C Frontend fails to convert forward-declared enum type as part of fn-signature

[fbrausse]

Example:

enum E;

enum E (*f)(void);

int main()
{
}

Currently:

ESBMC version 7.5.0 64-bit x86_64 linux
Target: 64-bit little-endian x86_64-unknown-linux with esbmclibc
Parsing enum-fwd-decl.c
Converting
esbmc: /home/kane/scorch/esbmc/src/clang-c-frontend/clang_c_convert.cpp:831: virtual bool clang_c_convertert::get_type(const clang::QualType&, typet&): Assertion `the_type' failed.

Essentially, the q_type is null here:

esbmc/src/clang-c-frontend/clang_c_convert.cpp

Line 1094 in 3a7ae4f

clang::QualType q_type = ent.getDecl()->getIntegerType();

Context: trying to translate an unmodified Linux driver.

[fbrausse]

With #1798 merged, the initial example works now. However, that PR only contains a work-around, which, in case the originally incomplete enum type is ever used within ESBMC, can lead to the same problems enum_type() has caused before. See #1758.