Document index:element initializers

Author: ntrel

spec/expression.dd

@@ -1702,7 +1702,7 @@ $(GNAME ArrayMemberInitialization):
     $(GLINK AssignExpression) $(D :) $(GLINK2 declaration, NonVoidInitializer)
 )
 
-    $(P Array literals are a comma-separated list of expressions
+    $(P An array literal is a comma-separated list of expressions
         between square brackets $(D [) and $(D ]).
         The expressions form the elements of a dynamic array.
         The length of the array is the number of elements.
@@ -1764,6 +1764,27 @@ $(GNAME ArrayMemberInitialization):
         }
         ---
 
+    $(P To initialize an element at a particular index, use the
+        *AssignExpression* `:` *NonVoidInitializer* syntax.
+        The *AssignExpression* must be known at compile-time.
+        Any missing elements will be initialized to the default value
+        of the element type.
+        Note that if the array type is not specified, the literal will
+        be parsed as an
+        $(RELATIVE_LINK2 associative_array_literals, associative array).)
+
+        $(SPEC_RUNNABLE_EXAMPLE_COMPILE
+        ---
+        int n = 4;
+        auto aa = [0:1, 3:n]; // associative array `int[int]`
+
+        int[] a = [1, 3:n, 5];
+        assert(a == [1, 0, 0, n, 5]);
+
+        //int[] e = [n:2]; // error, n not known at compile-time
+        ---
+        )
+
 $(H3 $(LNAME2 cast_array_literal, Casting))
 
     $(P When array literals are cast to another array type, each