typeAliases1.23b.go

     1  // -lang=go1.23 -gotypesalias=1 -goexperiment=aliastypeparams
     2  
     3  // Copyright 2024 The Go Authors. All rights reserved.
     4  // Use of this source code is governed by a BSD-style
     5  // license that can be found in the LICENSE file.
     6  
     7  package aliasTypes
     8  
     9  type _ = int
    10  type _[P any] = int
    11  
    12  // A type alias may have fewer type parameters than its RHS.
    13  type RHS[P any, Q ~int] struct {
    14  	p P
    15  	q Q
    16  }
    17  
    18  type _[P any] = RHS[P, int]
    19  
    20  // Or it may have more type parameters than its RHS.
    21  type _[P any, Q ~int, R comparable] = RHS[P, Q]
    22  
    23  // The type parameters of a type alias must implement the
    24  // corresponding type constraints of the type parameters
    25  // on the RHS (if any)
    26  type _[P any, Q ~int] = RHS[P, Q]
    27  type _[P any, Q int] = RHS[P, Q]
    28  type _[P int | float64] = RHS[P, int]
    29  type _[P, Q any] = RHS[P, Q /* ERROR "Q does not satisfy ~int" */]
    30  
    31  // A generic type alias may be used like any other generic type.
    32  type A[P any] = RHS[P, int]
    33  
    34  func _(a A[string]) {
    35  	a.p = "foo"
    36  	a.q = 42
    37  }
    38  
    39  // A generic alias may refer to another generic alias.
    40  type B[P any] = A[P]
    41  
    42  func _(a B[string]) {
    43  	a.p = "foo"
    44  	a.q = 42
    45  	// error messages print the instantiated alias type
    46  	a.r /* ERROR "a.r undefined (type B[string] has no field or method r)" */ = 0
    47  }
    48
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