      )",
+      Lang_CXX14);
+  auto *ToX = FirstDeclMatcher<VarTemplateDecl>().match(
+      ToTU, varTemplateDecl(hasName("X")));
+  ASSERT_FALSE(ToX->isThisDeclarationADefinition());
+
+  Decl *FromTU = getTuDecl(
+      R"(
+      struct A {
+        template <class U>
+        static int X;
+      };
+      template <class U>
+      int A::X = 2;
+      )",
+      Lang_CXX14, "input1.cc");
+  auto *FromXDef = LastDeclMatcher<VarTemplateDecl>().match(
+      FromTU, varTemplateDecl(hasName("X")));
+  ASSERT_TRUE(FromXDef->isThisDeclarationADefinition());
+  auto *ToXDef = Import(FromXDef, Lang_CXX14);
+  EXPECT_TRUE(ToXDef);
+  EXPECT_TRUE(ToXDef->isThisDeclarationADefinition());
+  EXPECT_EQ(ToXDef->getPreviousDecl(), ToX);
+}
+
 TEST_P(ASTImporterOptionSpecificTestBase, VarTemplateParameterDeclContext) {
   constexpr auto Code =
       R"(