[clang] [sema] Fix recursive concept detection (PR #206629)

[Daniel Christian Mandolang via cfe-commits]

https://github.com/danielcm585 updated https://github.com/llvm/llvm-project/pull/206629

>From 644978c7e0629638c4cce14aa6b284620538cf0b Mon Sep 17 00:00:00 2001
From: danielcm585 <danielchristianmandolang at gmail.com>
Date: Tue, 30 Jun 2026 09:43:04 +0800
Subject: [PATCH 1/2] [sema] Fix recursive concept detection

---
 clang/include/clang/Sema/Sema.h                |  2 ++
 clang/lib/Sema/SemaConcept.cpp                 | 12 +++++++++++-
 clang/test/SemaCXX/recursive-concept-alias.cpp |  3 +++
 3 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaCXX/recursive-concept-alias.cpp

diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 2cf16fac83282..f8dfc06d2dc1a 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -918,6 +918,8 @@ class Sema final : public SemaBase {
        CodeCompleteConsumer *CompletionConsumer = nullptr);
   ~Sema();
 
+  llvm::SmallPtrSet<const NamedDecl *, 8> ActiveConcepts;
+
   /// Perform initialization that occurs after the parser has been
   /// initialized but before it parses anything.
   void Initialize();
diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index f41f37b8a3d19..b558480d526b5 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -1051,6 +1051,16 @@ ExprResult ConstraintSatisfactionChecker::Evaluate(
     const MultiLevelTemplateArgumentList &MLTAL) {
 
   const ConceptReference *ConceptId = Constraint.getConceptId();
+  const NamedDecl *ConceptDecl = cast<clang::NamedDecl>(ConceptId->getNamedConcept()->getCanonicalDecl());
+
+  if (S.ActiveConcepts.contains(ConceptDecl)) {
+    S.Diag(ConceptId->getBeginLoc(), diag::err_recursive_concept)
+        << ConceptId->getNamedConcept()->getName();
+    Satisfaction.IsSatisfied = false;
+    Satisfaction.ContainsErrors = true;
+    return ExprError();
+  }
+
   Sema::InstantiatingTemplate InstTemplate(
       S, ConceptId->getBeginLoc(),
       Sema::InstantiatingTemplate::ConstraintsCheck{},
@@ -1069,7 +1079,7 @@ ExprResult ConstraintSatisfactionChecker::Evaluate(
   unsigned Size = Satisfaction.Details.size();
 
   llvm::SaveAndRestore PushConceptDecl(
-      ParentConcept, cast<ConceptDecl>(ConceptId->getNamedConcept()));
+      ParentConcept, cast<clang::ConceptDecl>(ConceptId->getNamedConcept()));
 
   ExprResult E = Evaluate(Constraint.getNormalizedConstraint(), MLTAL);
 
diff --git a/clang/test/SemaCXX/recursive-concept-alias.cpp b/clang/test/SemaCXX/recursive-concept-alias.cpp
new file mode 100644
index 0000000000000..f97889afb02e9
--- /dev/null
+++ b/clang/test/SemaCXX/recursive-concept-alias.cpp
@@ -0,0 +1,3 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+template<typename T> concept C1 = C1<T>; // expected-error {{a concept definition cannot refer to itself}} expected-note {{here}}
\ No newline at end of file

>From 06716f457538f187495c1e834a27357b70a78b2f Mon Sep 17 00:00:00 2001
From: danielcm585 <danielchristianmandolang at gmail.com>
Date: Tue, 30 Jun 2026 09:50:16 +0800
Subject: [PATCH 2/2] fix formatting

---
 clang/lib/Sema/SemaConcept.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index b558480d526b5..b71bb461d680a 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -1051,7 +1051,8 @@ ExprResult ConstraintSatisfactionChecker::Evaluate(
     const MultiLevelTemplateArgumentList &MLTAL) {
 
   const ConceptReference *ConceptId = Constraint.getConceptId();
-  const NamedDecl *ConceptDecl = cast<clang::NamedDecl>(ConceptId->getNamedConcept()->getCanonicalDecl());
+  const NamedDecl *ConceptDecl = 
+      cast<clang::NamedDecl>(ConceptId->getNamedConcept()->getCanonicalDecl());
 
   if (S.ActiveConcepts.contains(ConceptDecl)) {
     S.Diag(ConceptId->getBeginLoc(), diag::err_recursive_concept)