typesets.go

     1  // Copyright 2021 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // This file shows some examples of constraint literals with elided interfaces.
     6  // These examples are permitted if proposal issue #48424 is accepted.
     7  
     8  package p
     9  
    10  // Constraint type sets of the form T, ~T, or A|B may omit the interface.
    11  type (
    12  	_[T int]            struct{}
    13  	_[T ~int]           struct{}
    14  	_[T int | string]   struct{}
    15  	_[T ~int | ~string] struct{}
    16  )
    17  
    18  func min[T int | string](x, y T) T {
    19  	if x < y {
    20  		return x
    21  	}
    22  	return y
    23  }
    24  
    25  func lookup[M ~map[K]V, K comparable, V any](m M, k K) V {
    26  	return m[k]
    27  }
    28  
    29  func deref[P ~*E, E any](p P) E {
    30  	return *p
    31  }
    32  
    33  func _() int {
    34  	p := new(int)
    35  	return deref(p)
    36  }
    37  
    38  func addrOfCopy[V any, P *V](v V) P {
    39  	return &v
    40  }
    41  
    42  func _() *int {
    43  	return addrOfCopy(0)
    44  }
    45  
    46  // A type parameter may not be embedded in an interface;
    47  // so it can also not be used as a constraint.
    48  func _[A any, B A /* ERROR "cannot use a type parameter as constraint" */]()    {}
    49  func _[A any, B, C A /* ERROR "cannot use a type parameter as constraint" */]() {}
    50  
    51  // Error messages refer to the type constraint as it appears in the source.
    52  // (No implicit interface should be exposed.)
    53  func _[T string](x T) T {
    54  	return x /* ERROR "constrained by string" */ * x
    55  }
    56  
    57  func _[T int | string](x T) T {
    58  	return x /* ERROR "constrained by int | string" */ * x
    59  }
    60
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