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spec: clarify requirements for duplicate constants in interface-typed map literal keys and switches #28085

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mdempsky opened this Issue Oct 8, 2018 · 2 comments

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mdempsky commented Oct 8, 2018

The Go spec disallows duplicate constants in map literal keys, and allows compilers to reject duplicate constants in switch cases.

However, the Go spec does not formally allow interface-typed constants, and doesn't mention how to handle constants that are implicitly or explicitly converted to interface type. The existing compilers handle these situations in differing ways:

package p

// #1
var _ = map[interface{}]int{
	0: 0,
	0: 0,
}

// #2
var _ = map[interface{}]int{
	interface{}(0): 0,
	interface{}(0): 0,
}

func _() {
	// #3
	switch interface{}(0) {
	case 0:
	case 0:
	}

	// #4
	switch interface{}(0) {
	case interface{}(0):
	case interface{}(0):
	}
}

cmd/compile rejects 1, 2, and 3.

go/types rejects 1 and 3.

gccgo (8.0) rejects none.

/cc @griesemer @ianlancetaylor

@griesemer griesemer self-assigned this Oct 9, 2018

@griesemer griesemer added this to the Go1.12 milestone Oct 9, 2018

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mdempsky commented Oct 9, 2018

Caveat to gccgo not warning about 1 or 2: gccgo 8.0 apparently doesn't even warn about:

_ = map[int]int{0: 0, 0: 0}

Filed as #28104.

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gopherbot commented Dec 4, 2018

Change https://golang.org/cl/152544 mentions this issue: cmd/compile: unify duplicate const detection logic

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