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Source file test/cmp6.go

Documentation: test

  // errorcheck
  
  // Copyright 2010 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // Verify that incorrect comparisons are detected.
  // Does not compile.
  
  package main
  
  func use(bool) {}
  
  type T1 *int
  type T2 *int
  
  type T3 struct{ z []int }
  
  var t3 T3
  
  type T4 struct {
  	_ []int
  	a float64
  }
  
  var t4 T4
  
  func main() {
  	// Arguments to comparison must be
  	// assignable one to the other (or vice versa)
  	// so chan int can be compared against
  	// directional channels but channel of different
  	// direction cannot be compared against each other.
  	var c1 chan<- int
  	var c2 <-chan int
  	var c3 chan int
  
  	use(c1 == c2) // ERROR "invalid operation|incompatible"
  	use(c2 == c1) // ERROR "invalid operation|incompatible"
  	use(c1 == c3)
  	use(c2 == c2)
  	use(c3 == c1)
  	use(c3 == c2)
  
  	// Same applies to named types.
  	var p1 T1
  	var p2 T2
  	var p3 *int
  
  	use(p1 == p2) // ERROR "invalid operation|incompatible"
  	use(p2 == p1) // ERROR "invalid operation|incompatible"
  	use(p1 == p3)
  	use(p2 == p2)
  	use(p3 == p1)
  	use(p3 == p2)
  
  	// Arrays are comparable if and only if their element type is comparable.
  	var a1 [1]int
  	var a2 [1]func()
  	var a3 [0]func()
  	use(a1 == a1)
  	use(a2 == a2) // ERROR "invalid operation|invalid comparison"
  	use(a3 == a3) // ERROR "invalid operation|invalid comparison"
  
  	// Comparison of structs should have a good message
  	use(t3 == t3) // ERROR "struct|expected"
  	use(t4 == t4) // ERROR "cannot be compared|non-comparable"
  
  	// Slices, functions, and maps too.
  	var x []int
  	var f func()
  	var m map[int]int
  	use(x == x) // ERROR "slice can only be compared to nil"
  	use(f == f) // ERROR "func can only be compared to nil"
  	use(m == m) // ERROR "map can only be compared to nil"
  
  	// Comparison with interface that cannot return true
  	// (would panic).
  	var i interface{}
  	use(i == x) // ERROR "invalid operation"
  	use(x == i) // ERROR "invalid operation"
  	use(i == f) // ERROR "invalid operation"
  	use(f == i) // ERROR "invalid operation"
  	use(i == m) // ERROR "invalid operation"
  	use(m == i) // ERROR "invalid operation"
  }
  

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