From 0c7d46a7fd5b7956e285d385a6945153d6a06eb0 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109@gmail.com>
Date: Sat, 6 Jan 2024 22:26:34 +0800
Subject: [PATCH] [Clang] Correctly construct template arguments for template
 template parameters (#76811)

This fixes the bug introduced by

https://github.com/llvm/llvm-project/commit/6db007a0654ed7a6ed5c3aa3b61a937c19a6bc6b.

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 https://github.com/llvm/llvm-project/issues/57410.
Closes https://github.com/llvm/llvm-project/issues/76604. (The case is
slightly different than that in #57410. We should *not* assume the
surrounding context to be a file-scope one.)
---
 clang/docs/ReleaseNotes.rst                   |  5 ++++
 clang/lib/Sema/SemaTemplateInstantiate.cpp    | 25 +++++++++++-----
 .../temp/temp.arg/temp.arg.template/p3-2a.cpp | 29 +++++++++++++++++++
 3 files changed, 51 insertions(+), 8 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 9b6e00b231216b..154412144ef97a 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/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.
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 09dd119ed1b948..7f20413c104e97 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/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);
diff --git a/clang/test/CXX/temp/temp.arg/temp.arg.template/p3-2a.cpp b/clang/test/CXX/temp/temp.arg/temp.arg.template/p3-2a.cpp
index 449b6232542e24..f586069638614b 100644
--- a/clang/test/CXX/temp/temp.arg/temp.arg.template/p3-2a.cpp
+++ b/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]}}
+}
+
+}