c++: Add -Wnrvo

While looking at PRs about cases where we don't perform the named return
value optimization, it occurred to me that it might be useful to have a
warning for that.

This does not fix PR58487, but might be interesting to people watching it.

	PR c++/58487

gcc/c-family/ChangeLog:

	* c.opt: Add -Wnrvo.

gcc/ChangeLog:

	* doc/invoke.texi: Document it.

gcc/cp/ChangeLog:

	* typeck.cc (want_nrvo_p): New.
	(check_return_expr): Handle -Wnrvo.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/nrv25.C: New test.

1 files changed, 19 insertions, 0 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 9130104..6d08229 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -6678,6 +6678,25 @@ is only active when @option{-fdelete-null-pointer-checks} is active,
 which is enabled by optimizations in most targets.  The precision of
 the warnings depends on the optimization options used.
 
+@opindex Wnrvo
+@opindex Wno-nrvo
+@item -Wnrvo @r{(C++ and Objective-C++ only)}
+Warn if the compiler does not elide the copy from a local variable to
+the return value of a function in a context where it is allowed by
+[class.copy.elision].  This elision is commonly known as the Named
+Return Value Optimization.  For instance, in the example below the
+compiler cannot elide copies from both v1 and b2, so it elides neither.
+
+@smallexample
+std::vector<int> f()
+@{
+  std::vector<int> v1, v2;
+  // ...
+  if (cond) return v1;
+  else return v2; // warning: not eliding copy
+@}
+@end smallexample
+
 @opindex Winfinite-recursion
 @opindex Wno-infinite-recursion
 @item -Winfinite-recursion