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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
Open

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

mdempsky opened this issue Oct 8, 2018 · 2 comments
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@mdempsky
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mdempsky commented Oct 8, 2018 •
edited

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
@mdempsky
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mdempsky commented Oct 9, 2018 •
edited

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.

@mdempsky mdempsky mentioned this issue Oct 16, 2018
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Change https://golang.org/cl/152544 mentions this issue: cmd/compile: unify duplicate const detection logic

@griesemer griesemer modified the milestones: Go1.12, Go1.13 Dec 11, 2018
gopherbot pushed a commit that referenced this issue Feb 27, 2019
@mdempsky
cmd/compile: unify duplicate const detection logic
6fa7669 
Consistent logic for handling both duplicate map keys and case values,
and eliminates ad hoc value hashing code.

Also makes cmd/compile consistent with go/types's handling of
duplicate constants (see #28085), which is at least an improvement
over the status quo even if we settle on something different for the
spec.

As a side effect, this also suppresses cmd/compile's warnings about
duplicate nils in (non-interface expression) switch statements, which
was technically never allowed by the spec anyway.

Updates #28085.
Updates #28378.

Change-Id: I176a251e770c3c5bc11c2bf8d1d862db8f252a17
Reviewed-on: https://go-review.googlesource.com/c/152544
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
@andybons andybons modified the milestones: Go1.13, Go1.14 Jul 8, 2019
@rsc rsc modified the milestones: Go1.14, Backlog Oct 9, 2019
@mdempsky mdempsky mentioned this issue Jan 21, 2021
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