[diff.cpp17] Correct description of explicit(bool) compatibility.

Parenthesized names for explicit constructors and conversion functions
are still valid if the name doesn't immediately follow the explicit
keyword. Add an example using explicit(true) as a hint indicating how
to get the same effect.

Change affected subclauses from [dcl.fct] to the somewhat-more-precise
[class.ctor] and [class.conv.fct].

Author: zygoloid

source/compatibility.tex

@@ -1917,41 +1917,42 @@
 
 \rSec2[diff.cpp17.special]{\ref{special}: special member functions}
 
-\diffrefs{class.ctor}{class.dtor}
+\diffrefs{class.ctor}{class.conv.fct}
 \change
-A \grammarterm{simple-template-id}
-is no longer valid as the \grammarterm{declarator-id} of a constructor or destructor.
+The class name can no longer be used parenthesized
+immediately after an \tcode{explicit} \grammarterm{decl-specifier}
+in a constructor declaration.
+The \grammarterm{conversion-function-id} can no longer be used parenthesized
+immediately after an \tcode{explicit} \grammarterm{decl-specifier}
+in a conversion function declaration.
 \rationale
-Remove potentially error-prone option for redundancy.
+Necessary for new functionality.
 \effect
 Valid \CppXVII{} code may fail to compile
 in this International Standard. For example:
 \begin{codeblock}
-template<class T>
-struct A {
-  A<T>();          // error: \grammarterm{simple-template-id} not allowed for constructor
-  A(int);          // OK, \grammarterm{injected-class-name} used
-  ~A<T>();         // error: \grammarterm{simple-template-id} not allowed for destructor
+struct S {
+  explicit (S)(const S&);       // ill-formed; previously well-formed
+  explicit (operator int)();    // ill-formed; previously well-formed
+  explicit(true) (S)(int);      // OK
 };
 \end{codeblock}
 
-\rSec2[diff.cpp17.decl]{\ref{dcl.decl}: declarators}
-
-\diffref{dcl.fct}
+\diffrefs{class.ctor}{class.dtor}
 \change
-The class name can no longer be used parenthesized
-in an explicit constructor declaration.
-The \grammarterm{conversion-function-id} can no longer be used parenthesized
-in an explicit conversion function declaration.
+A \grammarterm{simple-template-id}
+is no longer valid as the \grammarterm{declarator-id} of a constructor or destructor.
 \rationale
-Necessary for new functionality.
+Remove potentially error-prone option for redundancy.
 \effect
 Valid \CppXVII{} code may fail to compile
 in this International Standard. For example:
 \begin{codeblock}
-struct S {
-  explicit (S)(const S&);       // ill-formed; previously well-formed
-  explicit (operator int)();    // ill-formed; previously well-formed
+template<class T>
+struct A {
+  A<T>();          // error: \grammarterm{simple-template-id} not allowed for constructor
+  A(int);          // OK, \grammarterm{injected-class-name} used
+  ~A<T>();         // error: \grammarterm{simple-template-id} not allowed for destructor
 };
 \end{codeblock}