spec: document interface satisfaction more explicitly

[dsymonds]

http://golang.org/ref/spec#Interface_types does not clearly spell out the requirements
for a type to implement an interface. It should.

[cznic]

Comment 1:

IMO, the first three sentences at the link do define both exactly and completely
(together -> clearly) the requirements:
"""
An interface type specifies a method set called its interface. A variable of interface
type can store a value of any type with a method set that is any superset of the
interface. Such a type is said to implement the interface. 
"""

[dsymonds]

Comment 2:

It does not define the set inclusion criteria. One might (incorrectly)
guess assignability or callability. Or might guess that parameter names
must match too.

[DanielMorsing]

Comment 3:

The section on method sets describes that the inclusion criteria is only related to
methods, but doesn't say what it is exactly.
The compilers use the rules for function identity for interface matching. You could
probably refer to the type identity section, but extending it to methods or members of
method sets might be confusing. Methods don't exist as types in the language, and
methods and members of method sets have different identity rules.
Example:
method identity:
func (f Foo) Bar(fb FooBar) == func (b Foo) Bar(bf FooBar)
func (f Foo) Bar(fb FooBar) != func (b FooBar) Bar(bf Foo)
member of method set identity:
func (f Foo) Bar(fb FooBar) == func (b FooBar) Bar(bf FooBar)

[rsc]

Comment 4:

Labels changed: added go1.2.

[rsc]

Comment 6:

Labels changed: added go1.2maybe, removed go1.2.

[rsc]

Comment 7:

Labels changed: added go1.3, removed go1.2maybe.

[rsc]

Comment 8:

Labels changed: added release-go1.3.

[rsc]

Comment 9:

Labels changed: removed go1.3.

[rsc]

Comment 10:

Labels changed: added repo-main.

[rsc]

Comment 11:

Labels changed: added release-go1.4, removed release-go1.3.

[rsc]

Comment 12:

Labels changed: added release-go1.4maybe, removed release-go1.4.

[rsc]

Comment 13:

Labels changed: added release-go1.5, removed release-go1.4maybe.

[griesemer]

The Go 1.18 spec has now a section on what it means to implement an interface (see the end of the section on Interface types). Closing.