A user-defined deduction guide steers class template argument deduction.
It is written at the same scope as the class template, either as a plain
declaration or, when generic, under a template parameter list:
Box(int) -> Box<int>;
template <typename T> Box(T) -> Box<T>;
A deduction guide is not a function: it has no body and emits no symbol,
and only steers argument deduction at compile time. There is nothing for
SWIG to wrap, so a new deduction_guide grammar rule parses the guide and
discards it. The optional explicit specifier and a C++20 trailing
requires-clause are accepted. Previously any deduction guide resulted in
a syntax error.
Assisted-by: Claude Code (Opus 4.8)
A variable whose declared type is a bare class template name uses class
template argument deduction (CTAD), added in C++17 - the arguments are
deduced from the initializer using guides synthesised from the class's
constructors. C++20 P1816 also allows CTAD for aggregates with no
deduction guide, e.g. 'Overloaded ov{...};'. SWIG performs no template
argument deduction, so it cannot determine the instantiated type; it
previously treated the bare template name as a concrete type and
generated a wrapper naming the template without arguments, which does
not compile.
cDeclaration now detects this, issues Warning 347 and skips the
declaration. Declarations alongside it are unaffected.
Assisted-by: Claude Code (Opus 4.8)
William S Fulton