[clang][ASTImporter] Fix of possible crash "Did not find base!". (#67680)

A problem with AST import could lead to multiple instances of the same
template class specialization, with different template arguments. The
difference was caused by pointers to different declarations of the same
function.
Problem is fixed by using the canonical declaration at import.

Co-authored-by: Balázs Kéri <[email protected]>

Author: balazske

clang/lib/AST/ASTImporter.cpp

@@ -811,7 +811,8 @@ ASTNodeImporter::import(const TemplateArgument &From) {
     ExpectedType ToTypeOrErr = import(From.getParamTypeForDecl());
     if (!ToTypeOrErr)
       return ToTypeOrErr.takeError();
-    return TemplateArgument(*ToOrErr, *ToTypeOrErr, From.getIsDefaulted());
+    return TemplateArgument(dyn_cast<ValueDecl>((*ToOrErr)->getCanonicalDecl()),
+                            *ToTypeOrErr, From.getIsDefaulted());
   }
 
   case TemplateArgument::NullPtr: {

clang/unittests/AST/ASTImporterTest.cpp

@@ -9175,6 +9175,64 @@ TEST_P(ASTImporterOptionSpecificTestBase,
   EXPECT_TRUE(ToXType->typeMatchesDecl());
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase,
+       ImportTemplateArgumentWithPointerToDifferentInstantiation) {
+  const char *CodeTo =
+      R"(
+      template<class A>
+      A f1() {
+       return A();
+      }
+      template<class A, A (B)()>
+      class X {};
+
+      X<int, f1<int>> x;
+      )";
+  const char *CodeFrom =
+      R"(
+      template<class A>
+      A f1();
+      template<class A, A (B)()>
+      class X {};
+
+      X<int, f1<int>> x;
+      )";
+  Decl *ToTU = getToTuDecl(CodeTo, Lang_CXX11);
+  Decl *FromTU = getTuDecl(CodeFrom, Lang_CXX11);
+
+  auto *ToF1 = FirstDeclMatcher<FunctionDecl>().match(
+      ToTU, functionDecl(hasName("f1"), isInstantiated()));
+  auto *FromF1 = FirstDeclMatcher<FunctionDecl>().match(
+      FromTU, functionDecl(hasName("f1"), isInstantiated()));
+  EXPECT_TRUE(ToF1->isThisDeclarationADefinition());
+  EXPECT_FALSE(FromF1->isThisDeclarationADefinition());
+
+  auto *ToX = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      ToTU, classTemplateSpecializationDecl(hasName("X")));
+  auto *FromX = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl(hasName("X")));
+
+  Decl *ToTArgF = ToX->getTemplateArgs().get(1).getAsDecl();
+  Decl *FromTArgF = FromX->getTemplateArgs().get(1).getAsDecl();
+  EXPECT_EQ(ToTArgF, ToF1);
+  EXPECT_EQ(FromTArgF, FromF1);
+
+  auto *ToXImported = Import(FromX, Lang_CXX11);
+  // The template argument 1 of 'X' in the "From" code points to a function
+  // that has no definition. The import must ensure that this template argument
+  // is imported in a way that it will point to the existing 'f1' function, not
+  // to the 'f1' that is imported. In this way when specialization of 'X' is
+  // imported it will have the same template arguments as the existing one.
+  EXPECT_EQ(ToXImported, ToX);
+  // FIXME: This matcher causes a crash "Tried to match orphan node".
+  // The code is removed until the problem is fixed.
+  // auto *ToF1Imported =
+  //    LastDeclMatcher<FunctionDecl>().match(ToTU,
+  //    functionDecl(hasName("f1"),isInstantiated()));
+  // EXPECT_NE(ToF1Imported, ToF1);
+  // EXPECT_EQ(ToF1Imported->getPreviousDecl(), ToF1);
+}
+
 INSTANTIATE_TEST_SUITE_P(ParameterizedTests, ASTImporterLookupTableTest,
                          DefaultTestValuesForRunOptions);