proxy-types.h: add static_assert to detect int/enum size mismatch

[ryanofsky]

Add static_assert to detect when an int field is too small to hold an enum value

This catches the bug TheCharlatan pointed out in bitcoin/bitcoin#29409 (comment)

[TheCharlatan]

lgtm ACK bbc80ab

[Sjors]

Looks correct to me based on the description, but I'm not very familiar with the libmultiprocess internals.

Maybe add a test to illustrate the fix?

[ryanofsky]

Maybe add a test to illustrate the fix?

Good idea. There was no coverage for enum/int conversions so I pushed a new commit 110349f to add some.

I don't think there's a good way to add test coverage for the new static_assert since it just triggers compile errors if enum and int types aren't compatible. But it is possible to trigger the static assert manually by changing:

--- a/test/mp/test/foo.capnp
+++ b/test/mp/test/foo.capnp
@@ -27,7 +27,7 @@ interface FooInterface $Proxy.wrap("mp::test::FooImplementation") {
     passEmpty @12 (arg :FooEmpty) -> (result :FooEmpty);
     passMessage @13 (arg :FooMessage) -> (result :FooMessage);
     passMutable @14 (arg :FooMutable) -> (arg :FooMutable);
-    passEnum @15 (arg :Int32) -> (result :Int32);
+    passEnum @15 (arg :Int16) -> (result :Int32);
 }
 
 interface FooCallback $Proxy.wrap("mp::test::FooCallback") {

Then there should be a compile error if you run make check:

include/mp/proxy-types.h:968:19: error: static assertion failed due to requirement 'std::numeric_limits<unsigned short>::max() >= std::numeric_limits<unsigned int>::max()': mismatched integral/enum types
  968 |     static_assert(std::numeric_limits<T>::max() >= std::numeric_limits<E>::max(), "mismatched integral/enum types");
[...]
include/mp/proxy-types.h:968:49: note: expression evaluates to '65535 >= 4294967295'
  968 |     static_assert(std::numeric_limits<T>::max() >= std::numeric_limits<E>::max(), "mismatched integral/enum types");