[Clang] Implement CWG2518 - static_assert(false)

This allows `static_assert(false)` to not be ill-formed
in template definitions.

This change is applied as a DR in all C++ modes.

Of notes, a couple of tests were relying of the eager nature
of static_assert

* test/SemaTemplate/instantiation-dependence.cpp
* test/SemaTemplate/instantiate-var-template.cpp

I don't know if the changes to `static_assert`
still allow that sort of tests to be expressed.

Reviewed By: #clang-language-wg, erichkeane, aaron.ballman

Differential Revision: https://reviews.llvm.org/D144285

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -73,6 +73,8 @@ C++ Language Changes
 - Improved ``-O0`` code generation for calls to ``std::forward_like``. Similarly to
   ``std::move, std::forward`` et al. it is now treated as a compiler builtin and implemented
   directly rather than instantiating the definition from the standard library.
+- Implemented `CWG2518 <https://wg21.link/CWG2518>`_ which allows ``static_assert(false)``
+  to not be ill-formed when its condition is evaluated in the context of a template definition.
 
 C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDeclCXX.cpp

@@ -16784,7 +16784,14 @@ Decl *Sema::BuildStaticAssertDeclaration(SourceLocation StaticAssertLoc,
                        FoldKind).isInvalid())
       Failed = true;
 
-    if (!Failed && !Cond) {
+    // CWG2518
+    // [dcl.pre]/p10  If [...] the expression is evaluated in the context of a
+    // template definition, the declaration has no effect.
+    bool InTemplateDefinition =
+        getLangOpts().CPlusPlus && CurContext->isDependentContext();
+
+    if (!Failed && !Cond && !InTemplateDefinition) {
+
       SmallString<256> MsgBuffer;
       llvm::raw_svector_ostream Msg(MsgBuffer);
       if (AssertMessage) {
@@ -16815,7 +16822,8 @@ Decl *Sema::BuildStaticAssertDeclaration(SourceLocation StaticAssertLoc,
         DiagnoseStaticAssertDetails(InnerCond);
       } else {
         Diag(StaticAssertLoc, diag::err_static_assert_failed)
-          << !AssertMessage << Msg.str() << AssertExpr->getSourceRange();
+            << !AssertMessage << Msg.str() << AssertExpr->getSourceRange();
+        PrintContextStack();
       }
       Failed = true;
     }

clang/test/CXX/drs/dr25xx.cpp

@@ -1,5 +1,39 @@
 // RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify
 
+namespace dr2518 { // dr2518: 17 review
+
+template <class T>
+void f(T t) {
+  if constexpr (sizeof(T) != sizeof(int)) {
+    static_assert(false, "must be int-sized"); // expected-error {{must be int-size}}
+  }
+}
+
+void g(char c) {
+  f(0);
+  f(c); // expected-note {{requested here}}
+}
+
+template <typename Ty>
+struct S {
+  static_assert(false); // expected-error {{static assertion failed}}
+};
+
+template <>
+struct S<int> {};
+
+template <>
+struct S<float> {};
+
+int test_specialization() {
+  S<int> s1;
+  S<float> s2;
+  S<double> s3; // expected-note {{in instantiation of template class 'dr2518::S<double>' requested here}}
+}
+
+}
+
+
 namespace dr2565 { // dr2565: 16 open
   template<typename T>
     concept C = requires (typename T::type x) {

clang/test/SemaCXX/access-base-class.cpp

@@ -96,14 +96,14 @@ struct flag {
 };
 
 template <class T>
-struct trait : flag<sizeof(T)> {};
+struct trait : flag<sizeof(T)> {}; // expected-note 2{{here}}
 
-template <class T, bool Inferred = trait<T>::value>
+template <class T, bool Inferred = trait<T>::value> // expected-note {{here}}
 struct a {};
 
 template <class T>
 class b {
-  a<T> x;
+  a<T> x; // expected-note {{here}}
   using U = a<T>;
 };
 
@@ -113,5 +113,5 @@ struct Impossible {
 };
 
 // verify "no member named 'value'" bogus diagnostic is not emitted.
-trait<b<Impossible<0>>>::value;
+trait<b<Impossible<0>>>::value; // expected-note {{here}}
 } // namespace T8

clang/test/SemaCXX/coroutines.cpp

@@ -1309,7 +1309,7 @@ struct DepTestType {
   }
 };
 
-template struct DepTestType<int>; // expected-note {{requested here}}
+template struct DepTestType<int>; // expected-note 2{{requested here}}
 template CoroMemberTag DepTestType<int>::test_member_template(long, const char *) const &&;
 
 template CoroMemberTag DepTestType<int>::test_static_template<void>(const char *volatile &, unsigned);

clang/test/SemaCXX/static-assert.cpp

@@ -52,9 +52,11 @@ static_assert(false, "🏳️‍🌈 🏴󠁧󠁢󠁥󠁮󠁧󠁿 🇪🇺"); //
 
 template<typename T> struct AlwaysFails {
   // Only give one error here.
-  static_assert(false, ""); // expected-error {{static assertion failed}}
+  static_assert(false, ""); // expected-error 2{{static assertion failed}}
 };
-AlwaysFails<int> alwaysFails;
+AlwaysFails<int> alwaysFails; // expected-note {{instantiation}}
+AlwaysFails<double> alwaysFails2; // expected-note {{instantiation}}
+
 
 template<typename T> struct StaticAssertProtected {
   static_assert(__is_literal(T), ""); // expected-error {{static assertion failed}}
@@ -217,6 +219,23 @@ static_assert(constexprNotBool, "message"); // expected-error {{value of type 'c
 
 static_assert(1 , "") // expected-error {{expected ';' after 'static_assert'}}
 
+namespace DependentAlwaysFalse {
+template <typename Ty>
+struct S {
+  static_assert(false); // expected-error{{static assertion failed}} \
+                        // expected-warning {{C++17 extension}}
+};
+
+template <typename Ty>
+struct T {
+  static_assert(false, "test"); // expected-error{{static assertion failed: test}}
+};
+
+int f() {
+  S<double> s; //expected-note {{in instantiation of template class 'DependentAlwaysFalse::S<double>' requested here}}
+  T<double> t; //expected-note {{in instantiation of template class 'DependentAlwaysFalse::T<double>' requested here}}
+}
+}
 
 namespace Diagnostics {
   /// No notes for literals.

clang/test/SemaTemplate/instantiate-var-template.cpp

@@ -31,8 +31,7 @@ namespace InstantiationDependent {
   static_assert(b<char> == 1, ""); // expected-note {{in instantiation of}}
 
   template<typename T> void f() {
-    static_assert(a<sizeof(sizeof(f(T())))> == 0, ""); // expected-error {{static assertion failed due to requirement 'a<sizeof (sizeof (f(type-parameter-0-0())))> == 0'}} \
-                                                       // expected-note {{evaluates to '1 == 0'}}
+    int check[a<sizeof(sizeof(f(T())))> == 0 ? 1 : -1]; // expected-error {{array with a negative size}}
   }
 }
 

clang/test/SemaTemplate/instantiation-dependence.cpp

@@ -37,8 +37,8 @@ namespace PR33655 {
   template<class ...Args> using indirect_void_t = typename indirect_void_t_imp<Args...>::type;
 
   template<class T> void foo() {
-    static_assert(!__is_void(indirect_void_t<T>)); // "ok", dependent
-    static_assert(!__is_void(void_t<T>)); // expected-error {{failed}}
+    int check1[__is_void(indirect_void_t<T>) == 0 ? 1 : -1]; // "ok", dependent
+    int check2[__is_void(void_t<T>) == 0 ? 1 : -1]; // expected-error {{array with a negative size}}
   }
 }
 

clang/www/cxx_dr_status.html

@@ -14915,7 +14915,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://wg21.link/cwg2518">2518</a></td>
     <td>review</td>
     <td>Conformance requirements and <TT>#error</TT>/<TT>#warning</TT></td>
-    <td align="center">Not resolved</td>
+    <td class="unreleased" align="center">Clang 17</td>
   </tr>
   <tr class="open" id="2519">
     <td><a href="https://wg21.link/cwg2519">2519</a></td>