[Clang] Correctly construct template arguments for template template parameters (#76811)

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.
Closes #76604. (The case is
slightly different than that in #57410. We should *not* assume the
surrounding context to be a file-scope one.)

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 +344,26 @@ MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
 
   using namespace TemplateInstArgsHelpers;
   const Decl *CurDecl = ND;
+
+  if (!CurDecl)
+    CurDecl = Decl::castFromDeclContext(DC);
+
   if (Innermost) {
     Result.addOuterTemplateArguments(const_cast<NamedDecl *>(ND),
                                      Innermost->asArray(), Final);
-    CurDecl = Response::UseNextDecl(ND).NextDecl;
+    // Populate placeholder template arguments for TemplateTemplateParmDecls.
+    // This is essential for the case e.g.
+    //
+    // template <class> concept Concept = false;
+    // template <template <Concept C> class T> void foo(T<int>)
+    //
+    // where parameter C has a depth of 1 but the substituting argument `int`
+    // has a depth of 0.
+    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 =
@@ -380,10 +391,8 @@ MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
       R = Response::ChangeDecl(CTD->getLexicalDeclContext());
     } else if (!isa<DeclContext>(CurDecl)) {
       R = Response::DontClearRelativeToPrimaryNextDecl(CurDecl);
-      if (CurDecl->getDeclContext()->isTranslationUnit()) {
-        if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(CurDecl)) {
-          R = HandleDefaultTempArgIntoTempTempParam(TTP, Result);
-        }
+      if (const auto *TTP = dyn_cast<TemplateTemplateParmDecl>(CurDecl)) {
+        R = HandleDefaultTempArgIntoTempTempParam(TTP, Result);
       }
     } else {
       R = HandleGenericDeclContext(CurDecl);

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

@@ -59,3 +59,32 @@ 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 <class> struct S {};
+
+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 <typename U, template<False> typename T>
+void foo(T<U>); // #foo
+
+void bar() {
+  foo<int>(S<int>{}); // expected-error {{no matching function for call to 'foo'}}
+  // expected-note@#foo {{substitution failure [with U = int]: constraints not satisfied for template template parameter 'T' [with $0 = int]}}
+}
+
+}