spec: introduce notion of basic interface, misc. fine-tuning

A basic interface is a classical Go interface containing only
methods or embedding basic interfaces.

Use this to simplify rule about what interfaces may be used
where. The term "basic interface" will also be useful when
talking about various interfaces in general.

Fix rule restricting union terms: as it was written it also
excluded interface terms with non-empty method sets due to
embedded non-interface types with methods.

Split the large section on interfaces into three smaller
pieces by introducing section titles.

Change-Id: I142a4d5609eb48aaa0f7800b5b85c1d6c0703fcb
Reviewed-on: https://go-review.googlesource.com/c/go/+/384994
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: griesemer

doc/go_spec.html

@@ -1226,11 +1226,15 @@ <h3 id="Interface_types">Interface types</h3>
 A type term is either a single type or a single underlying type.
 </p>
 
+<h4 id="Basic_interfaces">Basic interfaces</h4>
+
 <p>
 In its most basic form an interface specifies a (possibly empty) list of methods.
 The type set defined by such an interface is the set of types which implement all of
 those methods, and the corresponding <a href="#Method_sets">method set</a> consists
 exactly of the methods specified by the interface.
+Interfaces whose type sets can be defined entirely by a list of methods are called
+<i>basic interfaces.</i>
 </p>
 
 <pre>
@@ -1315,6 +1319,8 @@ <h3 id="Interface_types">Interface types</h3>
 as the <code>File</code> interface.
 </p>
 
+<h4 id="Embedded_interfaces">Embedded interfaces</h4>
+
 <p>
 In a slightly more general form
 an interface <code>T</code> may use a (possibly qualified) interface type
@@ -1359,8 +1365,10 @@ <h3 id="Interface_types">Interface types</h3>
 }
 </pre>
 
+<h4 id="Generenal_interfaces">General interfaces</h4>
+
 <p>
-Finally, in their most general form, an interface element may also be an arbitrary type term
+In their most general form, an interface element may also be an arbitrary type term
 <code>T</code>, or a term of the form <code>~T</code> specifying the underlying type <code>T</code>,
 or a union of terms <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>.
 Together with method specifications, these elements enable the precise
@@ -1462,21 +1470,21 @@ <h3 id="Interface_types">Interface types</h3>
 
 <p>
 Implementation restriction:
-A union with more than one term cannot contain interface types
-with non-empty <a href="#Method_sets">method sets</a> or which
-are or embed the <a href="#Predeclared_identifiers">predeclared identifier</a>
-<code>comparable</code>.
+A union with more than one term cannot contain the
+<a href="#Predeclared_identifiers">predeclared identifier</a> <code>comparable</code>
+or interfaces that specify methods, or embed <code>comparable</code> or interfaces
+that specify methods.
 </p>
 
 <p>
-Interfaces that contain non-interface types, terms of the form <code>~T</code>,
-or unions may only be used as type constraints, or as elements of other interfaces used
-as constraints. They cannot be the types of values or variables, or components of other,
+Interfaces that are not <a href="#Basic_interfaces">basic</a> may only be used as type
+constraints, or as elements of other interfaces used as constraints.
+They cannot be the types of values or variables, or components of other,
 non-interface types.
 </p>
 
 <pre>
-var x Floats                     // illegal: Floats is restricted by float32 and float64
+var x Floats                     // illegal: Floats is not a basic interface
 
 var x interface{} = Floats(nil)  // illegal
 
@@ -1714,7 +1722,7 @@ <h3 id="Underlying_types">Underlying types</h3>
 <h3 id="Core_types">Core types</h3>
 
 <p>
-Each non-interface type <code>T</code> has a <i>core</i> type, which is the
+Each non-interface type <code>T</code> has a <i>core type</i>, which is the same as the
 <a href="#Underlying_types">underlying type</a> of <code>T</code>.
 </p>
 
@@ -2665,9 +2673,9 @@ <h4 id="Type_constraints">Type constraints</h4>
 </pre>
 
 <p>
-If the constraint is an interface literal containing exactly one embedded type element
-<code>interface{E}</code>, in a type parameter list the enclosing <code>interface{ … }</code>
-may be omitted for convenience:
+If the constraint is an interface literal of the form <code>interface{E}</code> where
+<code>E</code> is an embedded type element (not a method), in a type parameter list
+the enclosing <code>interface{ … }</code> may be omitted for convenience:
 </p>
 
 <pre>