[Clang] Fix handling of qualified id-expressions in unevaluated contexts (#99807)

Summary:
In #89713, we made qualified, parenthesized id-expression ill-formed in
and address of expressions.

The expected behavior should instead be to form a pointer (rather than a
pointer to member)

The fix has been suggested by @zwuis and the tests by
@hubert-reinterpretcast.

It is worth pointing out that some of these tests seem rejected by all
compilers, however the tests do seem correct.

Fixes #89713
Fixes #40906

---------

Co-authored-by: YanzuoLiu <[email protected]>

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60251286

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -58,7 +58,8 @@ C++ Specific Potentially Breaking Changes
   versions of clang. The deprecation warning for the negative spelling can be
   disabled with `-Wno-deprecated-no-relaxed-template-template-args`.
 
-- Clang now rejects pointer to member from parenthesized expression in unevaluated context such as ``decltype(&(foo::bar))``. (#GH40906).
+- Clang no longer tries to form pointer-to-members from qualified and parenthesized unevaluated expressions
+  such as ``decltype(&(foo::bar))``. (#GH40906).
 
 - Clang now performs semantic analysis for unary operators with dependent operands
   that are known to be of non-class non-enumeration type prior to instantiation.

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -7596,9 +7596,6 @@ def err_nested_non_static_member_use : Error<
 def warn_cxx98_compat_non_static_member_use : Warning<
   "use of non-static data member %0 in an unevaluated context is "
   "incompatible with C++98">, InGroup<CXX98Compat>, DefaultIgnore;
-def err_form_ptr_to_member_from_parenthesized_expr : Error<
-  "cannot form pointer to member from a parenthesized expression; "
-  "did you mean to remove the parentheses?">;
 def err_invalid_incomplete_type_use : Error<
   "invalid use of incomplete type %0">;
 def err_builtin_func_cast_more_than_one_arg : Error<

clang/lib/Sema/SemaExpr.cpp

@@ -14117,7 +14117,14 @@ QualType Sema::CheckAddressOfOperand(ExprResult &OrigOp, SourceLocation OpLoc) {
       // Okay: we can take the address of a field.
       // Could be a pointer to member, though, if there is an explicit
       // scope qualifier for the class.
-      if (isa<DeclRefExpr>(op) && cast<DeclRefExpr>(op)->getQualifier()) {
+
+      // [C++26] [expr.prim.id.general]
+      // If an id-expression E denotes a non-static non-type member
+      // of some class C [...] and if E is a qualified-id, E is
+      // not the un-parenthesized operand of the unary & operator [...]
+      // the id-expression is transformed into a class member access expression.
+      if (isa<DeclRefExpr>(op) && cast<DeclRefExpr>(op)->getQualifier() &&
+          !isa<ParenExpr>(OrigOp.get())) {
         DeclContext *Ctx = dcl->getDeclContext();
         if (Ctx && Ctx->isRecord()) {
           if (dcl->getType()->isReferenceType()) {
@@ -14127,22 +14134,6 @@ QualType Sema::CheckAddressOfOperand(ExprResult &OrigOp, SourceLocation OpLoc) {
             return QualType();
           }
 
-          // C++11 [expr.unary.op] p4:
-          // A pointer to member is only formed when an explicit & is used and
-          // its operand is a qualified-id not enclosed in parentheses.
-          if (isa<ParenExpr>(OrigOp.get())) {
-            SourceLocation LeftParenLoc = OrigOp.get()->getBeginLoc(),
-                           RightParenLoc = OrigOp.get()->getEndLoc();
-
-            Diag(LeftParenLoc,
-                 diag::err_form_ptr_to_member_from_parenthesized_expr)
-                << SourceRange(OpLoc, RightParenLoc)
-                << FixItHint::CreateRemoval(LeftParenLoc)
-                << FixItHint::CreateRemoval(RightParenLoc);
-
-            // Continuing might lead to better error recovery.
-          }
-
           while (cast<RecordDecl>(Ctx)->isAnonymousStructOrUnion())
             Ctx = Ctx->getParent();
 

clang/test/CXX/expr/expr.unary/expr.unary.op/p4.cpp

@@ -43,18 +43,36 @@ namespace test2 {
 }
 
 namespace GH40906 {
-  struct A {
-    int val;
-    void func() {}
-  };
+struct S {
+    int x;
+    void func();
+    static_assert(__is_same_as(decltype((S::x)), int&), "");
+    static_assert(__is_same_as(decltype(&(S::x)), int*), "");
 
-  void test() {
-    decltype(&(A::val)) ptr1; // expected-error {{cannot form pointer to member from a parenthesized expression; did you mean to remove the parentheses?}}
-    int A::* ptr2 = &(A::val); // expected-error {{invalid use of non-static data member 'val'}}
+    // FIXME: provide better error messages
+    static_assert(__is_same_as(decltype((S::func)), int&), ""); // expected-error {{call to non-static member function without an object argument}}
+    static_assert(__is_same_as(decltype(&(S::func)), int*), ""); // expected-error {{call to non-static member function without an object argument}}
+};
+static_assert(__is_same_as(decltype((S::x)), int&), "");
+static_assert(__is_same_as(decltype(&(S::x)), int*), "");
+static_assert(__is_same_as(decltype((S::func)), int&), ""); // expected-error {{call to non-static member function without an object argument}}
+static_assert(__is_same_as(decltype(&(S::func)), int*), ""); // expected-error {{call to non-static member function without an object argument}}
+
+struct A { int x;};
+
+char q(int *);
+short q(int A::*);
+
+template <typename T>
+constexpr int f(char (*)[sizeof(q(&T::x))]) { return 1; }
+
+template <typename T>
+constexpr int f(char (*)[sizeof(q(&(T::x)))]) { return 2; }
+
+constexpr int g(char (*p)[sizeof(char)] = 0) { return f<A>(p); }
+constexpr int h(char (*p)[sizeof(short)] = 0) { return f<A>(p); }
+
+static_assert(g() == 2);
+static_assert(h() == 1);
 
-    // FIXME: Error messages in these cases are less than clear, we can do
-    // better.
-    int size = sizeof(&(A::func)); // expected-error {{call to non-static member function without an object argument}}
-    void (A::* ptr3)() = &(A::func); // expected-error {{call to non-static member function without an object argument}}
-  }
 }