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spec: more adjustments/corrections
- Change section title from "Type parameters lists" to "Type parameter declarations" as the enclosing section is about declarations. - Correct section on parsing ambiguity in type parameter lists. - Rephrase paragraphs on type parameters for method receivers and adjust examples. - Remove duplicate prose in section on function argument type inference. - Clarified "after substitution" column in Instantiations section. Change-Id: Id76be9804ad96a3f1221e5c4942552dde015dfcb Reviewed-on: https://go-review.googlesource.com/c/go/+/390994 Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>
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@ -807,7 +807,7 @@ TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType
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<p>
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The language <a href="#Predeclared_identifiers">predeclares</a> certain type names.
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Others are introduced with <a href="#Type_declarations">type declarations</a>
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or <a href="#Type_parameter_lists">type parameter lists</a>.
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or <a href="#Type_parameter_declarations">type parameter lists</a>.
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<i>Composite types</i>—array, struct, pointer, function,
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interface, slice, map, and channel types—may be constructed using
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type literals.
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@ -1459,7 +1459,7 @@ type Float interface {
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</pre>
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<p>
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In a union, a term cannot be a <a href="#Type_parameter_lists">type parameter</a>, and the type sets of all
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In a union, a term cannot be a <a href="#Type_parameter_declarations">type parameter</a>, and the type sets of all
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non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty).
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Given a type parameter <code>P</code>:
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</p>
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@ -1769,7 +1769,7 @@ depending on the direction of the directional channels present.
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<p>
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By definition, a core type is never a <a href="#Type_definitions">defined type</a>,
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<a href="#Type_parameter_lists">type parameter</a>, or
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<a href="#Type_parameter_declarations">type parameter</a>, or
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<a href="#Interface_types">interface type</a>.
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</p>
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@ -1965,7 +1965,7 @@ to <code>T</code>.
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<p>
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A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i>
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by a value of type <code>T</code>,
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where <code>T</code> is not a <a href="#Type_parameter_lists">type parameter</a>,
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where <code>T</code> is not a <a href="#Type_parameter_declarations">type parameter</a>,
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if one of the following conditions applies:
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</p>
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@ -2105,6 +2105,7 @@ Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>.
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A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a
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<a href="#Constant_declarations">constant</a>,
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<a href="#Type_declarations">type</a>,
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<a href="#Type_parameter_declarations">type parameter</a>,
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<a href="#Variable_declarations">variable</a>,
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<a href="#Function_declarations">function</a>,
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<a href="#Labeled_statements">label</a>, or
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@ -2502,7 +2503,7 @@ func (tz TimeZone) String() string {
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</pre>
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<p>
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If the type definition specifies <a href="#Type_parameter_lists">type parameters</a>,
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If the type definition specifies <a href="#Type_parameter_declarations">type parameters</a>,
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the type name denotes a <i>generic type</i>.
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Generic types must be <a href="#Instantiations">instantiated</a> when they
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are used.
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@ -2538,7 +2539,7 @@ present in the generic type definition.
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func (l *List[T]) Len() int { … }
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</pre>
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<h3 id="Type_parameter_lists">Type parameter lists</h3>
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<h3 id="Type_parameter_declarations">Type parameter declarations</h3>
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<p>
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A type parameter list declares the <i>type parameters</i> of a generic function or type declaration.
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@ -2577,22 +2578,22 @@ has a corresponding (meta-)type which is called its
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<p>
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A parsing ambiguity arises when the type parameter list for a generic type
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declares a single type parameter with a type constraint of the form <code>*C</code>
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or <code>(C)</code> where <code>C</code> is not a (possibly parenthesized)
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<a href="#Types">type literal</a>:
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declares a single type parameter <code>P</code> with a constraint <code>C</code>
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such that the text <code>P C</code> forms a valid expression:
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</p>
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<pre>
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type T[P *C] …
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type T[P (C)] …
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type T[P *C|Q] …
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…
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</pre>
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<p>
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In these rare cases, the type parameter declaration is indistinguishable from
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the expressions <code>P*C</code> or <code>P(C)</code> and the type declaration
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is parsed as an array type declaration.
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To resolve the ambiguity, embed the constraint in an interface or use a trailing
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comma:
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In these rare cases, the type parameter list is indistinguishable from an
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expression and the type declaration is parsed as an array type declaration.
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To resolve the ambiguity, embed the constraint in an
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<a href="#Interface_types">interface</a> or use a trailing comma:
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</p>
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<pre>
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@ -2606,6 +2607,11 @@ of a <a href="#Method_declarations">method declaration</a> associated
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with a generic type.
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</p>
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<!--
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This section needs to explain if and what kind of cycles are permitted
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using type parameters in a type parameter list.
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-->
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<h4 id="Type_constraints">Type constraints</h4>
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<p>
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@ -2625,10 +2631,10 @@ the enclosing <code>interface{ … }</code> may be omitted for convenience:
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</p>
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<pre>
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[T *P] // = [T interface{*P}]
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[T ~int] // = [T interface{~int}]
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[T int|string] // = [T interface{int|string}]
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type Constraint ~int // illegal: ~int is not inside a type parameter list
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[T []P] // = [T interface{[]P}]
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[T ~int] // = [T interface{~int}]
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[T int|string] // = [T interface{int|string}]
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type Constraint ~int // illegal: ~int is not inside a type parameter list
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</pre>
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<!--
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@ -2821,7 +2827,7 @@ func IndexRune(s string, r rune) int {
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</pre>
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<p>
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If the function declaration specifies <a href="#Type_parameter_lists">type parameters</a>,
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If the function declaration specifies <a href="#Type_parameter_declarations">type parameters</a>,
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the function name denotes a <i>generic function</i>.
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A generic function must be <a href="#Instantiations">instantiated</a> before it can be
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called or used as a value.
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@ -2911,13 +2917,10 @@ to the base type <code>Point</code>.
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If the receiver base type is a <a href="#Type_declarations">generic type</a>, the
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receiver specification must declare corresponding type parameters for the method
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to use. This makes the receiver type parameters available to the method.
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</p>
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<p>
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Syntactically, this type parameter declaration looks like an
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<a href="#Instantiations">instantiation</a> of the receiver base type, except that
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the type arguments are the type parameters being declared, one for each type parameter
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of the receiver base type.
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<a href="#Instantiations">instantiation</a> of the receiver base type: the type
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arguments must be identifiers denoting the type parameters being declared, one
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for each type parameter of the receiver base type.
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The type parameter names do not need to match their corresponding parameter names in the
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receiver base type definition, and all non-blank parameter names must be unique in the
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receiver parameter section and the method signature.
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@ -2931,8 +2934,8 @@ type Pair[A, B any] struct {
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b B
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}
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func (p Pair[A, B]) Swap() Pair[B, A] { return Pair[B, A]{p.b, p.a} }
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func (p Pair[First, _]) First() First { return p.a }
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func (p Pair[A, B]) Swap() Pair[B, A] { … } // receiver declares A, B
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func (p Pair[First, _]) First() First { … } // receiver declares First, corresponds to A in Pair
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</pre>
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<h2 id="Expressions">Expressions</h2>
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@ -2974,7 +2977,7 @@ operand only on the left-hand side of an <a href="#Assignments">assignment</a>.
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<p>
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Implementation restriction: A compiler need not report an error if an operand's
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type is a <a href="#Type_parameter_lists">type parameter</a> with an empty
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type is a <a href="#Type_parameter_declarations">type parameter</a> with an empty
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<a href="#Interface_types">type set</a>. Functions with such type parameters
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cannot be <a href="#Instantiations">instantiated</a>; any attempt will lead
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to an error at the instantiation site.
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@ -3759,7 +3762,7 @@ For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>:
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</ul>
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<p>
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For <code>a</code> of <a href="#Type_parameter_lists">type parameter type</a> <code>P</code>:
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For <code>a</code> of <a href="#Type_parameter_declarations">type parameter type</a> <code>P</code>:
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</p>
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<ul>
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<li>The index expression <code>a[x]</code> must be valid for values
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<p>
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For an expression <code>x</code> of <a href="#Interface_types">interface type</a>,
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but not a <a href="#Type_parameter_lists">type parameter</a>, and a type <code>T</code>,
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but not a <a href="#Type_parameter_declarations">type parameter</a>, and a type <code>T</code>,
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the primary expression
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</p>
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@ -4180,7 +4183,7 @@ including the type parameter list itself and any types in that list.
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<li>
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After substitution, each type argument must <a href="#Interface_types">implement</a>
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the <a href="#Type_parameter_lists">constraint</a> (instantiated, if necessary)
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the <a href="#Type_parameter_declarations">constraint</a> (instantiated, if necessary)
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of the corresponding type parameter. Otherwise instantiation fails.
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</li>
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</ol>
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<pre>
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type parameter list type arguments after substitution
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[P any] int [int any]
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[S ~[]E, E any] []int, int [[]int ~[]int, int any]
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[P io.Writer] string [string io.Writer] // illegal: string doesn't implement io.Writer
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[P any] int int implements any
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[S ~[]E, E any] []int, int []int implements ~[]int, int implements any
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[P io.Writer] string illegal: string doesn't implement io.Writer
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</pre>
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<p>
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<ul>
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<li>
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a <a href="#Type_parameter_lists">type parameter list</a>
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a <a href="#Type_parameter_declarations">type parameter list</a>
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</li>
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<li>
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a substitution map <i>M</i> initialized with the known type arguments, if any
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</p>
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<p>
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Function argument type inference can be used when the function has ordinary parameters
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whose types are defined using the function's type parameters. Inference happens in two
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separate phases; each phase operates on a specific list of (parameter, argument) pairs:
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Inference happens in two separate phases; each phase operates on a specific list of
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(parameter, argument) pairs:
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</p>
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<ol>
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</pre>
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<p>
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If the operand type is a <a href="#Type_parameter_lists">type parameter</a>,
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If the operand type is a <a href="#Type_parameter_declarations">type parameter</a>,
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the operator must apply to each type in that type set.
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The operands are represented as values of the type argument that the type parameter
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is <a href="#Instantiations">instantiated</a> with, and the operation is computed
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</p>
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<p>
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Converting a constant to a type that is not a <a href="#Type_parameter_lists">type parameter</a>
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Converting a constant to a type that is not a <a href="#Type_parameter_declarations">type parameter</a>
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yields a typed constant.
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</p>
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<li>
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ignoring struct tags (see below),
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<code>x</code>'s type and <code>T</code> are not
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<a href="#Type_parameter_lists">type parameters</a> but have
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<a href="#Type_parameter_declarations">type parameters</a> but have
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<a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>.
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</li>
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<li>
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Cases then match actual types <code>T</code> against the dynamic type of the
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expression <code>x</code>. As with type assertions, <code>x</code> must be of
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<a href="#Interface_types">interface type</a>, but not a
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<a href="#Type_parameter_lists">type parameter</a>, and each non-interface type
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<a href="#Type_parameter_declarations">type parameter</a>, and each non-interface type
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<code>T</code> listed in a case must implement the type of <code>x</code>.
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The types listed in the cases of a type switch must all be
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<a href="#Type_identity">different</a>.
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</pre>
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<p>
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A <a href="#Type_parameter_lists">type parameter</a> or a <a href="#Type_declarations">generic type</a>
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A <a href="#Type_parameter_declarations">type parameter</a> or a <a href="#Type_declarations">generic type</a>
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may be used as a type in a case. If upon <a href="#Instantiations">instantiation</a> that type turns
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out to duplicate another entry in the switch, the first matching case is chosen.
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</p>
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</pre>
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<p>
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If the argument type is a <a href="#Type_parameter_lists">type parameter</a> <code>P</code>,
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If the argument type is a <a href="#Type_parameter_declarations">type parameter</a> <code>P</code>,
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the call <code>len(e)</code> (or <code>cap(e)</code> respectively) must be valid for
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each type in <code>P</code>'s type set.
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The result is the length (or capacity, respectively) of the argument whose type
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</pre>
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<p>
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If the type of <code>m</code> is a <a href="#Type_parameter_lists">type parameter</a>,
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If the type of <code>m</code> is a <a href="#Type_parameter_declarations">type parameter</a>,
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all types in that type set must be maps, and they must all have identical key types.
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</p>
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