[Clang] Fail the constraint substitution early after CWG2369 (llvm#143096)

CWG2369 revealed another case where we were SFINAE'ing out the invalid
result of substitution, but the expression now makes the way into evaluation.

We switch to the concept specialization's context before we check it.
This ensures that we're able to realize the invalid expression earlier,
so we can avoid evaluating invalid expression, which is a hard error.

This also fixes llvm#115838

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -833,6 +833,7 @@ Bug Fixes to C++ Support
 - Fixed an access checking bug when initializing non-aggregates in default arguments (#GH62444), (#GH83608)
 - Fixed a pack substitution bug in deducing class template partial specializations. (#GH53609)
 - Fixed a crash when constant evaluating some explicit object member assignment operators. (#GH142835)
+- Fixed an access checking bug when substituting into concepts (#GH115838)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplate.cpp

@@ -4749,6 +4749,8 @@ Sema::CheckConceptTemplateId(const CXXScopeSpec &SS,
   EnterExpressionEvaluationContext EECtx{
       *this, ExpressionEvaluationContext::Unevaluated, CSD};
 
+  ContextRAII CurContext(*this, CSD->getDeclContext(),
+                         /*NewThisContext=*/false);
   if (!AreArgsDependent &&
       CheckConstraintSatisfaction(
           NamedConcept, AssociatedConstraint(NamedConcept->getConstraintExpr()),

clang/test/SemaTemplate/concepts.cpp

@@ -1200,3 +1200,57 @@ bool test_val_types() {
 }
 
 }
+
+namespace CWG2369_Regression {
+
+enum class KindEnum {
+  Unknown = 0,
+  Foo = 1,
+};
+
+template <typename T>
+concept KnownKind = T::kind() != KindEnum::Unknown;
+
+template <KnownKind T> struct KnownType;
+
+struct Type {
+  KindEnum kind() const;
+
+  static Type f(Type t);
+
+  template <KnownKind T> static KnownType<T> f(T t);
+
+  static void g() {
+    Type t;
+    f(t);
+  }
+};
+
+template <KnownKind T> struct KnownType {
+  static constexpr KindEnum kind() { return KindEnum::Foo; }
+};
+
+}
+
+namespace GH115838 {
+
+template<typename T> concept has_x = requires(T t) {{ t.x };};
+
+class Publ { public:    int x = 0; };
+class Priv { private:   int x = 0; };
+class Prot { protected: int x = 0; };
+class Same { protected: int x = 0; };
+
+template<typename T> class D;
+template<typename T> requires ( has_x<T>) class D<T>: public T { public: static constexpr bool has = 1; };
+template<typename T> requires (!has_x<T>) class D<T>: public T { public: static constexpr bool has = 0; };
+
+// "Same" is identical to "Prot" but queried before used.
+static_assert(!has_x<Same>,  "Protected should be invisible.");
+static_assert(!D<Same>::has, "Protected should be invisible.");
+
+static_assert( D<Publ>::has, "Public should be visible.");
+static_assert(!D<Priv>::has, "Private should be invisible.");
+static_assert(!D<Prot>::has, "Protected should be invisible.");
+
+}