[Clang][Parse] Diagnose requires expressions with explicit object parameters (#88974)

Clang currently allows the following:
```
auto x = requires (this int) { true; };
```
This patch addresses that.

1 files changed, 14 insertions, 1 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 274ee7b..5f26b5a 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -7660,8 +7660,21 @@ void Parser::ParseParameterDeclarationClause(
     // Parse a C++23 Explicit Object Parameter
     // We do that in all language modes to produce a better diagnostic.
     SourceLocation ThisLoc;
-    if (getLangOpts().CPlusPlus && Tok.is(tok::kw_this))
+    if (getLangOpts().CPlusPlus && Tok.is(tok::kw_this)) {
       ThisLoc = ConsumeToken();
+      // C++23 [dcl.fct]p6:
+      //   An explicit-object-parameter-declaration is a parameter-declaration
+      //   with a this specifier. An explicit-object-parameter-declaration
+      //   shall appear only as the first parameter-declaration of a
+      //   parameter-declaration-list of either:
+      //   - a member-declarator that declares a member function, or
+      //   - a lambda-declarator.
+      //
+      // The parameter-declaration-list of a requires-expression is not such
+      // a context.
+      if (DeclaratorCtx == DeclaratorContext::RequiresExpr)
+        Diag(ThisLoc, diag::err_requires_expr_explicit_object_parameter);
+    }
 
     ParseDeclarationSpecifiers(DS, /*TemplateInfo=*/ParsedTemplateInfo(),
                                AS_none, DeclSpecContext::DSC_normal,