[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

(cherry picked from commit 00e2098)

Author: cor3ntin

clang/lib/Sema/SemaDeclCXX.cpp

@@ -17146,21 +17146,14 @@ Decl *Sema::BuildStaticAssertDeclaration(SourceLocation StaticAssertLoc,
                        FoldKind).isInvalid())
       Failed = true;
 
-    // If the static_assert passes, only verify that
-    // the message is grammatically valid without evaluating it.
-    if (!Failed && AssertMessage && Cond.getBoolValue()) {
-      std::string Str;
-      EvaluateStaticAssertMessageAsString(AssertMessage, Str, Context,
-                                          /*ErrorOnInvalidMessage=*/false);
-    }
-
     // 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);
       bool HasMessage = AssertMessage;
@@ -17192,8 +17185,8 @@ Decl *Sema::BuildStaticAssertDeclaration(SourceLocation StaticAssertLoc,
             << InnerCond->getSourceRange();
         DiagnoseStaticAssertDetails(InnerCond);
       } else {
-        Diag(AssertExpr->getBeginLoc(), diag::err_static_assert_failed)
-            << !HasMessage << Msg.str() << AssertExpr->getSourceRange();
+        Diag(StaticAssertLoc, diag::err_static_assert_failed)
+            << !AssertMessage << Msg.str() << AssertExpr->getSourceRange();
         PrintContextStack();
       }
       Failed = true;

clang/test/CXX/drs/dr25xx.cpp

@@ -81,6 +81,40 @@ using ::dr2521::operator""_div;
 #endif
 } // namespace dr2521
 
+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
 #if __cplusplus >= 202002L
   template<typename T>