[0004] Adding spec language for unions

proposals/0004-unions.md

@@ -65,14 +65,77 @@ specific clarifications.
 
 * Following C++ Unions are always sized large enough to contain their largest
   member type.
-* Union members cannot have HLSL semantics applied to them.
+* Union members and objects cannot have HLSL semantics applied to them.
+  * As such, unions may not be used as input arguments to shaders.
+* Union objects may not be stored in constant buffers.
 * Union objects in buffer layouts are aligned as elements of their
   most-restrictive member type.
 * Union objects in struct layouts behave as defined in C++.
-* Union objects cannot have semantics applied to them.
 * Union's cannot have user-defined constructors or destructors until the
   language supports them for other user defined data types (See:
-  [0007](0007-constructors.md)).
+  [0032](0032-constructors.md)).
+
+## Detailed Design
+
+This proposal depends on the adoption of [Strict Initializer
+Lists](0005-strict-initializer-lists.md) because initialization of a union type
+is incompatible with HLSL's object initialization rules.
+
+> Note: With current rules which member of the union should be initialized is
+> ambiguous in all cases and unions with differing numbers of scalar members
+> would result in the whole initializer list being ambiguous.
+
+HLSL unions will adopt the behavior defined in the [ISO C++ specification
+(ISO/IEC 14882:2011)](https://timsong-cpp.github.io/cppwp/n3337/class.union),
+except with the following additions and clarifications.
+
+### Proposed Spec Language
+
+A union is a class defined with the `union` keyword.
+
+Non-static members of a union share overlapping storage. As such, a union may
+only store the value of one non-static data member at a time. The lifetime of a
+non-static data member begins when the member is initialized, and ends either
+when the value is destroyed via an explicit destructor call, a different
+non-static data member is initialized, or the union's lifetime ends.
+
+A union may not contain any non-static data members of intangible type.
+
+If a union contains more than one standard-layout structures which share a
+common initial sequence of non-static members, the common initial sequence may
+be accessed through any non-static union member containing the sequence.
+
+The size of a union is sufficient to contain the largest of its non-static data
+members, and each non-static data member is allocated at the union's base
+address. The required alignment of a union object is the most restrictive
+alignment of its non-static data members.
+
+> Note: Next paragraph written assuming [Constructors](0032-constructors.md).
+
+A union can have constructors, destructors and member functions. A union cannot
+have base classes or be used as a base class. At most one non-static data member
+of a union may have a default initializer specified as a
+_brace-or-equal-initializer_ on the member declaration.
+
+A union without a specified type or object name is an _anonymous union_. It
+defines an unnamed object of unnamed type. Anonymous unions may only contain
+non-static data members. Anonymous unions shall not have private or protected
+members. The names of members of an anonymous union are defined in the scope
+that encloses the union declaration, as such, the names of members of an
+anonymous union must be distinct from all names declared in the enclosing
+entity's scope.
+
+Anonymous unions must be declared with an appropriate storage class for their
+enclosing scope. When declared in namespaces scope an anonymous union must be
+declared `static`, when declared in block scope they shall be declared in any
+allowed storage class for the block or no storage class.
+
+Members of union types may not be annotated with semantic annotations or other
+attributes. No union type or class containing non-static data members of union
+types shall be used as an input to an entry function.
+
+Objects of union type, or of classes which contain non-static union members may
+not be stored in constant buffers.
 
 ## Acknowledgments