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spec: add type parameter types
- add section on type parameters - added two sections on the scope of type parameters - expanded general section on types accordingly - introduced the notion of a named type which will help in simplifying various rules (subsequent CLs) Change-Id: I49c1ed7d6d4f951d751f0a3ca5dfb637e49829f2 Reviewed-on: https://go-review.googlesource.com/c/go/+/365414 Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>
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@ -787,32 +787,46 @@ If a variable has not yet been assigned a value, its value is the
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<p>
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A type determines a set of values together with operations and methods specific
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to those values. A type may be denoted by a <i>type name</i>, if it has one,
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or specified using a <i>type literal</i>, which composes a type from existing types.
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to those values. A type may be denoted by a <i>type name</i>, if it has one, which must be
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followed by <a href="#Instantiations">type arguments</a> if the type is parameterized.
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A type may also be specified using a <i>type literal</i>, which composes a type
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from existing types.
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</p>
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<pre class="ebnf">
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Type = TypeName | TypeLit | "(" Type ")" .
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Type = TypeName [ TypeArgs ] | TypeLit | "(" Type ")" .
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TypeName = identifier | QualifiedIdent .
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TypeArgs = "[" TypeList [ "," ] "]" .
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TypeList = Type { "," Type } .
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TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
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SliceType | MapType | ChannelType .
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</pre>
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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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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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<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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</p>
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<p>
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Predeclared types, defined types, and type parameters are called <i>named types</i>.
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An alias denotes a named type if the type given in the alias declaration is a named type.
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</p>
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<h3 id="Underlying_types">Underlying types</h3>
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<p>
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Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code>
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is one of the predeclared boolean, numeric, or string types, or a type literal,
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the corresponding underlying
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type is <code>T</code> itself. Otherwise, <code>T</code>'s underlying type
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is the underlying type of the type to which <code>T</code> refers in its
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<a href="#Type_declarations">type declaration</a>.
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the corresponding underlying type is <code>T</code> itself.
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Otherwise, <code>T</code>'s underlying type is the underlying type of the
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type to which <code>T</code> refers in its <a href="#Type_declarations">type
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declaration</a>. Accordingly, the underlying type of a type parameter is the
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underlying type of its <a href="#Type_constraints">type constraint</a>, which
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is always an interface.
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</p>
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<pre>
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@ -827,12 +841,15 @@ type (
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B3 []B1
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B4 B3
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)
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func f[P any](x P) { … }
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</pre>
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<p>
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The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>,
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and <code>B2</code> is <code>string</code>.
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The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>.
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The underlying type of <code>P</code> is <code>interface{}</code>.
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</p>
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<h3 id="Method_sets">Method sets</h3>
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@ -1706,6 +1723,25 @@ and a second goroutine receives them, the values are
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received in the order sent.
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</p>
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<h3 id="Type_parameters">Type parameters</h3>
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<p>
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A <i>type parameter</i> is an (unqualified) type name declared in the
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<a href="#Type_parameter_lists">type parameter list</a> of a
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<a href="#Function_declarations">function declaration</a> or
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<a href="#Type_definitions">type definition</a>; or in the receiver specification
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of a <a href="#Method_declarations">method declaration</a> that is associated
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with a parameterized type.
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A type parameter acts as a place holder for an (as of yet) unknown type in the declaration;
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the type parameter is replaced with a <i>type argument</i> upon
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<a href="#Instantiations">instantiation</a> of the parameterized function or type.
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</p>
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<p>
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The properties of a type parameter are determined by its
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<a href="#Type_constraints">type constraint</a>.
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</p>
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<h2 id="Properties_of_types_and_values">Properties of types and values</h2>
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<h3 id="Type_identity">Type identity</h3>
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@ -1983,6 +2019,15 @@ Go is lexically scoped using <a href="#Blocks">blocks</a>:
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<li>The scope of an identifier denoting a method receiver, function parameter,
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or result variable is the function body.</li>
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<li>The scope of an identifier denoting a type parameter of a type-parameterized function
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or declared by a method receiver is the function body and all parameter lists of the
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function.
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</li>
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<li>The scope of an identifier denoting a type parameter of a parameterized type
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begins after the name of the parameterized type and ends at the end
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of the TypeSpec.</li>
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<li>The scope of a constant or variable identifier declared
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inside a function begins at the end of the ConstSpec or VarSpec
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(ShortVarDecl for short variable declarations)
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@ -5384,7 +5429,6 @@ TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClau
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TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
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TypeCaseClause = TypeSwitchCase ":" StatementList .
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TypeSwitchCase = "case" TypeList | "default" .
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TypeList = Type { "," Type } .
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</pre>
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<p>
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