[Clang] Fix the template argument collection after CWG2369

Since the function template isn't instantiated before constraint
checking, we'll not be able to find the outer template arguments through
function specialization when evaluating the inner constraint that is
nested within a larger constraint expression.

The only practical solution is to get them back through the code
synthesis context, which also allows us to eliminate an overload of
getTemplateInstantiationArgs.

Author: zyn0217

clang/include/clang/Sema/Sema.h

@@ -13315,18 +13315,6 @@ class Sema final : public SemaBase {
   /// \param ForDefaultArgumentSubstitution indicates we should continue looking
   /// when encountering a specialized member function template, rather than
   /// returning immediately.
-  void getTemplateInstantiationArgs(
-      MultiLevelTemplateArgumentList &Result, const NamedDecl *D,
-      const DeclContext *DC = nullptr, bool Final = false,
-      std::optional<ArrayRef<TemplateArgument>> Innermost = std::nullopt,
-      bool RelativeToPrimary = false, const FunctionDecl *Pattern = nullptr,
-      bool ForConstraintInstantiation = false,
-      bool SkipForSpecialization = false,
-      bool ForDefaultArgumentSubstitution = false);
-
-  /// This creates a new \p MultiLevelTemplateArgumentList and invokes the other
-  /// overload with it as the first parameter. Prefer this overload in most
-  /// situations.
   MultiLevelTemplateArgumentList getTemplateInstantiationArgs(
       const NamedDecl *D, const DeclContext *DC = nullptr, bool Final = false,
       std::optional<ArrayRef<TemplateArgument>> Innermost = std::nullopt,

clang/lib/Sema/SemaConcept.cpp

@@ -1078,15 +1078,19 @@ static bool CheckFunctionConstraintsWithoutInstantiation(
   // template. We need the entire list, since the constraint is completely
   // uninstantiated at this point.
 
-  // FIXME: Add TemplateArgs through the 'Innermost' parameter once
-  // the refactoring of getTemplateInstantiationArgs() relands.
-  MultiLevelTemplateArgumentList MLTAL;
-  MLTAL.addOuterTemplateArguments(Template, {}, /*Final=*/false);
-  SemaRef.getTemplateInstantiationArgs(
-      MLTAL, /*D=*/FD, FD,
+  // getTemplateInstantiationArgs uses this instantiation context to find out
+  // template arguments for uninstantiated functions.
+  std::optional<Sema::InstantiatingTemplate> Inst(
+      std::in_place, SemaRef, PointOfInstantiation,
+      Sema::InstantiatingTemplate::ConstraintsCheck{}, Template, TemplateArgs,
+      PointOfInstantiation);
+  if (Inst->isInvalid())
+    return true;
+  MultiLevelTemplateArgumentList MLTAL = SemaRef.getTemplateInstantiationArgs(
+      /*D=*/FD, FD,
       /*Final=*/false, /*Innermost=*/{}, /*RelativeToPrimary=*/true,
       /*Pattern=*/nullptr, /*ForConstraintInstantiation=*/true);
-  MLTAL.replaceInnermostTemplateArguments(Template, TemplateArgs);
+  Inst = std::nullopt;
 
   Sema::ContextRAII SavedContext(SemaRef, FD);
   std::optional<Sema::CXXThisScopeRAII> ThisScope;

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -309,10 +309,24 @@ Response HandleFunction(Sema &SemaRef, const FunctionDecl *Function,
         isGenericLambdaCallOperatorOrStaticInvokerSpecialization(Function))
       return Response::Done();
 
-  } else if (Function->getDescribedFunctionTemplate()) {
+  } else if (auto *Template = Function->getDescribedFunctionTemplate()) {
     assert(
         (ForConstraintInstantiation || Result.getNumSubstitutedLevels() == 0) &&
         "Outer template not instantiated?");
+    if (ForConstraintInstantiation) {
+      for (auto &Inst : llvm::reverse(SemaRef.CodeSynthesisContexts)) {
+        if (Inst.Kind == Sema::CodeSynthesisContext::ConstraintsCheck &&
+            Inst.Entity == Template) {
+          // After CWG2369, the outer templates are not instantiated when
+          // checking its associated constraints. So add them back through the
+          // synthesis context; this is useful for e.g. nested constraints
+          // involving lambdas.
+          Result.addOuterTemplateArguments(Template, Inst.template_arguments(),
+                                           /*Final=*/false);
+          break;
+        }
+      }
+    }
   }
   // If this is a friend or local declaration and it declares an entity at
   // namespace scope, take arguments from its lexical parent
@@ -474,21 +488,6 @@ MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
   assert((ND || DC) && "Can't find arguments for a decl if one isn't provided");
   // Accumulate the set of template argument lists in this structure.
   MultiLevelTemplateArgumentList Result;
-  getTemplateInstantiationArgs(
-      Result, ND, DC, Final, Innermost, RelativeToPrimary, Pattern,
-      ForConstraintInstantiation, SkipForSpecialization,
-      ForDefaultArgumentSubstitution);
-  return Result;
-}
-
-void Sema::getTemplateInstantiationArgs(
-    MultiLevelTemplateArgumentList &Result, const NamedDecl *ND,
-    const DeclContext *DC, bool Final,
-    std::optional<ArrayRef<TemplateArgument>> Innermost, bool RelativeToPrimary,
-    const FunctionDecl *Pattern, bool ForConstraintInstantiation,
-    bool SkipForSpecialization, bool ForDefaultArgumentSubstitution) {
-  assert((ND || DC) && "Can't find arguments for a decl if one isn't provided");
-  // Accumulate the set of template argument lists in this structure.
 
   using namespace TemplateInstArgsHelpers;
   const Decl *CurDecl = ND;
@@ -548,12 +547,13 @@ void Sema::getTemplateInstantiationArgs(
     }
 
     if (R.IsDone)
-      return;
+      return Result;
     if (R.ClearRelativeToPrimary)
       RelativeToPrimary = false;
     assert(R.NextDecl);
     CurDecl = R.NextDecl;
   }
+  return Result;
 }
 
 bool Sema::CodeSynthesisContext::isInstantiationRecord() const {

clang/test/SemaTemplate/concepts-lambda.cpp

@@ -340,3 +340,21 @@ void foo() {
 }
 
 }
+
+namespace GH147772 {
+
+template<int...>
+struct seq {};
+
+using arr = char[1];
+
+struct foo {
+	template<int... i>
+	constexpr foo(seq<i...>) requires requires {
+		arr { [](auto) requires(i, true) { return 0; }(i)... };
+	} {}
+};
+
+constexpr auto bar = foo(seq<0>());
+
+}