cmd/gc: error for x.(T) should offer *T if it would work

[rsc]

x.go:10: impossible type assertion: tables[0] (type Table) cannot have dynamic type
diskTable (missing Close method)

if *Table is not impossible, then the error should mention that

[rsc]

Comment 1:

if *diskTable is not impossible, that is

[DanielMorsing]

Comment 2:

I took a look at this code, and it looks like it should be fairly easy to fix. The
implements() function already has a way to find out if a pointer to the checked type
would implement an interface.
Somewhat unrelated, the implements() error reporting is unwieldy. It takes a couple of
pointer arguments that are filled with the various properties of the error. The function
is only used in a couple of spots and the callers either ignore the error reporting, or
format the properties into strings in pretty much the same way.
Does a CL that changes the error reporting to just be a formatted string make sense? It
would make it easier to make sure that the error reporting doesn't go out of sync, like
it did in this case.

[rsc]

Comment 3:

Sure, if the new call sites suggest a different way to format the
error, that seems fine.

[DanielMorsing]

Comment 4:

Got a fix at http://golang.org/cl/6500064

[rsc]

Comment 5:

This issue was closed by revision 1c2021c.

Status changed to Fixed.