c++: avoid -Wdangling-reference for std::span-like classes [PR110358]

Real-world experience shows that -Wdangling-reference triggers for
user-defined std::span-like classes a lot.  We can easily avoid that
by considering classes like

    template<typename T>
    struct Span {
      T* data_;
      std::size len_;
    };

to be std::span-like, and not warning for them.  Unlike the previous
patch, this one considers a non-union class template that has a pointer
data member and a trivial destructor as std::span-like.

	PR c++/110358
	PR c++/109640

gcc/cp/ChangeLog:

	* call.cc (reference_like_class_p): Don't warn for std::span-like
	classes.

gcc/ChangeLog:

	* doc/invoke.texi: Update -Wdangling-reference description.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wdangling-reference18.C: New test.
	* g++.dg/warn/Wdangling-reference19.C: New test.
	* g++.dg/warn/Wdangling-reference20.C: New test.

1 files changed, 14 insertions, 0 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 819a75d..eb931b9 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -3916,6 +3916,20 @@ where @code{std::minmax} returns @code{std::pair<const int&, const int&>}, and
 both references dangle after the end of the full expression that contains
 the call to @code{std::minmax}.
 
+The warning does not warn for @code{std::span}-like classes.  We consider
+classes of the form:
+
+@smallexample
+template<typename T>
+struct Span @{
+  T* data_;
+  std::size len_;
+@};
+@end smallexample
+
+as @code{std::span}-like; that is, the class is a non-union class template
+that has a pointer data member and a trivial destructor.
+
 This warning is enabled by @option{-Wall}.
 
 @opindex Wdelete-non-virtual-dtor