cmd/vet: ignore unrecognized flags for fmt.Formatter

Fixes #22608.

Change-Id: Id85eb86b0b262156646e55f102fe888b345b20cf
Reviewed-on: https://go-review.googlesource.com/77230
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/vet/print.go

@@ -483,15 +483,17 @@ func (f *File) okPrintfArg(call *ast.CallExpr, state *formatState) (ok bool) {
 		}
 	}
 
-	if !found && !formatter {
-		f.Badf(call.Pos(), "%s format %s has unknown verb %c", state.name, state.format, state.verb)
-		return false
-	}
-	for _, flag := range state.flags {
-		if !strings.ContainsRune(v.flags, rune(flag)) {
-			f.Badf(call.Pos(), "%s format %s has unrecognized flag %c", state.name, state.format, flag)
+	if !formatter {
+		if !found {
+			f.Badf(call.Pos(), "%s format %s has unknown verb %c", state.name, state.format, state.verb)
 			return false
 		}
+		for _, flag := range state.flags {
+			if !strings.ContainsRune(v.flags, rune(flag)) {
+				f.Badf(call.Pos(), "%s format %s has unrecognized flag %c", state.name, state.format, flag)
+				return false
+			}
+		}
 	}
 	// Verb is good. If len(state.argNums)>trueArgs, we have something like %.*s and all
 	// but the final arg must be an integer.

src/cmd/vet/testdata/print.go

@@ -163,11 +163,12 @@ func PrintfTests() {
 	Printf(format, "hi")              // ERROR "Printf format %s reads arg #2, but call has only 1 arg$"
 	Printf("%s %d %.3v %q", "str", 4) // ERROR "Printf format %.3v reads arg #3, but call has only 2 args"
 	f := new(stringer)
-	f.Warn(0, "%s", "hello", 3)  // ERROR "Warn call has possible formatting directive %s"
-	f.Warnf(0, "%s", "hello", 3) // ERROR "Warnf call needs 1 arg but has 2 args"
-	f.Warnf(0, "%r", "hello")    // ERROR "Warnf format %r has unknown verb r"
-	f.Warnf(0, "%#s", "hello")   // ERROR "Warnf format %#s has unrecognized flag #"
-	Printf("d%", 2)              // ERROR "Printf format % is missing verb at end of string"
+	f.Warn(0, "%s", "hello", 3)           // ERROR "Warn call has possible formatting directive %s"
+	f.Warnf(0, "%s", "hello", 3)          // ERROR "Warnf call needs 1 arg but has 2 args"
+	f.Warnf(0, "%r", "hello")             // ERROR "Warnf format %r has unknown verb r"
+	f.Warnf(0, "%#s", "hello")            // ERROR "Warnf format %#s has unrecognized flag #"
+	fmt.Printf("%#s", FormatterVal(true)) // correct (the type is responsible for formatting)
+	Printf("d%", 2)                       // ERROR "Printf format % is missing verb at end of string"
 	Printf("%d", percentDV)
 	Printf("%d", &percentDV)
 	Printf("%d", notPercentDV)  // ERROR "Printf format %d has arg notPercentDV of wrong type testdata.notPercentDStruct"