[Clang] Don't allow implicit this access when checking function constraints (#151276)

We allowed implicit this access when checking associated constraints
after CWG2369. As a result, some of the invalid function call
expressions were not properly SFINAE'ed out and ended up as hard errors
at evaluation time.

We tried fixing that by mucking around the CurContext, but that spawned
additional breakages and I think it's probably safe to revert to the
previous behavior to avoid churns.

Though there is CWG2589, which justifies the previous change, it's not
what we're pursuing now.

Fixes #151271
Fixes #145505

Author: zyn0217

clang/lib/Sema/SemaConcept.cpp

@@ -1105,10 +1105,6 @@ static bool CheckFunctionConstraintsWithoutInstantiation(
   }
 
   Sema::ContextRAII SavedContext(SemaRef, FD);
-  std::optional<Sema::CXXThisScopeRAII> ThisScope;
-  if (auto *Method = dyn_cast<CXXMethodDecl>(FD))
-    ThisScope.emplace(SemaRef, /*Record=*/Method->getParent(),
-                      /*ThisQuals=*/Method->getMethodQualifiers());
   return SemaRef.CheckConstraintSatisfaction(
       Template, TemplateAC, MLTAL, PointOfInstantiation, Satisfaction);
 }

clang/lib/Sema/SemaTemplate.cpp

@@ -4749,8 +4749,6 @@ 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

@@ -1228,26 +1228,26 @@ template <KnownKind T> struct KnownType {
 
 }
 
-namespace GH115838 {
+namespace CWG2369_Regression_2 {
 
-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; };
+template <typename T>
+concept HasFastPropertyForAttribute =
+    requires(T element, int name) { element.propertyForAttribute(name); };
+
+template <typename OwnerType>
+struct SVGPropertyOwnerRegistry {
+  static int fastAnimatedPropertyLookup() {
+    static_assert (HasFastPropertyForAttribute<OwnerType>);
+    return 1;
+  }
+};
 
-// "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.");
+class SVGCircleElement {
+  friend SVGPropertyOwnerRegistry<SVGCircleElement>;
+  void propertyForAttribute(int);
+};
 
-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.");
+int i = SVGPropertyOwnerRegistry<SVGCircleElement>::fastAnimatedPropertyLookup();
 
 }