cmd/vet: report uncalled functions in Printf %v

Given, say, var f *os.File, a new vet check in CL 14122 diagnoses:

	fmt.Printf("%s\n", f.Name)
	fmt.Println(f.Name)

but not

	fmt.Printf("%v\n", f.Name)

In all three cases the error is that the argument should be f.Name().

Diagnosing Println but not Printf %v seems oddly inconsistent,
so I changed %v to have the check too. In fact, all verbs now have
the check except %p and %T.

Fixes Dave Cheney's confusion when trying to write an example
of the new vet check advertised in the Go 1.6 release notes.

Change-Id: I92fa6a7a1d5d9339a6a59ae4e587a254e633f500
Reviewed-on: https://go-review.googlesource.com/19101
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/print.go

@@ -445,12 +445,12 @@ func (f *File) okPrintfArg(call *ast.CallExpr, state *formatState) (ok bool) {
 		return false
 	}
 	arg := call.Args[argNum]
+	if f.isFunctionValue(arg) && state.verb != 'p' && state.verb != 'T' {
+		f.Badf(call.Pos(), "arg %s in printf call is a function value, not a function call", f.gofmt(arg))
+		return false
+	}
 	if !f.matchArgType(v.typ, nil, arg) {
 		typeString := ""
-		if f.isFunctionValue(arg) {
-			f.Badf(call.Pos(), "arg %s in printf call is a function value, not a function call", f.gofmt(arg))
-			return false
-		}
 		if typ := f.pkg.types[arg].Type; typ != nil {
 			typeString = typ.String()
 		}

src/cmd/vet/testdata/print.go

@@ -197,7 +197,10 @@ func PrintfTests() {
 	et5.error() // ok, not an error method.
 	// Can't print a function.
 	Printf("%d", someFunction) // ERROR "arg someFunction in printf call is a function value, not a function call"
+	Printf("%v", someFunction) // ERROR "arg someFunction in printf call is a function value, not a function call"
 	Println(someFunction)      // ERROR "arg someFunction in Println call is a function value, not a function call"
+	Printf("%p", someFunction) // ok: maybe someone wants to see the pointer
+	Printf("%T", someFunction) // ok: maybe someone wants to see the type
 	// Bug: used to recur forever.
 	Printf("%p %x", recursiveStructV, recursiveStructV.next)
 	Printf("%p %x", recursiveStruct1V, recursiveStruct1V.next)