Perform access checking to private members in simple requirement.

> Dependent access checks.

Fixes: https://github.com/llvm/llvm-project/issues/53364

We previously ignored dependent access checks to private members.
These are visible only to the `RequiresExprBodyExpr` (through `PerformDependentDiagnositcs`) and not to the individual requirements.

---

> Non-dependent access checks.
Fixes: https://github.com/llvm/llvm-project/issues/53334
Access to members in a non-dependent context would always yield an
invalid expression. When it appears in a requires-expression, then this
is a hard error as this would always result in a substitution failure.

https://eel.is/c++draft/expr.prim.req#general-note-1
> Note 1: If a requires-expression contains invalid types or expressions in its requirements, and it does not appear within the declaration of a templated entity, then the program is ill-formed. — end note]
> If the substitution of template arguments into a requirement would always result in a substitution failure, the program is ill-formed; no diagnostic required.

The main issue here is the delaying of the diagnostics.
Use a `ParsingDeclRAIIObject` creates a separate diagnostic pool for diagnositcs associated to the `RequiresExprBodyDecl`.
This is important because dependent diagnostics should not be leaked/delayed to higher scopes (Eg. inside a template function or in a trailing requires). These dependent diagnostics must be attached to the `DeclContext` of the parameters of `RequiresExpr` (which is the `RequiresExprBodyDecl` in this case).
Non dependent diagnostics, on the other hand, should not delayed and surfaced as hard errors.

Differential Revision: https://reviews.llvm.org/D140547

clang/docs/ReleaseNotes.rst

@@ -735,7 +735,8 @@ C++20 Feature Support
 - Do not hide templated base members introduced via using-decl in derived class
   (useful specially for constrained members). Fixes `GH50886 <https://github.com/llvm/llvm-project/issues/50886>`_.
 - Implemented CWG2635 as a Defect Report, which prohibits structured bindings from being constrained.
-
+- Correctly handle access-checks in requires expression. Fixes `GH53364 <https://github.com/llvm/llvm-project/issues/53364>`_,
+  `GH53334 <https://github.com/llvm/llvm-project/issues/53334>`_.
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/AST/ExprConcepts.h

@@ -528,6 +528,12 @@ public:
     return RequiresExprBits.IsSatisfied;
   }
 
+  void setSatisfied(bool IsSatisfied) {
+    assert(!isValueDependent() &&
+           "setSatisfied called on a dependent RequiresExpr");
+    RequiresExprBits.IsSatisfied = IsSatisfied;
+  }
+
   SourceLocation getRequiresKWLoc() const {
     return RequiresExprBits.RequiresKWLoc;
   }

clang/lib/Parse/ParseExprCXX.cpp

@@ -3509,6 +3509,10 @@ ExprResult Parser::ParseRequiresExpression() {
       Actions, Sema::ExpressionEvaluationContext::Unevaluated);
 
   ParseScope BodyScope(this, Scope::DeclScope);
+  // Create a separate diagnostic pool for RequiresExprBodyDecl.
+  // Dependent diagnostics are attached to this Decl and non-depenedent
+  // diagnostics are surfaced after this parse.
+  ParsingDeclRAIIObject ParsingBodyDecl(*this, ParsingDeclRAIIObject::NoParent);
   RequiresExprBodyDecl *Body = Actions.ActOnStartRequiresExpr(
       RequiresKWLoc, LocalParameterDecls, getCurScope());
 
@@ -3746,6 +3750,7 @@ ExprResult Parser::ParseRequiresExpression() {
   }
   Braces.consumeClose();
   Actions.ActOnFinishRequiresExpr();
+  ParsingBodyDecl.complete(Body);
   return Actions.ActOnRequiresExpr(RequiresKWLoc, Body, LocalParameterDecls,
                                    Requirements, Braces.getCloseLocation());
 }

clang/lib/Sema/SemaAccess.cpp

@@ -1493,6 +1493,8 @@ void Sema::HandleDelayedAccessCheck(DelayedDiagnostic &DD, Decl *D) {
   } else if (TemplateDecl *TD = dyn_cast<TemplateDecl>(D)) {
     if (isa<DeclContext>(TD->getTemplatedDecl()))
       DC = cast<DeclContext>(TD->getTemplatedDecl());
+  } else if (auto *RD = dyn_cast<RequiresExprBodyDecl>(D)) {
+    DC = RD;
   }
 
   EffectiveContext EC(DC);

clang/lib/Sema/SemaConcept.cpp

@@ -1002,6 +1002,7 @@ static void diagnoseWellFormedUnsatisfiedConstraintExpr(Sema &S,
     S.DiagnoseUnsatisfiedConstraint(CSE->getSatisfaction());
     return;
   } else if (auto *RE = dyn_cast<RequiresExpr>(SubstExpr)) {
+    // FIXME: RequiresExpr should store dependent diagnostics.
     for (concepts::Requirement *Req : RE->getRequirements())
       if (!Req->isDependent() && !Req->isSatisfied()) {
         if (auto *E = dyn_cast<concepts::ExprRequirement>(Req))

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -15,6 +15,7 @@
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTLambda.h"
 #include "clang/AST/ASTMutationListener.h"
+#include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprConcepts.h"
@@ -1362,7 +1363,22 @@ namespace {
 
     ExprResult TransformRequiresExpr(RequiresExpr *E) {
       LocalInstantiationScope Scope(SemaRef, /*CombineWithOuterScope=*/true);
-      return inherited::TransformRequiresExpr(E);
+      auto TransReq = inherited::TransformRequiresExpr(E);
+      if (TransReq.isInvalid())
+        return TransReq;
+      assert(TransReq.get() != E &&
+             "Do not change value of isSatisfied for the existing expression. "
+             "Create a new expression instead.");
+      if (E->getBody()->isDependentContext()) {
+        Sema::SFINAETrap Trap(SemaRef);
+        // We recreate the RequiresExpr body, but not by instantiating it.
+        // Produce pending diagnostics for dependent access check.
+        SemaRef.PerformDependentDiagnostics(E->getBody(), TemplateArgs);
+        // FIXME: Store SFINAE diagnostics in RequiresExpr for diagnosis.
+        if (Trap.hasErrorOccurred())
+          TransReq.getAs<RequiresExpr>()->setSatisfied(false);
+      }
+      return TransReq;
     }
 
     bool TransformRequiresExprRequirements(

clang/test/CXX/expr/expr.prim/expr.prim.req/simple-requirement.cpp

@@ -104,3 +104,138 @@ class X { virtual ~X(); };
 constexpr bool b = requires (X &x) { static_cast<int(*)[(typeid(x), 0)]>(nullptr); };
 // expected-error@-1{{constraint variable 'x' cannot be used in an evaluated context}}
 // expected-note@-2{{'x' declared here}}
+
+namespace access_checks {
+namespace in_requires_expression {
+template<auto>
+struct A {
+    static constexpr bool foo();
+    static constexpr bool bar();
+    static constexpr bool baz();
+    static constexpr bool faz();
+};
+
+class C{};
+
+class B {
+    void p() {}
+    bool data_member = true;
+    static const bool static_member = true;
+    friend struct A<0>;
+};
+
+template<auto x>
+constexpr bool A<x>::foo() {
+    return requires(B b) { b.p(); };
+}
+static_assert(!A<1>::foo());
+static_assert(A<0>::foo());
+
+template<auto x>
+constexpr bool A<x>::bar() {
+    return requires() { B::static_member; };
+}
+static_assert(!A<1>::bar());
+static_assert(A<0>::bar());
+
+template<auto x>
+constexpr bool A<x>::baz() {
+    return requires(B b) { b.data_member; };
+}
+static_assert(!A<1>::baz());
+static_assert(A<0>::baz());
+
+template<auto x>
+constexpr bool A<x>::faz() {
+    return requires(B a, B b) { 
+      a.p();
+      b.data_member;
+      B::static_member;
+    };
+}
+static_assert(!A<1>::faz());
+static_assert(A<0>::faz());
+} // namespace in_requires_expression
+
+namespace in_concepts {
+// Dependent access does not cause hard errors.
+template<int N> class A;
+
+template <> class A<0> {
+  static void f() {}
+};
+template<int N>
+concept C1 = requires() { A<N>::f(); };
+static_assert(!C1<0>);
+
+template <> class A<1> {
+public: 
+  static void f() {}
+};
+static_assert(C1<1>);
+
+// Non-dependent access to private member is a hard error.
+class B{
+   static void f() {} // expected-note 2{{implicitly declared private here}}
+};
+template<class T>
+concept C2 = requires() { B::f(); }; // expected-error {{'f' is a private member}}
+
+constexpr bool non_template_func() {
+  return requires() {
+      B::f(); // expected-error {{'f' is a private member}}
+  };
+}
+template<int x>
+constexpr bool template_func() {
+  return requires() {
+      A<x>::f();
+  };
+}
+static_assert(!template_func<0>());
+static_assert(template_func<1>());
+} // namespace in_concepts
+
+namespace in_trailing_requires {
+template <class> struct B;
+class A {
+   static void f();
+   friend struct B<short>;
+};
+ 
+template <class T> struct B {
+  static constexpr int index() requires requires{ A::f(); } {
+    return 1;
+  }
+  static constexpr int index() {
+    return 2;
+  }
+};
+
+static_assert(B<short>::index() == 1);
+static_assert(B<int>::index() == 2);
+
+namespace missing_member_function {
+template <class T> struct Use;
+class X { 
+  int a;
+  static int B;
+  friend struct Use<short>;
+};
+template <class T> struct Use {
+  constexpr static int foo() requires requires(X x) { x.a; } {
+    return 1;
+  }
+  constexpr static int bar() requires requires { X::B; } {
+    return 1;
+  }
+};
+
+void test() {
+  // FIXME: Propagate diagnostic.
+  Use<int>::foo(); //expected-error {{invalid reference to function 'foo': constraints not satisfied}}
+  static_assert(Use<short>::foo() == 1);
+}
+} // namespace missing_member_function
+} // namespace in_trailing_requires
+} // namespace access_check

clang/test/SemaCXX/invalid-requirement-requires-expr.cpp

@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 %s -I%S -std=c++2a -verify
+
+// RequiresExpr contains invalid requirement. (Eg. Highly recurisive template).
+template<int x>
+struct A { static constexpr bool far(); };
+class B {
+    bool data_member;
+    friend struct A<1>;
+};
+
+template<>
+constexpr bool A<0>::far() { return true; }
+
+template<int x>
+constexpr bool A<x>::far() {
+    return requires(B b) {
+      b.data_member;
+      requires A<x-1>::far(); //expected-note 3{{in instantiation}} // expected-note 6{{while}} expected-note {{contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all}}
+      // expected-error@-1{{recursive template instantiation exceeded maximum depth}}
+    };
+}
+static_assert(A<1>::far());
+static_assert(!A<10001>::far()); // expected-note {{in instantiation of member function}}