spec: clarify variable declaration type rules

Not a language change.

Several inaccuracies were fixed:

1) A variable declaration may declare more than just one
variable.

2) Variable initialization follows the rules of assignments,
including n:1 assignments. The existing wording implied a 1:1
or n:n rule and generally was somewhat unspecific.

3) The rules for variable declarations with no types and
untyped initialization expressions had minor holes (issue 8088).

4) Clarified the special cases of assignments of untyped values
(we don't just have untyped constants, but also untyped bools,
e.g. from comparisons). The new wording is more direct.

To that end, introduced the notion of an untyped constant's
"default type" so that the same concept doesn't have to be
repeatedly introduced.

Fixes #8088.

LGTM=iant, r, rsc
R=r, rsc, iant, ken
CC=golang-codereviews
https://golang.org/cl/142320043

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of September 29, 2014",
+	"Subtitle": "Version of September 30, 2014",
 	"Path": "/ref/spec"
 }-->
 
@@ -577,7 +577,7 @@ Numeric constants represent values of arbitrary precision and do not overflow.
 </p>
 
 <p>
-Constants may be <a href="#Types">typed</a> or untyped.
+Constants may be <a href="#Types">typed</a> or <i>untyped</i>.
 Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>,
 and certain <a href="#Constant_expressions">constant expressions</a>
 containing only untyped constant operands are untyped.
@@ -597,6 +597,17 @@ can be given the types <code>float32</code>, <code>float64</code>, or <code>uint
 not <code>int32</code> or <code>string</code>.
 </p>
 
+<p>
+An untyped constant has a <i>default type</i> which is the type to which the
+constant is implicitly converted in contexts where a typed value is required,
+for instance, in a <a href="#Short_variable_declarations">short variable declaration</a>
+such as <code>i := 0</code> where there is no explicit type.
+The default type of an untyped constant is <code>bool</code>, <code>rune</code>,
+<code>int</code>, <code>float64</code>, <code>complex128</code> or <code>string</code>
+respectively, depending on whether it is a boolean, rune, integer, floating-point,
+complex, or string constant.
+</p>
+
 <p>
 There are no constants denoting the IEEE-754 infinity and not-a-number values,
 but the <a href="/pkg/math/"><code>math</code> package</a>'s
@@ -1882,9 +1893,10 @@ func (tz TimeZone) String() string {
 <h3 id="Variable_declarations">Variable declarations</h3>
 
 <p>
-A variable declaration creates a variable, binds an identifier to it and
-gives it a type and optionally an initial value.
+A variable declaration creates one or more variables, binds corresponding
+identifiers to them, and gives each a type and an initial value.
 </p>
+
 <pre class="ebnf">
 VarDecl     = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
 VarSpec     = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
@@ -1905,22 +1917,27 @@ var _, found = entries[name]  // map lookup; only interested in "found"
 
 <p>
 If a list of expressions is given, the variables are initialized
-by <a href="#Assignments">assigning</a> the expressions to the variables
-in order; all expressions must be consumed and all variables initialized from them.
+with the expressions following the rules for <a href="#Assignments">assignments</a>.
 Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>.
 </p>
 
 <p>
-If the type is present, each variable is given that type.
-Otherwise, the types are deduced from the assignment
-of the expression list.
+If a type is present, each variable is given that type.
+Otherwise, each variable is given the type of the corresponding
+initialization value in the assignment.
+If that value is an untyped constant, it is first
+<a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>;
+if it is an untyped boolean value, it is first converted to type <code>bool</code>.
+The predeclared value <code>nil</code> cannot be used to initialize a variable
+with no explicit type.
 </p>
 
-<p>
-If the type is absent and the corresponding expression evaluates to an
-untyped <a href="#Constants">constant</a>, the type of the declared variable
-is as described in §<a href="#Assignments">Assignments</a>.
-</p>
+<pre>
+var d = math.Sin(0.5)  // d is int64
+var i = 42             // i is int
+var t, ok = x.(T)      // t is T, ok is bool
+var n = nil            // illegal
+</pre>
 
 <p>
 Implementation restriction: A compiler may make it illegal to declare a variable
@@ -4318,7 +4335,7 @@ a[i] = 23
 
 <p>
 An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
-<code>y</code> where <i>op</i> is a binary arithmetic operation equivalent
+<code>y</code> where <i>op</i> is a binary arithmetic operation is equivalent
 to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
 <code>y</code> but evaluates <code>x</code>
 only once.  The <i>op</i><code>=</code> construct is a single token.
@@ -4336,8 +4353,8 @@ i &amp;^= 1&lt;&lt;n
 A tuple assignment assigns the individual elements of a multi-valued
 operation to a list of variables.  There are two forms.  In the
 first, the right hand operand is a single multi-valued expression
-such as a function evaluation or <a href="#Channel_types">channel</a> or
-<a href="#Map_types">map</a> operation or a <a href="#Type_assertions">type assertion</a>.
+such as a function call, a <a href="#Channel_types">channel</a> or
+<a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>.
 The number of operands on the left
 hand side must match the number of values.  For instance, if
 <code>f</code> is a function returning two values,
@@ -4411,23 +4428,21 @@ to the type of the operand to which it is assigned, with the following special c
 </p>
 
 <ol>
-<li><p>
-	If an untyped <a href="#Constants">constant</a>
+<li>
+	Any typed value may be assigned to the blank identifier.
+</li>
+
+<li>
+	If an untyped constant
 	is assigned to a variable of interface type or the blank identifier,
-	the constant is first <a href="#Conversions">converted</a> to type
-	<code>bool</code>, <code>rune</code>, <code>int</code>, <code>float64</code>,
-	<code>complex128</code> or <code>string</code> respectively, depending on
-	whether the value is a boolean, rune, integer, floating-point, complex, or
-	string constant.
-</p></li>
-
-<li><p>
-	<!-- Note that the result of a comparison is an untyped bool that may not be constant. -->
-	If a left-hand side is the blank identifier, any typed or non-constant
-	value except for the predeclared identifier
-	<a href="#Predeclared_identifiers"><code>nil</code></a>
-	may be assigned to it.
-</p></li>
+	the constant is first <a href="#Conversions">converted</a> to its
+	 <a href="#Constants">default type</a>.
+</li>
+
+<li>
+	If an untyped boolean value is assigned to a variable of interface type or
+	the blank identifier, it is first converted to type <code>bool</code>.
+</li>
 </ol>
 
 <h3 id="If_statements">If statements</h3>