Make simple requirements starting with requires ill-formed in in requ…

…irement body

This patch implements P2092

Simple requirements in requirement body shall not start with requires.
A warning was already in place so we just turn this warning into an error.

In addition, we add tests to make sure typename is optional in
requirement-parameter-list as per the same paper.

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -806,10 +806,10 @@ def err_requires_expr_expected_type_constraint : Error<
 def err_requires_expr_simple_requirement_noexcept : Error<
   "'noexcept' can only be used in a compound requirement (with '{' '}' around "
   "the expression)">;
-def warn_requires_expr_in_simple_requirement : Warning<
-  "this requires expression will only be checked for syntactic validity; did "
+def err_requires_expr_in_simple_requirement : Error<
+  "requires expression in requirement body; did "
   "you intend to place it in a nested requirement? (add another 'requires' "
-  "before the expression)">, InGroup<DiagGroup<"requires-expression">>;
+  "before the expression)">;
 
 def err_missing_dependent_template_keyword : Error<
   "use 'template' keyword to treat '%0' as a dependent template name">;

clang/lib/Parse/ParseExprCXX.cpp

@@ -3602,7 +3602,7 @@ ExprResult Parser::ParseRequiresExpression() {
           break;
         }
         if (!Expression.isInvalid() && PossibleRequiresExprInSimpleRequirement)
-          Diag(StartLoc, diag::warn_requires_expr_in_simple_requirement)
+          Diag(StartLoc, diag::err_requires_expr_in_simple_requirement)
               << FixItHint::CreateInsertion(StartLoc, "requires");
         if (auto *Req = Actions.ActOnSimpleRequirement(Expression.get()))
           Requirements.push_back(Req);

clang/test/Parser/cxx2a-concepts-requires-expr.cpp

@@ -134,13 +134,13 @@ void r37(bool b) requires requires { 1 } {}
 // expected-error@-1 {{expected ';' at end of requirement}}
 
 bool r38 = requires { requires { 1; }; };
-// expected-warning@-1 {{this requires expression will only be checked for syntactic validity; did you intend to place it in a nested requirement? (add another 'requires' before the expression)}}
+// expected-error@-1 {{requires expression in requirement body; did you intend to place it in a nested requirement? (add another 'requires' before the expression)}}
 
 bool r39 = requires { requires () { 1; }; };
-// expected-warning@-1 {{this requires expression will only be checked for syntactic validity; did you intend to place it in a nested requirement? (add another 'requires' before the expression)}}
+// expected-error@-1 {{requires expression in requirement body; did you intend to place it in a nested requirement? (add another 'requires' before the expression)}}
 
 bool r40 = requires { requires (int i) { i; }; };
-// expected-warning@-1 {{this requires expression will only be checked for syntactic validity; did you intend to place it in a nested requirement? (add another 'requires' before the expression)}}
+// expected-error@-1 {{requires expression in requirement body; did you intend to place it in a nested requirement? (add another 'requires' before the expression)}}
 
 bool r41 = requires { requires (); };
 // expected-error@-1 {{expected expression}}

clang/www/cxx_status.html

@@ -934,7 +934,7 @@ <h2 id="cxx20">C++20 implementation status</h2>
       </tr>
       <tr> <!-- from Belfast -->
         <td><a href="https://wg21.link/p1972r0">P1972R0</a></td>
-        <td rowspan="5" class="none" align="center">No</td>
+        <td rowspan="3" class="none" align="center">No</td>
       </tr>
       <tr>
         <td><a href="https://wg21.link/p1980r0">P1980R0</a></td>
@@ -944,9 +944,11 @@ <h2 id="cxx20">C++20 implementation status</h2>
       </tr>
       <tr>
         <td><a href="https://wg21.link/p2092r0">P2092R0</a></td>
+        <td rowspan="1" class="partial" align="center">Clang 13</td>
       </tr>
       <tr>
         <td><a href="https://wg21.link/p2113r0">P2113R0</a></td>
+        <td rowspan="1" class="none" align="center">No</td>
       </tr>
     <!-- Albuquerque papers -->
     <tr>