mirror of https://github.com/golang/go.git
ee402e9882
This change defines two unification modes used to control unification: - assign set when unifying types involved in an assignment - exact if set, types unify if they can be made identical Currently, unification is inexact: when a defined type is compared against a type literal, the underlying type of the defined type is considered. When channel types are compared, the channel direction is ignored. And when defined types are compared where one (or both) are interfaces, interface unification is used. By contrast, exact unification requires types to match exactly: if they can be unified, the types must be identical (with suitable type arguments). Exact unification is required when comparing component types. For instance, when unifying func(x P) with func(x Q), the two signatures unify only if P is identical to Q per Go's assignment rules. Until now we have ignored exact unification and made due with inexact unification everywhere, even for component types. In some cases this led to infinite recursions in the unifier, which we guarded against with a depth limit (and unification failure). Go's assignmemt rules allow inexact matching at the top-level but require exact matching for element types. This change passes 'assign' to the unifier when unifying parameter against argument types because those follow assignment rules. When comparing constraints, inexact unification is used as before. In 'assign' mode, when comparing element types, the unifyier is called recursively, this time with the 'exact' mode set, causing element types to be compared exactly. If unification succeeds for element types, they are identical (with suitable type arguments). This change fixes #60460. It also fixes a bug in the test for issue #60377. We also don't need to rely anymore on the recursion depth limit (a temporary fix) for #59740. Finally, because we use exact unification when comparing element types which are channels, errors caused by assignment failures (due to inexact inference which succeeded when it shouldn't have) now produce the correct inference error. Fixes #60460. For #60377. For #59740. Change-Id: Icb6a9b4dbd34294f99328a06d52135cb499cab85 Reviewed-on: https://go-review.googlesource.com/c/go/+/498895 Reviewed-by: Robert Findley <rfindley@google.com> Auto-Submit: Robert Griesemer <gri@google.com> Run-TryBot: Robert Griesemer <gri@google.com> Reviewed-by: Robert Griesemer <gri@google.com> TryBot-Result: Gopher Robot <gobot@golang.org> |
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README.vendor
Vendoring in std and cmd
========================
The Go command maintains copies of external packages needed by the
standard library in the src/vendor and src/cmd/vendor directories.
There are two modules, std and cmd, defined in src/go.mod and
src/cmd/go.mod. When a package outside std or cmd is imported
by a package inside std or cmd, the import path is interpreted
as if it had a "vendor/" prefix. For example, within "crypto/tls",
an import of "golang.org/x/crypto/cryptobyte" resolves to
"vendor/golang.org/x/crypto/cryptobyte". When a package with the
same path is imported from a package outside std or cmd, it will
be resolved normally. Consequently, a binary may be built with two
copies of a package at different versions if the package is
imported normally and vendored by the standard library.
Vendored packages are internally renamed with a "vendor/" prefix
to preserve the invariant that all packages have distinct paths.
This is necessary to avoid compiler and linker conflicts. Adding
a "vendor/" prefix also maintains the invariant that standard
library packages begin with a dotless path element.
The module requirements of std and cmd do not influence version
selection in other modules. They are only considered when running
module commands like 'go get' and 'go mod vendor' from a directory
in GOROOT/src.
Maintaining vendor directories
==============================
Before updating vendor directories, ensure that module mode is enabled.
Make sure that GO111MODULE is not set in the environment, or that it is
set to 'on' or 'auto'.
Requirements may be added, updated, and removed with 'go get'.
The vendor directory may be updated with 'go mod vendor'.
A typical sequence might be:
cd src
go get golang.org/x/net@latest
go mod tidy
go mod vendor
Use caution when passing '-u' to 'go get'. The '-u' flag updates
modules providing all transitively imported packages, not only
the module providing the target package.
Note that 'go mod vendor' only copies packages that are transitively
imported by packages in the current module. If a new package is needed,
it should be imported before running 'go mod vendor'.