staticcheck: improve handling of byte slices and arrays in printf che…

…cker

Closes gh-714

code/code.go

@@ -101,6 +101,7 @@ func IsCallToAny(call *ir.CallCommon, names ...string) bool {
 	return false
 }
 
+// OPT(dh): IsType is kind of expensive; should we really use it?
 func IsType(T types.Type, name string) bool { return types.TypeString(T, nil) == name }
 
 func FilterDebug(instr []ir.Instruction) []ir.Instruction {

staticcheck/lint.go

@@ -527,8 +527,26 @@ func checkPrintfCallImpl(carg *Argument, f ir.Value, args []ir.Value) {
 			return true
 		}
 
-		if flags&isString != 0 && (code.IsType(T, "[]byte") || isStringer(T, ms) || isError(T, ms)) {
-			return true
+		if flags&isString != 0 {
+			isStringyElem := func(typ types.Type) bool {
+				if typ, ok := typ.Underlying().(*types.Basic); ok {
+					return typ.Kind() == types.Byte
+				}
+				return false
+			}
+			switch T := T.(type) {
+			case *types.Slice:
+				if isStringyElem(T.Elem()) {
+					return true
+				}
+			case *types.Array:
+				if isStringyElem(T.Elem()) {
+					return true
+				}
+			}
+			if isStringer(T, ms) || isError(T, ms) {
+				return true
+			}
 		}
 
 		if flags&isPointer != 0 && code.IsPointerLike(T) {

staticcheck/testdata/src/CheckPrintf/CheckPrintf.go

@@ -399,3 +399,17 @@ func dbg(format string, args ...interface{}) {
 	}
 	fmt.Printf(format, args...)
 }
+
+// https://github.com/dominikh/go-tools/issues/714
+func fn2() {
+	type String string
+	type Byte byte
+
+	var a string = "a"
+	var b []byte = []byte{'b'}
+	var c [1]byte = [1]byte{'c'}
+	var d String = "d"
+	var e []uint8 = []uint8{'e'}
+	var f []Byte = []Byte{'h'}
+	fmt.Printf("%s %s %s %s %s %s %s", a, b, c, &c, d, e, f)
+}