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[dev.typeparams] cmd/compile: explain how pkgReader.typIdx handles alias cyclic

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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>
This commit is contained in:
Cuong Manh Le 2021-06-20 02:06:45 +07:00
parent 3f7a3133da
commit d24c90a153
1 changed files with 39 additions and 2 deletions
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41
src/cmd/compile/internal/noder/reader.go
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@ -295,9 +295,46 @@ func (pr *pkgReader) typIdx(idx int, implicits, explicits []*types.Type) *types.
typ := r.doTyp() typ := r.doTyp()
assert(typ != nil) assert(typ != nil)
// For recursive type declarations involving interfaces and aliases,
// above r.doTyp() call may have already set pr.typs[idx], so just
// double check and return the type.
//
// Example:
//
// type F = func(I)
//
// type I interface {
// m(F)
// }
//
// The writer writes data types in following index order:
//
// 0: func(I)
// 1: I
// 2: interface{m(func(I))}
//
// The reader resolves it in following index order:
//
// 0 -> 1 -> 2 -> 0 -> 1
//
// and can divide in logically 2 steps:
//
// - 0 -> 1 : first time the reader reach type I,
// it creates new named type with symbol I.
//
// - 2 -> 0 -> 1: the reader ends up reaching symbol I again,
// now the symbol I was setup in above step, so
// the reader just return the named type.
//
// Now, the functions called return, the pr.typs looks like below:
//
// - 0 -> 1 -> 2 -> 0 : [<T> I <T>]
// - 0 -> 1 -> 2 : [func(I) I <T>]
// - 0 -> 1 : [func(I) I interface { "".m(func("".I)) }]
//
// The idx 1, corresponding with type I was resolved successfully
// after r.doTyp() call.
if typ := pr.typs[idx]; typ != nil { if typ := pr.typs[idx]; typ != nil {
// This happens in fixedbugs/issue27232.go.
// TODO(mdempsky): Explain why/how this happens.
return typ return typ
} }