[clang] [Clang] Correctly construct template arguments for file-scope template template parameters (PR #76811)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 created https://github.com/llvm/llvm-project/pull/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.

>From 1164c705a8515d39bc9d4404e8523da8876d81cf Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Wed, 3 Jan 2024 19:33:01 +0800
Subject: [PATCH] [Clang] Correctly construct template arguments for file-scope
 template template parameters

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.
---
 clang/docs/ReleaseNotes.rst                   |  3 +++
 clang/lib/Sema/SemaTemplateInstantiate.cpp    | 12 ++++++---
 .../temp/temp.arg/temp.arg.template/p3-2a.cpp | 25 +++++++++++++++++++
 3 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 0c8fec691bf3c9..7193d711333780 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -843,6 +843,9 @@ Bug Fixes to C++ Support
 - Fix crash when parsing nested requirement. Fixes:
   (`#73112 <https://github.com/llvm/llvm-project/issues/73112>`_)
 
+- Fix a regression where clang forgets how to substitute into constraints on template-template
+  parameters. Fixes: (`#57410 <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 df6b40999e645c..4420280efebb86 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -345,15 +345,19 @@ 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 =
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..277935f6b3b2f0 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 +59,28 @@ 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}}
+
+}