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[dev.typeparams] cmd/compile: explain how pkgReader.typIdx handles alias cyclic
Change-Id: Ib9357c21bb010abf0d5fd17c3bee3197854c3a8a Reviewed-on: https://go-review.googlesource.com/c/go/+/329570 Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com> Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com> TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com>
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@ -295,9 +295,46 @@ func (pr *pkgReader) typIdx(idx int, implicits, explicits []*types.Type) *types.
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typ := r.doTyp()
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assert(typ != nil)
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// For recursive type declarations involving interfaces and aliases,
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// above r.doTyp() call may have already set pr.typs[idx], so just
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// double check and return the type.
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//
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// Example:
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//
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// type F = func(I)
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//
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// type I interface {
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// m(F)
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// }
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//
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// The writer writes data types in following index order:
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//
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// 0: func(I)
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// 1: I
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// 2: interface{m(func(I))}
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//
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// The reader resolves it in following index order:
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//
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// 0 -> 1 -> 2 -> 0 -> 1
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//
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// and can divide in logically 2 steps:
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//
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// - 0 -> 1 : first time the reader reach type I,
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// it creates new named type with symbol I.
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//
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// - 2 -> 0 -> 1: the reader ends up reaching symbol I again,
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// now the symbol I was setup in above step, so
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// the reader just return the named type.
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//
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// Now, the functions called return, the pr.typs looks like below:
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//
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// - 0 -> 1 -> 2 -> 0 : [<T> I <T>]
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// - 0 -> 1 -> 2 : [func(I) I <T>]
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// - 0 -> 1 : [func(I) I interface { "".m(func("".I)) }]
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//
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// The idx 1, corresponding with type I was resolved successfully
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// after r.doTyp() call.
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if typ := pr.typs[idx]; typ != nil {
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// This happens in fixedbugs/issue27232.go.
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// TODO(mdempsky): Explain why/how this happens.
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return typ
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}
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