go spec: relaxed syntax for array, slice, and map composite literals

For elements which are themselves composite literals, the type may
be omitted if it is identical to the element type of the containing
composite literal.

R=r, rsc, iant, ken2
CC=golang-dev
https://golang.org/cl/2661041

doc/go_spec.html

 <!-- title The Go Programming Language Specification -->
-<!-- subtitle Version of Sep 28, 2010 -->
+<!-- subtitle Version of Oct 21, 2010 -->
 
 <!--
 TODO
@@ -1971,15 +1971,16 @@ a single expression or a key-value pair.
 </p>
 
 <pre class="ebnf">
-CompositeLit  = LiteralType "{" [ ElementList [ "," ] ] "}" .
+CompositeLit  = LiteralType LiteralValue .
 LiteralType   = StructType | ArrayType | "[" "..." "]" ElementType |
                 SliceType | MapType | TypeName .
+LiteralValue  = "{" [ ElementList [ "," ] ] "}" .
 ElementList   = Element { "," Element } .
 Element       = [ Key ":" ] Value .
 Key           = FieldName | ElementIndex .
 FieldName     = identifier .
 ElementIndex  = Expression .
-Value         = Expression .
+Value         = Expression | LiteralValue .
 </pre>
 
 <p>
@@ -2093,6 +2094,17 @@ and is a shortcut for a slice operation applied to an array literal:
 [n]T{x1, x2, ... xn}[0 : n]
 </pre>
 
+<p>
+Within a composite literal of array, slice, or map type <code>T</code>,
+elements that are themselves composite literals may elide the respective
+literal type if it is identical to the element type of <code>T</code>.
+</p>
+
+<pre>
+[...]Point{{1.5, -3.5}, {0, 0}}  // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}}
+[][]int{{1, 2, 3}, {4, 5}}       // same as [][]int{[]int{1, 2, 3}, []int{4, 5}}
+</pre>
+
 <p>
 A parsing ambiguity arises when a composite literal using the
 TypeName form of the LiteralType appears between the