Fix $Proxy.wrap mapping for empty structs with no fields #104
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Trying to pass an empty $Proxy.wrap struct with no fields fails because BuildOne overload for the last field number is incorrectly declared to accept an lvalue Builder reference instead of a univeral reference, and the CustomBuildField overload which calls BuildOne passes an rvalue Builder. The problem did not happen as long as the wrapped struct had at least one field because CustomBuildField would call the other BuildOne overload for the first field number which accepts an rvalue Builder, and that BuildOne overload would call the next BuildOne overload for the next field number, passing the builder as an lvalue. A test is included which triggers the bug. Compiler output before the bugfix looked like: include/mp/proxy-types.h:1066:5: error: no matching function for call to 'BuildOne' 1066 | BuildOne<0>(local_type, invoke_context, output.init(), value); include/mp/proxy-types.h:1050:6: note: candidate function [with index = 0, LocalType = mp::test::FooEmpty, Value = mp::test::FooEmpty, Output = mp::test::messages::FooEmpty::Builder] not viable: expects an lvalue for 3rd argument 1050 | void BuildOne(TypeList<LocalType> param,
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Trying to pass an empty struct that is annotated with
$Proxy.wrapand has no data fields fails because theBuildOneoverload for the last field number is incorrectly declared to accept an lvalue builder reference instead of a universal reference, and theCustomBuildFieldoverload which callsBuildOnepasses anrvaluebuilder, so the call fails with error:The problem did not happen as long as the wrapped struct had at least one field because
CustomBuildFieldwould call a differentBuildOneoverload for the first field number which does accept rvalue builders, and thatBuildOneoverload would call the nextBuildOneoverload for the next field number passing the builder as an lvalue.