PR c/71560 - union compound literal initializes wrong union field

gcc/ChangeLog:
	* doc/extend.texi (Compound Literals): Correct and clarify.
	(Cast to Union): Same.

From-SVN: r238651

gcc/ChangeLog

+2016-07-22 Martin Sebor  <msebor@redhat.com>
+
+	PR c/71560
+	* doc/extend.texi (Compound Literals): Correct and clarify.
+	(Cast to Union): Same.
+
 2016-07-22  Kelvin Nilsen  <kelvin@gcc.gnu.org>
 
 	* config/rs6000/rs6000.c (rs6000_option_override_internal): Add

gcc/doc/extend.texi

@@ -1868,15 +1868,16 @@ foo (float f, float g)
 @cindex compound literals
 @c The GNU C name for what C99 calls compound literals was "constructor expressions".
 
-ISO C99 supports compound literals.  A compound literal looks like
-a cast containing an initializer.  Its value is an object of the
-type specified in the cast, containing the elements specified in
-the initializer; it is an lvalue.  As an extension, GCC supports
-compound literals in C90 mode and in C++, though the semantics are
-somewhat different in C++.
+A compound literal looks like a cast of a brace-enclosed aggregate
+initializer list.  Its value is an object of the type specified in
+the cast, containing the elements specified in the initializer.
+Unlike the result of a cast, a compound literal is an lvalue.  ISO
+C99 and later support compound literals.  As an extension, GCC
+supports compound literals also in C90 mode and in C++, although
+as explained below, the C++ semantics are somewhat different.
 
-Usually, the specified type is a structure.  Assume that
-@code{struct foo} and @code{structure} are declared as shown:
+Usually, the specified type of a compound literal is a structure.  Assume
+that @code{struct foo} and @code{structure} are declared as shown:
 
 @smallexample
 struct foo @{int a; char b[2];@} structure;
@@ -1901,7 +1902,7 @@ This is equivalent to writing the following:
 
 You can also construct an array, though this is dangerous in C++, as
 explained below.  If all the elements of the compound literal are
-(made up of) simple constant expressions, suitable for use in
+(made up of) simple constant expressions suitable for use in
 initializers of objects of static storage duration, then the compound
 literal can be coerced to a pointer to its first element and used in
 such an initializer, as shown here:
@@ -1910,18 +1911,23 @@ such an initializer, as shown here:
 char **foo = (char *[]) @{ "x", "y", "z" @};
 @end smallexample
 
-Compound literals for scalar types and union types are
-also allowed, but then the compound literal is equivalent
-to a cast.
+Compound literals for scalar types and union types are also allowed.  In
+the following example the variable @code{i} is initialized to the value
+@code{2}, the result of incrementing the unnamed object created by
+the compound literal.
+
+@smallexample
+int i = ++(int) @{ 1 @};
+@end smallexample
 
 As a GNU extension, GCC allows initialization of objects with static storage
-duration by compound literals (which is not possible in ISO C99, because
+duration by compound literals (which is not possible in ISO C99 because
 the initializer is not a constant).
-It is handled as if the object is initialized only with the bracket
-enclosed list if the types of the compound literal and the object match.
-The initializer list of the compound literal must be constant.
+It is handled as if the object were initialized only with the brace-enclosed
+list if the types of the compound literal and the object match.
+The elements of the compound literal must be constant.
 If the object being initialized has array type of unknown size, the size is
-determined by compound literal size.
+determined by the size of the compound literal.
 
 @smallexample
 static struct foo x = (struct foo) @{1, 'a', 'b'@};
@@ -1939,22 +1945,21 @@ static int z[] = @{1, 0, 0@};
 
 In C, a compound literal designates an unnamed object with static or
 automatic storage duration.  In C++, a compound literal designates a
-temporary object, which only lives until the end of its
-full-expression.  As a result, well-defined C code that takes the
-address of a subobject of a compound literal can be undefined in C++,
-so the C++ compiler rejects the conversion of a temporary array to a pointer.
-For instance, if the array compound literal example above appeared
-inside a function, any subsequent use of @samp{foo} in C++ has
-undefined behavior because the lifetime of the array ends after the
-declaration of @samp{foo}.  
-
-As an optimization, the C++ compiler sometimes gives array compound
-literals longer lifetimes: when the array either appears outside a
-function or has const-qualified type.  If @samp{foo} and its
-initializer had elements of @samp{char *const} type rather than
-@samp{char *}, or if @samp{foo} were a global variable, the array
-would have static storage duration.  But it is probably safest just to
-avoid the use of array compound literals in code compiled as C++.
+temporary object that only lives until the end of its full-expression.
+As a result, well-defined C code that takes the address of a subobject
+of a compound literal can be undefined in C++, so G++ rejects
+the conversion of a temporary array to a pointer.  For instance, if
+the array compound literal example above appeared inside a function,
+any subsequent use of @code{foo} in C++ would have undefined behavior
+because the lifetime of the array ends after the declaration of @code{foo}.
+
+As an optimization, G++ sometimes gives array compound literals longer
+lifetimes: when the array either appears outside a function or has
+a @code{const}-qualified type.  If @code{foo} and its initializer had
+elements of type @code{char *const} rather than code{char *}, or if
+@code{foo} were a global variable, the array would have static storage
+duration.  But it is probably safest just to avoid the use of array
+compound literals in C++ code.
 
 @node Designated Inits
 @section Designated Initializers
@@ -2145,11 +2150,12 @@ case 1...5:
 @cindex cast to a union
 @cindex union, casting to a
 
-A cast to union type is similar to other casts, except that the type
-specified is a union type.  You can specify the type either with
-@code{union @var{tag}} or with a typedef name.  A cast to union is actually
-a constructor, not a cast, and hence does not yield an lvalue like
-normal casts.  (@xref{Compound Literals}.)
+A cast to union type looks similar to other casts, except that the type
+specified is a union type.  You can specify the type either with the
+@code{union} keyword or with a @code{typedef} name that refers to
+a union.  A cast to a union actually creates a compound literal and
+yields an lvalue, not an rvalue like true casts do.
+(@xref{Compound Literals}.)
 
 The types that may be cast to the union type are those of the members
 of the union.  Thus, given the following union and variables: