[clang] [Clang] Fix a crash on invalid concept (PR #150186)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin created https://github.com/llvm/llvm-project/pull/150186

Fixes #149986

>From 44e0a15203f0d579c6c78ddf135a4f76e164b642 Mon Sep 17 00:00:00 2001
From: Corentin Jabot <corentinjabot at gmail.com>
Date: Wed, 23 Jul 2025 10:31:49 +0200
Subject: [PATCH] [Clang] Fix a crash on invalid concept

Fixes #149986
---
 clang/lib/Sema/SemaConcept.cpp       | 3 +++
 clang/test/SemaTemplate/concepts.cpp | 8 ++++++++
 2 files changed, 11 insertions(+)

diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 5205ca0bca6fa..044cf5cf18d0a 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -588,6 +588,9 @@ static bool CheckConstraintSatisfaction(
     return true;
 
   for (const AssociatedConstraint &AC : AssociatedConstraints) {
+    if (AC.isNull())
+      return true;
+
     Sema::ArgPackSubstIndexRAII _(S, AC.ArgPackSubstIndex);
     ExprResult Res = calculateConstraintSatisfaction(
         S, Template, TemplateIDRange.getBegin(), TemplateArgsLists,
diff --git a/clang/test/SemaTemplate/concepts.cpp b/clang/test/SemaTemplate/concepts.cpp
index 62a4f95d79c74..663bc984ece2e 100644
--- a/clang/test/SemaTemplate/concepts.cpp
+++ b/clang/test/SemaTemplate/concepts.cpp
@@ -1250,3 +1250,11 @@ static_assert(!D<Priv>::has, "Private should be invisible.");
 static_assert(!D<Prot>::has, "Protected should be invisible.");
 
 }
+
+
+namespace GH149986 {
+template <typename T> concept PerfectSquare = [](){} // expected-note 2{{here}}
+([](auto) { return true; }) < PerfectSquare <class T>;
+// expected-error at -1 {{declaration of 'T' shadows template parameter}} \
+// expected-error at -1 {{a concept definition cannot refer to itself}}
+}