Error: a reinterpret_cast is not a constant expression

[marcovannoord]

Hi,
I'm been changing from an ESP32/Arduino environment into an ESP32/esp-idf environment that has support for Arduino-code, and so far everything has been smooth, except for this library.
Somehow, the compiler trips over the following code:

#define DEFINE_FIELD(fieldname, value_t, obis, field_t, field_args...) \
  struct fieldname : field_t<fieldname, ##field_args> { \
    value_t fieldname; \
    bool fieldname ## _present = false; \
    static constexpr ObisId id = obis; \
    static constexpr char name_progmem[] DSMR_PROGMEM = #fieldname; \
    static constexpr const __FlashStringHelper *name = reinterpret_cast<const __FlashStringHelper*>(&name_progmem); \
    value_t& val() { return fieldname; } \
    bool& present() { return fieldname ## _present; } \
  }

at the reinterpret_cast<.... part it gives me this error:

.pio/libdeps/switch-1/Dsmr/src/dsmr/fields.h:185:56: error: a reinterpret_cast is not a constant expression
     static constexpr const __FlashStringHelper *name = reinterpret_cast<const __FlashStringHelper*>(&name_progmem); \
                                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.pio/libdeps/switch-1/Dsmr/src/dsmr/fields.h:231:1: note: in expansion of macro 'DEFINE_FIELD'
 DEFINE_FIELD(electricity_long_failures, uint32_t, ObisId(0, 0, 96, 7, 9), IntField, units::none);

What i've tried/checked

• The build flags for the GCC compiler (C++11) for the Arduino environment and for the esp-idf environment are the same: gcc 8.2.0
• the __FlashStringHelper is present, and is the same as before
• The DEFINE_FIELD() functions where the DEFINE_FIELD macro is used, expand to the same code in both environments.

Could you give me any hints where i should look to solve this issue?

[matthijskooijman]

I was just running into this same error compiling DSMR on a regular PC instead of for Arduino. Good to know this also occurs with certain ESP cores.

The problem is probably that newer gcc versions (correctly) check the constness of expressions per the standard, where older versions would (incorrectly) allow this expression.

I tried the following patch, which seems to compile (but I haven't managed linking successfully, so I'm not sure if it actually works). If you try it, let me know if it works for you:

--- a/src/dsmr/fields.h
+++ b/src/dsmr/fields.h
@@ -176,13 +176,19 @@ const uint8_t WATER_MBUS_ID = 2;
 const uint8_t THERMAL_MBUS_ID = 3;
 const uint8_t SLAVE_MBUS_ID = 4;
 
+template <typename FieldT>
+struct NameConverter {
+  public:
+    operator const __FlashStringHelper*() const { return reinterpret_cast<const __FlashStringHelper*>(&FieldT::name_progmem); }
+};
+
 #define DEFINE_FIELD(fieldname, value_t, obis, field_t, field_args...) \
   struct fieldname : field_t<fieldname, ##field_args> { \
     value_t fieldname; \
     bool fieldname ## _present = false; \
     static constexpr ObisId id = obis; \
     static constexpr char name_progmem[] DSMR_PROGMEM = #fieldname; \
-    static constexpr const __FlashStringHelper *name = reinterpret_cast<const __FlashStringHelper*>(&name_progmem); \
+    static constexpr NameConverter<dsmr::fields::fieldname> name = {}; \
     value_t& val() { return fieldname; } \
     bool& present() { return fieldname ## _present; } \
   }

I'll probably continue work on this in the coming days, hopefully I can push a fix out soon.

[marcovannoord]

Thanks for the quick response. I was already hitting my head at the wall, wondering what i did wrong with my build environment, but indeed good to hear that it's indeed intentionally failing.
I was already looking for compiler flags that would prevent it from checking the constexpr, but didn't find any yet.

For me, it still fails compiling, although it finds a conflicting declaration.

conflicting declaration 'constexpr const __FlashStringHelper* const dsmr::fields::identification::name'
 constexpr const __FlashStringHelper *identification::name;
                                                      ^~~~
In file included from .pio\libdeps\switch-1\Dsmr\src\dsmr\fields.cpp:31:
.pio\libdeps\switch-1\Dsmr\src\dsmr\fields.h:192:61: note: previous declaration as 'constexpr const dsmr::fields::NameConverter<dsmr::fields::identification> dsmr::fields::identification::name'
     static constexpr NameConverter<dsmr::fields::fieldname> name = {}; \
.pio\libdeps\switch-1\Dsmr\src\dsmr\fields.h:199:1: note: in expansion of macro 'DEFINE_FIELD'
 DEFINE_FIELD(identification, String, ObisId(255, 255, 255, 255, 255, 255), RawField);

[matthijskooijman]

Ah, I see I missed one. Try removing this line from fields.cpp:

constexpr const __FlashStringHelper *identification::name;

If that still breaks, maybe replace it with:

constexpr const dsmr::fields::NameConverter<dsmr::fields::identification> identification::name;

All of the other ::name lines need the same treatment.

[marcovannoord]

Ah, that first one did the trick.
I will see if parsing works correctly, but at least compilation and linking works perfectly now.

[marcovannoord]

Yep, parsing seems to work perfectly with the patch you mentioned, plus removing the
constexpr const __FlashStringHelper *identification::name; line.

[matthijskooijman]

Awesome, thanks for confirming. I'll try to push out this change to the master branch in the coming days.