CWG2653 Can an explicit object parameter have a default argument?

Also fixes NB GB 051 (C++23 CD).

Author: burblebee

source/declarations.tex

@@ -3380,9 +3380,9 @@
 \begin{bnf}
 \nontermdef{parameter-declaration}\br
     \opt{attribute-specifier-seq} \opt{\keyword{this}} decl-specifier-seq declarator\br
-    \opt{attribute-specifier-seq} \opt{\keyword{this}} decl-specifier-seq declarator \terminal{=} initializer-clause\br
+    \opt{attribute-specifier-seq} decl-specifier-seq declarator \terminal{=} initializer-clause\br
     \opt{attribute-specifier-seq} \opt{\keyword{this}} decl-specifier-seq \opt{abstract-declarator}\br
-    \opt{attribute-specifier-seq} \opt{\keyword{this}} decl-specifier-seq \opt{abstract-declarator} \terminal{=} initializer-clause
+    \opt{attribute-specifier-seq} decl-specifier-seq \opt{abstract-declarator} \terminal{=} initializer-clause
 \end{bnf}
 
 The optional \grammarterm{attribute-specifier-seq} in a \grammarterm{parameter-declaration}