Start working on HLSL initialization lists (microsoft#329)

* Start working on HLSL initialization lists

This one is a bit gnarly because it involves a lot of depth-first traversing of declarations, types, and initialization expressions.

Questions and suggestions on how to clarify are greatly appreciated!

Co-authored-by: Greg Roth <grroth@microsoft.com>

Author: llvm-beanz

specs/language/declarations.tex

@@ -46,6 +46,66 @@
 
 \Sec{Declarators}{Decl.Decl}
 \Sec{Initializers}{Decl.Init}
+
+\p The process of initialization described in this section applies to all
+initializers regardless of the context.
+
+\begin{grammar}
+  \define{initializer}\br
+  brace-or-equal-initializer\br
+  \terminal{(} expression-list \terminal{)}\br
+
+  \define{brace-or-equal-initializer}\br
+  \terminal{=} initializer-clause\br
+  braced-init-list\br
+
+  \define{initializer-clause}\br
+  assignment-expression\br
+  braced-init-list\br
+
+  \define{braced-init-list}\br
+  \terminal{\{} initializer-list \opt{\terminal{,}} \terminal{\}}\br
+  \terminal{\{} \terminal{\}}\br
+
+  \define{initializer-list}\br
+  initializer-clause\br
+  initializer-list \terminal{,} initializer-clause\br
+\end{grammar}
+
+\Sub{Aggregate Initialization}{Decl.Init.Agg}
+
+\p An \textit{aggregate} is a vector, matrix, array, or class.
+
+\p The subobjects of an aggregate have a defined order. For vectors and arrays
+the order is increasing subscript order. For matrices it is increasing subscript
+order with the subscript nesting such that in the notation
+\texttt{Mat[M][N]}, the ordering is \(Mat[0][0]...Mat[0][N]...
+Mat[M][0]...Mat[M][N]\). For classes the order is base class, followed by member
+subobjects in declaration order.
+
+\p A \textit{flattened ordering} of subobjects can be produced by performing a
+depth-first traversal of the subobjects of an object following the defined
+subobject ordering.
+
+\p Each \textit{braced initializer list} is comprised of zero or more
+\textit{initializer-clause} expressions, which is either another braced
+initializer list or an expression which generates a value that either is or can
+be implicitly converted to an rvalue. Each assignment-expression is an object,
+which may be a scalar or aggregate type. A \textit{flattened initializer
+sequence} is constructed by a depth-first traversal over each
+assignment-expression in an initializer-list and performing a depth-first
+traversal accessing each subobject of the assignment-expression.
+
+\p An initializer-list is a valid initializer if for each element \(E_n\) in the
+target object's flattened ordering there is a corresponding initializer \(I_n\)
+in the flattened initializer sequence which can be implicitly converted to the
+element's type.
+
+\p An initializer-list is invalid if the flattened initializer sequence contains
+more or fewer elements than the target object's flattened ordering, or if any
+initializer \(I_n\) cannot be implicitly converted to the corresponding element
+\(E_n\)'s type.
+
 \Sec{Function Definitions}{Decl.Function}
 \Sec{Attributes}{Decl.Attr}
 \Sub{Semantic Annotations}{Decl.Attr.Semantic}