cmd/vet: failed import causes spurious printf error

[alandonovan]

$ cat a.go
package main

import "nosuchpkg"

var format = "%s %s"
var _ = nosuchpkg.Sprintf(format, "a", "b")

$ vet ~/a.go
/usr/local/google/home/adonovan/a.go:6: no formatting directive in Sprintf call

The printf check issues a spurious error when type information for an imported printf-like function is not available. Without the function's type signature, the checker heuristically assumes that the first string constant is the format string, such as "a" in the example above.

[josharian]

cc @valyala

[valyala]

Prepared the CL.

[gopherbot]

CL https://golang.org/cl/29014 mentions this issue.