c++: reference cast, conversion fn [PR113141]

The second testcase in 113141 is a separate issue: we first decide that the
conversion is ill-formed, but then when recalculating the special c_cast_p
handling makes us think it's OK.  We don't want that, it should continue to
fall back to the reinterpret_cast interpretation.  And while we're here,
let's warn that we're not using the conversion function.

Note that the standard seems to say that in this case we should
treat (Matrix &) as const_cast<Matrix &>(static_cast<const Matrix &>(X)),
which would use the conversion operator, but that doesn't match existing
practice, so let's resolve that another day.  I've raised this issue with
CWG; at the moment I lean toward never binding a temporary in a C-style cast
to reference type, which would also be a change from existing practice.

	PR c++/113141

gcc/c-family/ChangeLog:

	* c.opt: Add -Wcast-user-defined.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Wcast-user-defined.

gcc/cp/ChangeLog:

	* call.cc (reference_binding): For an invalid cast, warn and don't
	recalculate.

gcc/testsuite/ChangeLog:

	* g++.dg/conversion/ref12.C: New test.

Co-authored-by: Patrick Palka <ppalka@redhat.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 5d5e70c..e328558 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -9391,6 +9391,19 @@ In a cast involving pointer to member types this warning warns whenever
 the type cast is changing the pointer to member type.
 This warning is enabled by @option{-Wextra}.
 
+@opindex Wcast-user-defined
+@opindex Wno-cast-user-defined
+@item -Wcast-user-defined
+Warn when a cast to reference type does not involve a user-defined
+conversion that the programmer might expect to be called.
+
+@smallexample
+struct A @{ operator const int&(); @} a;
+auto r = (int&)a; // warning
+@end smallexample
+
+This warning is enabled by default.
+
 @opindex Wwrite-strings
 @opindex Wno-write-strings
 @item -Wwrite-strings