CWG2322 Substitution failure and lexical order

Author: tkoeppe

source/templates.tex

@@ -7173,6 +7173,9 @@
 inside \tcode{sizeof}, \tcode{decltype}, and other contexts that allow non-constant
 expressions. The substitution proceeds in lexical order and stops when
 a condition that causes deduction to fail is encountered.
+If substitution into different declarations of the same function template would
+cause template instantiations to occur in a different order or not at all,
+the program is ill-formed; no diagnostic required.
 \begin{note}
 The equivalent substitution in exception specifications is
 done only when the \grammarterm{noexcept-specifier} is instantiated,
@@ -7186,10 +7189,14 @@
 template <class T> void f(...) { }
 template <class T> auto g(typename A<T>::X) -> typename T::X;
 template <class T> void g(...) { }
+template <class T> typename T::X h(typename A<T>::X);
+template <class T> auto h(typename A<T>::X) -> typename T::X;   // redeclaration
+template <class T> void h(...) { }
 
-void h() {
+void x() {
   f<int>(0);        // OK, substituting return type causes deduction to fail
   g<int>(0);        // error, substituting parameter type instantiates \tcode{A<int>}
+  h<int>(0);        // ill-formed, no diagnostic required
 }
 \end{codeblock}
 \end{example}