Support parsing integer literals (#529)

mapper.go

@@ -387,7 +387,7 @@ func intDecoder(bits int) MapperFunc { //nolint: dupl
 		default:
 			return fmt.Errorf("expected an int but got %q (%T)", t, t.Value)
 		}
-		n, err := strconv.ParseInt(sv, 10, bits)
+		n, err := strconv.ParseInt(sv, 0, bits)
 		if err != nil {
 			return fmt.Errorf("expected a valid %d bit int but got %q", bits, sv)
 		}
@@ -416,7 +416,7 @@ func uintDecoder(bits int) MapperFunc { //nolint: dupl
 		default:
 			return fmt.Errorf("expected an int but got %q (%T)", t, t.Value)
 		}
-		n, err := strconv.ParseUint(sv, 10, bits)
+		n, err := strconv.ParseUint(sv, 0, bits)
 		if err != nil {
 			return fmt.Errorf("expected a valid %d bit uint but got %q", bits, sv)
 		}

mapper_test.go

@@ -395,6 +395,31 @@ func TestNumbers(t *testing.T) {
 			I64: math.MinInt64,
 		}, cli)
 	})
+	t.Run("Integer literals", func(t *testing.T) {
+		integerLiterals := "10_0"
+
+		_, err := p.Parse([]string{
+			fmt.Sprintf("--i-8=%s", integerLiterals),
+			fmt.Sprintf("--i-16=%s", integerLiterals),
+			fmt.Sprintf("--i-32=%s", integerLiterals),
+			fmt.Sprintf("--i-64=%s", integerLiterals),
+			fmt.Sprintf("--u-8=%s", integerLiterals),
+			fmt.Sprintf("--u-16=%s", integerLiterals),
+			fmt.Sprintf("--u-32=%s", integerLiterals),
+			fmt.Sprintf("--u-64=%s", integerLiterals),
+		})
+		assert.NoError(t, err)
+		assert.Equal(t, CLI{
+			I8:  int8(100),
+			I16: int16(100),
+			I32: int32(100),
+			I64: int64(100),
+			U8:  uint8(100),
+			U16: uint16(100),
+			U32: uint32(100),
+			U64: uint64(100),
+		}, cli)
+	})
 }
 
 func TestJSONLargeNumber(t *testing.T) {