PR c++/81327 - cast to void* does not suppress -Wclass-memaccess

gcc/ChangeLog:
	PR c++/81327
	* doc/invoke.texi (-Wlass-memaccess): Document suppression by casting.

From-SVN: r256677

gcc/ChangeLog

+2018-01-14  Martin Sebor  <msebor@redhat.com>
+
+	PR c++/81327
+	* doc/invoke.texi (-Wlass-memaccess): Document suppression by casting.
+
 2018-01-14  Jakub Jelinek  <jakub@redhat.com>
 
 	* config.gcc (i[34567]86-*-*): Remove one duplicate gfniintrin.h

gcc/doc/invoke.texi

@@ -2956,20 +2956,23 @@ C++17 it calls @code{f<void(*)()noexcept>}.
 @item -Wclass-memaccess @r{(C++ and Objective-C++ only)}
 @opindex Wclass-memaccess
 Warn when the destination of a call to a raw memory function such as
-@code{memset} or @code{memcpy} is an object of class type writing into which
-might bypass the class non-trivial or deleted constructor or copy assignment,
-violate const-correctness or encapsulation, or corrupt the virtual table.
-Modifying the representation of such objects may violate invariants maintained
-by member functions of the class.  For example, the call to @code{memset}
-below is undefined because it modifies a non-trivial class object and is,
-therefore, diagnosed.  The safe way to either initialize or clear the storage
-of objects of such types is by using the appropriate constructor or assignment
-operator, if one is available.
+@code{memset} or @code{memcpy} is an object of class type, and when writing
+into such an object might bypass the class non-trivial or deleted constructor
+or copy assignment, violate const-correctness or encapsulation, or corrupt
+the virtual table.  Modifying the representation of such objects may violate
+invariants maintained by member functions of the class.  For example, the call
+to @code{memset} below is undefined because it modifies a non-trivial class
+object and is, therefore, diagnosed.  The safe way to either initialize or
+clear the storage of objects of such types is by using the appropriate
+constructor or assignment operator, if one is available.
 @smallexample
 std::string str = "abc";
-memset (&str, 0, 3);
+memset (&str, 0, sizeof str);
 @end smallexample
 The @option{-Wclass-memaccess} option is enabled by @option{-Wall}.
+Explicitly casting the pointer to the class object to @code{void *} or
+to a type that can be safely accessed by the raw memory function suppresses
+the warning.
 
 @item -Wnon-virtual-dtor @r{(C++ and Objective-C++ only)}
 @opindex Wnon-virtual-dtor