[Clang] Improve support for expression messages in static_assert (#73234)

- Support non-member functions and callable objects for size and data().
We previously tried to (badly) pick the best overload ourselves, in a
way that would only support member functions. We now leave clang
construct an unresolved member expression and call that, properly
performing overload resolution with callable objects and static
functions, consistent with the logic for `get` calls for structured
bindings.
- Support UDLs as message expression.
- Add tests and mark CWG2798 as resolved

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -635,6 +635,9 @@ Bug Fixes in This Version
 - Fix crash during code generation of C++ coroutine initial suspend when the return
   type of await_resume is not trivially destructible.
   Fixes (`#63803 <https://github.com/llvm/llvm-project/issues/63803>`_)
+- Fix crash when the object used as a ``static_assert`` message has ``size`` or ``data`` members
+  which are not member functions.
+- Support UDLs in ``static_assert`` message.
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Parse/ParseDeclCXX.cpp

@@ -1023,7 +1023,7 @@ Decl *Parser::ParseStaticAssertDeclaration(SourceLocation &DeclEnd) {
         const Token &T = GetLookAheadToken(I);
         if (T.is(tok::r_paren))
           break;
-        if (!tokenIsLikeStringLiteral(T, getLangOpts())) {
+        if (!tokenIsLikeStringLiteral(T, getLangOpts()) || T.hasUDSuffix()) {
           ParseAsExpression = true;
           break;
         }

clang/lib/Sema/SemaDeclCXX.cpp

@@ -17291,33 +17291,15 @@ bool Sema::EvaluateStaticAssertMessageAsString(Expr *Message,
 
   auto FindMember = [&](StringRef Member, bool &Empty,
                         bool Diag = false) -> std::optional<LookupResult> {
-    QualType ObjectType = Message->getType();
-    Expr::Classification ObjectClassification =
-        Message->Classify(getASTContext());
-
     DeclarationName DN = PP.getIdentifierInfo(Member);
     LookupResult MemberLookup(*this, DN, Loc, Sema::LookupMemberName);
     LookupQualifiedName(MemberLookup, RD);
     Empty = MemberLookup.empty();
     OverloadCandidateSet Candidates(MemberLookup.getNameLoc(),
                                     OverloadCandidateSet::CSK_Normal);
-    for (NamedDecl *D : MemberLookup) {
-      AddMethodCandidate(DeclAccessPair::make(D, D->getAccess()), ObjectType,
-                         ObjectClassification, /*Args=*/{}, Candidates);
-    }
-    OverloadCandidateSet::iterator Best;
-    switch (Candidates.BestViableFunction(*this, Loc, Best)) {
-    case OR_Success:
-      return std::move(MemberLookup);
-    default:
-      if (Diag)
-        Candidates.NoteCandidates(
-            PartialDiagnosticAt(
-                Loc, PDiag(diag::err_static_assert_invalid_mem_fn_ret_ty)
-                         << (Member == "data")),
-            *this, OCD_AllCandidates, /*Args=*/{});
-    }
-    return std::nullopt;
+    if (MemberLookup.empty())
+      return std::nullopt;
+    return MemberLookup;
   };
 
   bool SizeNotFound, DataNotFound;

clang/test/CXX/drs/dr27xx.cpp

@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 -std=c++2c -verify %s
+
+namespace dr2798 { // dr2798: 17 drafting
+#if __cpp_static_assert >= 202306
+struct string {
+  constexpr string() {
+    data_ = new char[6]();
+    __builtin_memcpy(data_, "Hello", 5);
+    data_[5] = 0;
+  }
+  constexpr ~string() { delete[] data_; }
+  constexpr unsigned long size() const { return 5; };
+  constexpr const char *data() const { return data_; }
+
+  char *data_;
+};
+struct X {
+  string s;
+};
+consteval X f() { return {}; }
+
+static_assert(false, f().s); // expected-error {{static assertion failed: Hello}}
+#endif
+} // namespace dr2798

clang/test/SemaCXX/static-assert-cxx26.cpp

@@ -179,18 +179,20 @@ static_assert(false, Message{}); // expected-error {{static assertion failed: He
 }
 
 struct MessageInvalidSize {
-    constexpr auto size(int) const; // expected-note {{candidate function not viable: requires 1 argument, but 0 were provided}}
-    constexpr auto data() const;
+    constexpr unsigned long size(int) const; // expected-note {{'size' declared here}}
+    constexpr const char* data() const;
 };
 struct MessageInvalidData {
-    constexpr auto size() const;
-    constexpr auto data(int) const; // expected-note {{candidate function not viable: requires 1 argument, but 0 were provided}}
+    constexpr unsigned long size() const;
+    constexpr const char* data(int) const; // expected-note {{'data' declared here}}
 };
 
 static_assert(false, MessageInvalidSize{});  // expected-error {{static assertion failed}} \
-                                             // expected-error {{the message in a static assertion must have a 'size()' member function returning an object convertible to 'std::size_t'}}
+                                             // expected-error {{the message in a static assertion must have a 'size()' member function returning an object convertible to 'std::size_t'}} \
+                                             // expected-error {{too few arguments to function call, expected 1, have 0}}
 static_assert(false, MessageInvalidData{});  // expected-error {{static assertion failed}} \
-                                             // expected-error {{the message in a static assertion must have a 'data()' member function returning an object convertible to 'const char *'}}
+                                             // expected-error {{the message in a static assertion must have a 'data()' member function returning an object convertible to 'const char *'}} \
+                                             // expected-error {{too few arguments to function call, expected 1, have 0}}
 
 struct NonConstMembers {
     constexpr int size() {
@@ -227,14 +229,14 @@ static_assert(false, Variadic{}); // expected-error {{static assertion failed: O
 
 template <typename T>
 struct DeleteAndRequires {
-    constexpr int size() = delete; // expected-note {{candidate function has been explicitly deleted}}
-    constexpr const char* data() requires false; // expected-note {{candidate function not viable: constraints not satisfied}} \
-                                                 // expected-note {{because 'false' evaluated to false}}
+    constexpr int size() = delete; // expected-note {{'size' has been explicitly marked deleted here}}
+    constexpr const char* data() requires false; // expected-note {{because 'false' evaluated to false}}
 };
 static_assert(false, DeleteAndRequires<void>{});
 // expected-error@-1 {{static assertion failed}} \
 // expected-error@-1 {{the message in a static assertion must have a 'size()' member function returning an object convertible to 'std::size_t'}}\
-// expected-error@-1 {{the message in a static assertion must have a 'data()' member function returning an object convertible to 'const char *'}}
+// expected-error@-1 {{invalid reference to function 'data': constraints not satisfied}} \
+// expected-error@-1 {{attempt to use a deleted function}}
 
 class Private {
     constexpr int size(int i = 0) { // expected-note {{implicitly declared private here}}
@@ -294,6 +296,9 @@ struct Frobble {
   constexpr const char *data() const { return "hello"; }
 };
 
+constexpr Frobble operator ""_myd (const char *, unsigned long) { return Frobble{}; }
+static_assert (false, "foo"_myd); // expected-error {{static assertion failed: hello}}
+
 Good<Frobble> a; // expected-note {{in instantiation}}
 Bad<int> b; // expected-note {{in instantiation}}
 
@@ -307,3 +312,32 @@ static_assert((char8_t)-128 == (char8_t)-123, ""); // expected-error {{failed}}
 static_assert((char16_t)0xFEFF == (char16_t)0xDB93, ""); // expected-error {{failed}} \
                                                          // expected-note {{evaluates to 'u'' (0xFEFF, 65279) == u'\xDB93' (0xDB93, 56211)'}}
 }
+
+struct Static {
+  static constexpr int size() { return 5; }
+  static constexpr const char *data() { return "hello"; }
+};
+static_assert(false, Static{}); // expected-error {{static assertion failed: hello}}
+
+struct Data {
+    unsigned long size = 0;
+    const char* data = "hello";
+};
+static_assert(false, Data{}); // expected-error {{called object type 'unsigned long' is not a function or function pointer}} \
+                              // expected-error {{called object type 'const char *' is not a function or function pointer}} \
+                              // expected-error {{the message in a static assertion must have a 'size()' member function returning an object convertible to 'std::size_t'}} \
+                              // expected-error {{static assertion failed}}
+
+struct Callable {
+    struct {
+        constexpr auto operator()() const {
+            return 5;
+        };
+    } size;
+    struct {
+        constexpr auto operator()() const {
+            return "hello";
+        };
+    } data;
+};
+static_assert(false, Callable{}); // expected-error {{static assertion failed: hello}}

clang/www/cxx_dr_status.html

@@ -7145,7 +7145,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1223.html">1223</a></td>
     <td>drafting</td>
     <td>Syntactic disambiguation and <I>trailing-return-type</I>s</td>
-    <td class="unreleased" align="center">Clang 17</td>
+    <td class="full" align="center">Clang 17</td>
   </tr>
   <tr id="1224">
     <td><a href="https://cplusplus.github.io/CWG/issues/1224.html">1224</a></td>