[clang] [clang][ASTImporter] skip TemplateTypeParmDecl in VisitTypeAliasTemplateDecl (PR #74919)

[Qizhi Hu via cfe-commits]

jcsxky wrote:

> The `VisitTypeAliasTemplateDecl` function should be re-designed to check for structural equivalence at import. The following test does not pass because an existing `TypeAliasTemplateDecl` declaration with the same name is always found and returned, without check for structural equivalence.
> 
> ```
> TEST_P(ASTImporterOptionSpecificTestBase, ImportTypeAliasTemplateDecl1) {
>   const char *ToCode =
>       R"(
>       struct S;
>       template <typename, typename>
>       using Callable = S;
>       )";
>   const char *Code =
>       R"(
>       struct S;
>       template <typename>
>       using Callable = S;
>       )";
>   Decl *ToTU = getToTuDecl(ToCode, Lang_CXX17);
>   Decl *FromTU = getTuDecl(Code, Lang_CXX17);
> 
>   auto *FromCallable = FirstDeclMatcher<TypeAliasTemplateDecl>().match(
>       FromTU, typeAliasTemplateDecl(hasName("Callable")));
> 
>   auto *ToCallable = FirstDeclMatcher<TypeAliasTemplateDecl>().match(
>       ToTU, typeAliasTemplateDecl(hasName("Callable")));
> 
>   auto *ImportedCallable = Import(FromCallable, Lang_CXX17);
>   EXPECT_TRUE(ImportedCallable);
>   EXPECT_NE(ImportedCallable, ToCallable);
> }
> ```
> 
> Additionally I discovered that import of `ClassTemplateDecl` is not correct too: If there is an object with the same name that is not a `ClassTemplateDecl`, it is just ignored at import. This is not correct, the existing object may cause name conflict (for example it can be a non-template `RecordDecl`). (I found this when checking the questions in my last comment.) This is an independent problem but should be fixed.

I pulled the latest code of main branch and found it seems incorrect with the test case I have supplied. I will have a double check this pr and issues you mentioned here. Thank for your detail comments and guidance.

https://github.com/llvm/llvm-project/pull/74919