go spec: Base comparison compatibility on assignment compatibility.

Specifically:
- Simplified definition of comparison compatibility and folded into
  section on comparison operators since it's only used there.

This is a small language change/cleanup. As a consequence:
- An interface value may now be compared against a non-interface value.
- Channels with opposite directions cannot be compared directly anymore
  (per discussion with rsc).

R=rsc, r, iant, ken2
CC=golang-dev
https://golang.org/cl/1462041

doc/go_spec.html

@@ -1,9 +1,8 @@
 <!-- title The Go Programming Language Specification -->
-<!-- subtitle Version of May 26, 2010 -->
+<!-- subtitle Version of June 1, 2010 -->
 
 <!--
-Todo
+TODO
-[ ] clarify: two equal lower-case identifiers from different packages denote different objects
 [ ] need language about function/method calls and parameter passing rules
 [ ] last paragraph of #Assignments (constant promotion) should be elsewhere
     and mention assignment to empty interface.
@@ -17,6 +16,7 @@ Todo
 	though obvious
 [ ] specify iteration direction for range clause
 [ ] review language on implicit dereferencing
+[ ] clarify what it means for two functions to be "the same" when comparing them
 -->
 
 
@@ -1121,9 +1121,9 @@ KeyType     = Type .
 
 <p>
 The comparison operators <code>==</code> and <code>!=</code>
-(§<a href="#Comparison_operators">Comparison operators</a>) must be fully defined for operands of the
+(§<a href="#Comparison_operators">Comparison operators</a>) must be fully defined
-key type; thus the key type must be a boolean, numeric, string, pointer, function, interface,
+for operands of the key type; thus the key type must not be a struct, array or slice.
-map, or channel type. If the key type is an interface type, these
+If the key type is an interface type, these
 comparison operators must be defined for the dynamic key values;
 failure will cause a <a href="#Run_time_panics">run-time panic</a>.
 
@@ -1374,58 +1374,6 @@ represents the <a href="#The_zero_value">zero value</a> for that type.
 Any value may be assigned to the <a href="#Blank_identifier">blank identifier</a>.
 </p>
 
-<h3 id="Comparison_compatibility">Comparison compatibility</h3>
-
-<p>
-Except as noted, values of any type may be compared to other values of
-<a href="#Type_compatibility">compatible static type</a>.
-Values of integer, floating-point, and string type may be compared using the
-full range of <a href="#Comparison_operators;">comparison operators</a>;
-booleans and complex values may be compared only for equality or inequality.
-</p>
-
-<p>
-Values of composite type may be
-compared for equality or inequality using the <code>==</code> and
-<code>!=</code> operators, with the following provisos:
-</p>
-<ul>
-<li>
-Arrays and structs may not be compared to anything.
-</li>
-<li>
-A slice value may only be compared explicitly against <code>nil</code>.
-A slice value is equal to <code>nil</code> if it has been assigned the explicit
-value <code>nil</code>, if it is uninitialized, or if it has
-been assigned another slice value equal to <code>nil</code>·
-</li>
-<li>
-An interface value is equal to <code>nil</code> if it has
-been assigned the explicit value <code>nil</code>, if it is uninitialized,
-or if it has been assigned another interface value equal to <code>nil</code>.
-</li>
-<li>
-For types that can be compared to <code>nil</code>,
-two values of the same type are equal if they both equal <code>nil</code>,
-unequal if one equals <code>nil</code> and one does not.
-</li>
-<li>
-Pointer values are equal if they point to the same location.
-</li>
-<li>
-Function values are equal if they refer to the same function.
-</li>
-<li>
-Channel and map values are equal if they were created by the same call to <code>make</code>
-(§<a href="#Making_slices">Making slices</a>, maps, and channels).
-When comparing two values of channel type, the channel value types
-must be identical but the channel direction is ignored.
-</li>
-<li>
-Interface values may be compared if they have compatible static types.
-They will be equal only if they have the same dynamic type and the underlying values are equal.
-</li>
-</ul>
 
 <h2 id="Blocks">Blocks</h2>
 
@@ -2960,11 +2908,7 @@ not occur. For instance, it may not assume that <code>x &lt; x + 1</code> is alw
 <h3 id="Comparison_operators">Comparison operators</h3>
 
 <p>
-Comparison operators yield a value of type <code>bool</code>.
+Comparison operators compare two operands and yield a value of type <code>bool</code>.
-The operators <code>==</code> and <code>!=</code> apply
-to operands of all types except arrays and structs.
-All other comparison operators apply only to integer, floating-point
-and string values.
 </p>
 
 <pre class="grammar">
@@ -2977,20 +2921,71 @@ and string values.
 </pre>
 
 <p>
-Operands of numeric type are compared in the usual way.
+The operands must be <i>comparable</i>; that is, the first operand
-</p>
+must be <a href="#Assignment_compatibility">assignment compatible</a>
+with the type of the second operand, or vice versa.
 <p>
-Operands of string type are compared byte-wise (lexically).
 </p>
-<p>
+The operators <code>==</code> and <code>!=</code> apply
-Operands of boolean type are equal if they are either both <code>true</code>
+to operands of all types except arrays and structs.
-or both <code>false</code>.
+All other comparison operators apply only to integer, floating-point
-</p>
+and string values. The result of a comparison is defined as follows:
-<p>
-The rules for comparison of composite types are described in the
-section on §<a href="#Comparison_compatibility">Comparison compatibility</a>.
 </p>
 
+<ul>
+	<li>
+	Integer values are compared in the usual way.
+	</li>
+	<li>
+	Floating point values are compared as defined by the IEEE-754
+	standard.
+	</li>
+	<li>
+	Two complex values <code>u</code>, <code>v</code> are
+	equal if both <code>real(u) == real(v)</code> and
+	<code>imag(u) == imag(v)</code>.
+	</li>
+	<li>
+	String values are compared byte-wise (lexically).
+	</li>
+	<li>
+	Boolean values are are equal if they are either both
+	<code>true</code> or both <code>false</code>.
+	</li>
+	<li>
+	Pointer values are equal if they point to the same location
+	or if both are <code>nil</code>.
+	</li>
+	<li>
+	Function values are equal if they refer to the same function
+	or if both are <code>nil</code>.
+	</li>
+	<li>
+	A slice value may only be compared to <code>nil</code>.
+	</li>
+	<li>
+	Channel and map values are equal if they were created by the same call to <code>make</code>
+	(§<a href="#Making_slices_maps_and_channels">Making slices, maps, and channels</a>)
+	or if both are <code>nil</code>.
+	</li>
+	<li>
+	Interface values are equal if they have identical dynamic types and
+	equal dynamic values or if both are <code>nil</code>.
+	</li>
+	<li>
+	An interface value <code>x</code> is equal to a non-interface value
+	<code>y</code> if the dynamic type of <code>x</code> is identical to
+	the static type of <code>y</code> and the dynamic value of <code>x</code>
+	is equal to <code>y</code>.
+	</li>
+	<li>
+	A pointer, function, slice, channel, map, or interface value is equal
+	to <code>nil</code> if it has been assigned the explicit value
+	<code>nil</code>, if it is uninitialized, or if it has been assigned
+	another value equal to <code>nil</code>.
+	</li>
+</ul>
+
 
 <h3 id="Logical_operators">Logical operators</h3>