sa9004

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Variables

var Analyzer = SCAnalyzer.Analyzer

var SCAnalyzer = lint.InitializeAnalyzer(&lint.Analyzer{
	Analyzer: &analysis.Analyzer{
		Name:     "SA9004",
		Run:      run,
		Requires: []*analysis.Analyzer{inspect.Analyzer},
	},
	Doc: &lint.RawDocumentation{
		Title: `Only the first constant has an explicit type`,

		Text: `In a constant declaration such as the following:

    const (
        First byte = 1
        Second     = 2
    )

the constant Second does not have the same type as the constant First.
This construct shouldn't be confused with

    const (
        First byte = iota
        Second
    )

where \'First\' and \'Second\' do indeed have the same type. The type is only
passed on when no explicit value is assigned to the constant.

When declaring enumerations with explicit values it is therefore
important not to write

    const (
          EnumFirst EnumType = 1
          EnumSecond         = 2
          EnumThird          = 3
    )

This discrepancy in types can cause various confusing behaviors and
bugs.


Wrong type in variable declarations

The most obvious issue with such incorrect enumerations expresses
itself as a compile error:

    package pkg

    const (
        EnumFirst  uint8 = 1
        EnumSecond       = 2
    )

    func fn(useFirst bool) {
        x := EnumSecond
        if useFirst {
            x = EnumFirst
        }
    }

fails to compile with

    ./const.go:11:5: cannot use EnumFirst (type uint8) as type int in assignment


Losing method sets

A more subtle issue occurs with types that have methods and optional
interfaces. Consider the following:

    package main

    import "fmt"

    type Enum int

    func (e Enum) String() string {
        return "an enum"
    }

    const (
        EnumFirst  Enum = 1
        EnumSecond      = 2
    )

    func main() {
        fmt.Println(EnumFirst)
        fmt.Println(EnumSecond)
    }

This code will output

    an enum
    2

as \'EnumSecond\' has no explicit type, and thus defaults to \'int\'.`,
		Since:    "2019.1",
		Severity: lint.SeverityWarning,
		MergeIf:  lint.MergeIfAny,
	},
})