[Clang] Correctly construct template arguments for file-scope template template parameters

This fixes the bug introduced by
6db007a.

We construct placeholder template arguments for template-template parameters to
avoid mismatching argument substitution since they have different depths
with their corresponding template arguments. In this case,

```cpp
template <template <Concept C> class T> void foo(T<int>);
```

T lies at the depth 0, and C lies at 1. The corresponding argument, of which
there is exactly one, int, is at depth 0. If we consider the
argument as the outermost one, then we would end up substituting 'int'
into the wrong parameter T.

We used to perform such placeholder construction during the context walk-up.
In the previous patch, we slipped through that inadvertently because we would
walk up to the parent, which is precisely a FileContext for template-template
parameters, after adding innermost arguments.

Besides, this patch moves the sanity check up to the context switch.
That way, we avoid dereferencing null pointers if ND is unspecified.

Closes #57410.

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -857,6 +857,11 @@ Bug Fixes to C++ Support
   (`#64607 <https://github.com/llvm/llvm-project/issues/64607>`_)
   (`#64086 <https://github.com/llvm/llvm-project/issues/64086>`_)
 
+- Fixed a regression where clang forgets how to substitute into constraints on template-template
+  parameters. Fixes: 
+  (`#57410 <https://github.com/llvm/llvm-project/issues/57410>`_) and
+  (`#76604 <https://github.com/llvm/llvm-project/issues/57410>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -344,15 +344,19 @@ MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
 
   using namespace TemplateInstArgsHelpers;
   const Decl *CurDecl = ND;
+
+  if (!ND)
+    CurDecl = Decl::castFromDeclContext(DC);
+
   if (Innermost) {
     Result.addOuterTemplateArguments(const_cast<NamedDecl *>(ND),
                                      Innermost->asArray(), Final);
-    CurDecl = Response::UseNextDecl(ND).NextDecl;
+    if (CurDecl->getDeclContext()->isFileContext())
+      if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(CurDecl))
+        HandleDefaultTempArgIntoTempTempParam(TTP, Result);
+    CurDecl = Response::UseNextDecl(CurDecl).NextDecl;
   }
 
-  if (!ND)
-    CurDecl = Decl::castFromDeclContext(DC);
-
   while (!CurDecl->isFileContextDecl()) {
     Response R;
     if (const auto *VarTemplSpec =

clang/test/CXX/temp/temp.arg/temp.arg.template/p3-2a.cpp

@@ -59,3 +59,28 @@ struct Nothing {};
 
 // FIXME: Wait the standard to clarify the intent.
 template<> template<> Z<Nothing> S5<Z>::V<Nothing>;
+
+namespace GH57410 {
+
+template<typename T>
+concept True = true;
+
+template<typename T>
+concept False = false; // #False
+
+template<template<True T> typename Wrapper>
+using Test = Wrapper<int>;
+
+template<template<False T> typename Wrapper> // #TTP-Wrapper
+using Test = Wrapper<int>; // expected-error {{constraints not satisfied for template template parameter 'Wrapper' [with T = int]}}
+
+// expected-note@#TTP-Wrapper {{'int' does not satisfy 'False'}}
+// expected-note@#False {{evaluated to false}}
+
+template <template<False> typename T> // #TTP-foo
+void foo(T<int>); // expected-error {{constraints not satisfied for template template parameter 'T' [with $0 = int]}}
+
+// expected-note@#TTP-foo {{'int' does not satisfy 'False'}}
+// expected-note@#False {{evaluated to false}}
+
+}