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cmd/compile, go/parser: cannot parse certain constraint literals in generic types #49175

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griesemer opened this issue Oct 27, 2021 · 4 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done. release-blocker
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@griesemer
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griesemer commented Oct 27, 2021

The following generic type declaration cannot be parsed:

package p
type _[_ []t]t // syntax error: unexpected ], expecting operand

Not a release blocker because the typical

type _[_ ~[]t]t

is accepted.

See also #49174 .

@griesemer griesemer added the NeedsFix The path to resolution is known, but the work has not been done. label Oct 27, 2021
@griesemer griesemer added this to the Go1.18 milestone Oct 27, 2021
@griesemer griesemer self-assigned this Oct 27, 2021
@findleyr
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This affects go/parser as well.

@findleyr findleyr changed the title cmd/compile: cannot parse certain constraint literals in generic types cmd/compile, go/parser: cannot parse certain constraint literals in generic types Oct 27, 2021
@findleyr
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Another case:

type _[_ [1]t]t

@gopherbot
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Change https://golang.org/cl/359134 mentions this issue: go/parser: fix parsing of array or slice constraint types

gopherbot pushed a commit that referenced this issue Oct 28, 2021
Now that we allow eliding 'interface' from constraint types, we need to
be a bit more careful about not consuming a '[' when parsing the next
expression after "type T [". We want to check if the next expression is
an identifier not followed by ']', in which case we're in a generic
type, but need to avoid parsing index or slice expressions. Such
expressions aren't valid array lengths because these expressions are
never constant, so when encountering a following '[' we can instead
assume that this is a type parameter field with array or slice type
constraint.

Test cases are added for the related issues #49174 and #49175, along
with a flag to enable tracing error tests.

For #49174
For #49175

Change-Id: I0476ef20c4c134ac537118272f20caaf123ee70e
Reviewed-on: https://go-review.googlesource.com/c/go/+/359134
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
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Change https://golang.org/cl/360135 mentions this issue: cmd/compile/internal/syntax: fix parsing of array or slice constraint types

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