spec: remove notion of specific types

Specific types were introduced to explain rules for operands of
type parameter type. Specific types are really an implementation
mechanism to represent (possibly infinite) type sets in the machine;
they are not needed in the specification.

A specific type is either standing for a single named or unnamed
type, or it is the underlying (unnamed) type of an infinite set of
types. Each rule that applies to a type T of the set of specific
types must also apply to all types T' in the type set for which T
is a representative of. Thus, in the spec we can simply refer to
the type set directly, infinite or not.

Rather then excluding operands with empty type sets in each instance,
leave unspecified what happens when such an operand is used. Instead
give an implementation some leeway with an implementation restriction.

(The implementation restriction also needs to be formulated for types,
such as in conversions, which technically are not "operands". Left for
another CL.)

Minor: Remove the two uses of the word "concrete" to refer to non-
interface types; instead just say "non-interface type" for clarity.

Change-Id: I67ac89a640c995369c9d421a03820a0c0435835a
Reviewed-on: https://go-review.googlesource.com/c/go/+/390694
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification - Go 1.18 Draft",
-	"Subtitle": "Version of March 7, 2022",
+	"Subtitle": "Version of March 9, 2022",
 	"Path": "/ref/spec"
 }-->
 
@@ -761,7 +761,7 @@ <h2 id="Variables">Variables</h2>
 <code>new</code> call or composite literal, or the type of
 an element of a structured variable.
 Variables of interface type also have a distinct <i>dynamic type</i>,
-which is the concrete type of the value assigned to the variable at run time
+which is the (non-interface) type of the value assigned to the variable at run time
 (unless the value is the predeclared identifier <code>nil</code>,
 which has no type).
 The dynamic type may vary during execution but values stored in interface
@@ -1799,70 +1799,6 @@ <h3 id="Core_types">Core types</h3>
 interface{ &lt;-chan int | chan&lt;- int }      // directional channels have different directions
 </pre>
 
-<h3 id="Specific_types">Specific types</h3>
-
-<p><b>
-[The definition of specific types is not quite correct yet.]
-</b></p>
-
-<p>
-An interface specification that contains <a href="#Interface_types">type elements</a>
-defines a (possibly empty) set of <i>specific types</i>.
-Loosely speaking, these are the types <code>T</code> that appear in the
-interface definition in terms of the form <code>T</code>, <code>~T</code>,
-or in unions of such terms.
-</p>
-
-<p>
-More precisely, for a given interface, the set of specific types corresponds to
-the set 𝑅 of representative types of the interface, if 𝑅 is non-empty and finite.
-Otherwise, if 𝑅 is empty or infinite, the interface has <i>no specific types</i>.
-</p>
-
-<p>
-For a given interface, type element or type term, the set 𝑅 of representative types is defined as follows:
-</p>
-
-<ul>
-	<li>For an interface with no type elements, 𝑅 is the (infinite) set of all types.
-	</li>
-
-	<li>For an interface with type elements,
-		𝑅 is the intersection of the representative types of its type elements.
-	</li>
-
-	<li>For a non-interface type term <code>T</code> or a term of the form <code>~T</code>,
-		𝑅 is the set consisting of the type <code>T</code>.
-	</li>
-
-	<li>For a <i>union</i> of terms
-		<code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>,
-		𝑅 is the union of the representative types of the terms.
-	</li>
-</ul>
-
-<p>
-An interface may have specific types even if its <a href="#Interface_types">type set</a>
-is empty.
-</p>
-
-<p>
-Examples of interfaces with their specific types:
-</p>
-
-<pre>
-interface{}                    // no specific types
-interface{ int }               // int
-interface{ ~string }           // string
-interface{ int|~string }       // int, string
-interface{ Celsius|Kelvin }    // Celsius, Kelvin
-interface{ float64|any }       // no specific types (union is all types)
-interface{ int; m() }          // int (but type set is empty because int has no method m)
-interface{ ~int; m() }         // int (but type set is infinite because many integer types have a method m)
-interface{ int; any }          // int
-interface{ int; string }       // no specific types (intersection is empty)
-</pre>
-
 <h3 id="Type_identity">Type identity</h3>
 
 <p>
@@ -2002,25 +1938,24 @@ <h3 id="Assignability">Assignability</h3>
 </ul>
 
 <p>
-Additionally, if <code>x</code>'s type <code>V</code> or <code>T</code> are type parameters
-with <a href="#Specific_types">specific types</a>, <code>x</code>
+Additionally, if <code>x</code>'s type <code>V</code> or <code>T</code> are type parameters, <code>x</code>
 is assignable to a variable of type <code>T</code> if one of the following conditions applies:
 </p>
 
 <ul>
 <li>
 <code>x</code> is the predeclared identifier <code>nil</code>, <code>T</code> is
-a type parameter, and <code>x</code> is assignable to each specific type of
-<code>T</code>.
+a type parameter, and <code>x</code> is assignable to each type in
+<code>T</code>'s type set.
 </li>
 <li>
 <code>V</code> is not a <a href="#Types">named type</a>, <code>T</code> is
-a type parameter, and <code>x</code> is assignable to each specific type of
-<code>T</code>.
+a type parameter, and <code>x</code> is assignable to each type in
+<code>T</code>'s type set.
 </li>
 <li>
 <code>V</code> is a type parameter and <code>T</code> is not a named type,
-and values of each specific type of <code>V</code> are assignable
+and values of each type in <code>V</code>'s type set are assignable
 to <code>T</code>.
 </li>
 </ul>
@@ -2055,9 +1990,9 @@ <h3 id="Representability">Representability</h3>
 </ul>
 
 <p>
-If <code>T</code> is a type parameter with <a href="#Specific_types">specific types</a>,
+If <code>T</code> is a type parameter,
 <code>x</code> is representable by a value of type <code>T</code> if <code>x</code> is representable
-by a value of each specific type of <code>T</code>.
+by a value of each type in <code>T</code>'s type set.
 </p>
 
 <pre>
@@ -2705,7 +2640,7 @@ <h4 id="Type_constraints">Type constraints</h4>
 <p>
 The <a href="#Predeclared_identifiers">predeclared</a>
 <a href="#Interface_types">interface type</a> <code>comparable</code>
-denotes the set of all concrete (non-interface) types that are
+denotes the set of all non-interface types that are
 <a href="#Comparison_operators">comparable</a>. Specifically,
 a type <code>T</code> implements <code>comparable</code> if:
 </p>
@@ -3037,6 +2972,14 @@ <h3 id="Operands">Operands</h3>
 operand only on the left-hand side of an <a href="#Assignments">assignment</a>.
 </p>
 
+<p>
+Implementation restriction: A compiler need not report an error if an operand's
+type is a <a href="#Type_parameter_lists">type parameter</a> with an empty
+<a href="#Interface_types">type set</a>. Functions with such type parameters
+cannot be <a href="#Instantiations">instantiated</a>; any attempt will lead
+to an error at the instantiation site.
+</p>
+
 <h3 id="Qualified_identifiers">Qualified identifiers</h3>
 
 <p>
@@ -3819,20 +3762,19 @@ <h3 id="Index_expressions">Index expressions</h3>
 For <code>a</code> of <a href="#Type_parameter_lists">type parameter type</a> <code>P</code>:
 </p>
 <ul>
-	<li><code>P</code> must have <a href="#Specific_types">specific types</a>.</li>
 	<li>The index expression <code>a[x]</code> must be valid for values
-	    of all specific types of <code>P</code>.</li>
-	<li>The element types of all specific types of <code>P</code> must be identical.
+	    of all types in <code>P</code>'s type set.</li>
+	<li>The element types of all types in <code>P</code>'s type set must be identical.
 	    In this context, the element type of a string type is <code>byte</code>.</li>
-	<li>If there is a map type among the specific types of <code>P</code>,
-	    all specific types must be map types, and the respective key types
+	<li>If there is a map type in the type set of <code>P</code>,
+	    all types in that type set must be map types, and the respective key types
 	    must be all identical.</li>
 	<li><code>a[x]</code> is the array, slice, or string element at index <code>x</code>,
 	    or the map element with key <code>x</code> of the type argument
 	    that <code>P</code> is instantiated with, and the type of <code>a[x]</code> is
 	    the type of the (identical) element types.</li>
-	<li><code>a[x]</code> may not be assigned to if the specific types of <code>P</code>
-	    include string types.
+	<li><code>a[x]</code> may not be assigned to if <code>P</code>'s type set
+	    includes string types.
 </ul>
 
 <p>
@@ -4728,6 +4670,10 @@ <h3 id="Operators">Operators</h3>
 to the type of the other operand.
 </p>
 
+<p><b>
+[The rules for shifts need adjustments for type parameters. Issue #51182.]
+</b></p>
+
 <p>
 The right operand in a shift expression must have <a href="#Numeric_types">integer type</a>
 or be an untyped constant <a href="#Representability">representable</a> by a
@@ -4832,9 +4778,8 @@ <h3 id="Arithmetic_operators">Arithmetic operators</h3>
 </pre>
 
 <p>
-Excluding shifts, if the operand type is a <a href="#Type_parameter_lists">type parameter</a>,
-it must have <a href="#Specific_types">specific types</a>, and the operator must
-apply to each specific type.
+If the operand type is a <a href="#Type_parameter_lists">type parameter</a>,
+the operator must apply to each type in that type set.
 The operands are represented as values of the type argument that the type parameter
 is <a href="#Instantiations">instantiated</a> with, and the operation is computed
 with the precision of that type argument. For example, given the function:
@@ -4857,11 +4802,6 @@ <h3 id="Arithmetic_operators">Arithmetic operators</h3>
 respectively, depending on the type argument for <code>F</code>.
 </p>
 
-<p>
-For shifts, the <a href="#Core_types">core type</a> of both operands must be
-an integer.
-</p>
-
 <h4 id="Integer_operators">Integer operators</h4>
 
 <p>
@@ -5374,23 +5314,23 @@ <h3 id="Conversions">Conversions</h3>
 
 <p>
 Additionally, if <code>T</code> or <code>x</code>'s type <code>V</code> are type
-parameters with <a href="#Specific_types">specific types</a>, <code>x</code>
+parameters, <code>x</code>
 can also be converted to type <code>T</code> if one of the following conditions applies:
 </p>
 
 <ul>
 <li>
 Both <code>V</code> and <code>T</code> are type parameters and a value of each
-specific type of <code>V</code> can be converted to each specific type
-of <code>T</code>.
+type in <code>V</code>'s type set can be converted to each type in <code>T</code>'s
+type set.
 </li>
 <li>
 Only <code>V</code> is a type parameter and a value of each
-specific type of <code>V</code> can be converted to <code>T</code>.
+type in <code>V</code>'s type set can be converted to <code>T</code>.
 </li>
 <li>
 Only <code>T</code> is a type parameter and <code>x</code> can be converted to each
-specific type of <code>T</code>.
+type in <code>T</code>'s type set.
 </li>
 </ul>
 
@@ -7085,9 +7025,8 @@ <h3 id="Length_and_capacity">Length and capacity</h3>
 
 <p>
 If the argument type is a <a href="#Type_parameter_lists">type parameter</a> <code>P</code>,
-<code>P</code> must have <a href="#Specific_types">specific types</a>, and
 the call <code>len(e)</code> (or <code>cap(e)</code> respectively) must be valid for
-each specific type of <code>P</code>.
+each type in <code>P</code>'s type set.
 The result is the length (or capacity, respectively) of the argument whose type
 corresponds to the type argument with which <code>P</code> was
 <a href="#Instantiations">instantiated</a>.
@@ -7309,8 +7248,7 @@ <h3 id="Deletion_of_map_elements">Deletion of map elements</h3>
 
 <p>
 If the type of <code>m</code> is a <a href="#Type_parameter_lists">type parameter</a>,
-it must have <a href="#Specific_types">specific types</a>, all specific types
-must be maps, and they must all have identical key types.
+all types in that type set must be maps, and they must all have identical key types.
 </p>
 
 <p>