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cmd/go2go: chan X(T) does not parse #39670

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ianlancetaylor opened this issue Jun 17, 2020 · 1 comment
Closed

cmd/go2go: chan X(T) does not parse #39670

ianlancetaylor opened this issue Jun 17, 2020 · 1 comment
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@ianlancetaylor
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@ianlancetaylor ianlancetaylor commented Jun 17, 2020

This initializer for c1 does not parse, and I'm not sure why. The initialize for c2 does work.

package main

type X(type T) T

var c1 = make(chan X(int))
var c2 = make(chan (X(int)))

I get

type checking failed for main
foo.go2:5:20: cannot use generic type X(type T) without instantiation
@griesemer
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@griesemer griesemer commented Jun 17, 2020

Note that chan x(y) is a legal type conversion (chan x)(y). Even though we are expecting a type here, for the parser make(chan X(int)) looks like an ordinary function call (it doesn't know anything about make being special) and so it expects a (non-type) expression as argument. By that logic, that expression must be a conversion. Hence you need the extra parentheses. If the parser knew to expect a type, the parentheses wouldn't be needed.

I suppose the type-checker could re-associate the expression once it knows we're dealing with the built-in make call, but that seems a bit iffy (to me).

Closing as working as intended.

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