Fixes#11797.
Imported scalar and pointer aliases inherit their external C spelling,
but
receive a different Nim symbol. Signature hashing previously used that
symbol
identity, so aliases that emit exactly the same C type could produce
different
backend names for tuples, sequences, and other generic types.
For example, `cint` and `type CIntAlias = cint` both emit `int`, but
`seq[cint]` and `seq[CIntAlias]` could be emitted as incompatible C
structs.
The Nim type checker nevertheless permits assignments and calls between
them,
causing the generated C or C++ compilation to fail.
This changes the backend hash to use the external type spelling when
available.
A symbol-based fallback remains for imported types without a resolved
spelling.
The change deliberately does not collapse imported types into their
underlying
Nim builtin. Types such as `pid_t`, imported pointers with qualifiers,
and
platform typedefs may require distinct backend representations.
## NIF and incremental compilation
This does not change NIF serialization, NIF type keys, or the IC cache
format.
The bug is in backend type-name generation. An IC regression test is
included
to ensure that the corrected backend identity is preserved when
compilation
passes through the NIF pipeline.
## Tests
The regressions cover:
- tuple and sequence assignments between an imported type and its alias
- cross-module sequence parameters and mutation
- C and C++ backends
- NIF-backed incremental compilation
Existing C-type tests were also run under C/C++, refc, and ARC.
## Remaining scope
This does not solve the broader question of compatibility between
imported and
builtin types that have different backend identities, such as
`seq[cdouble]` and `seq[float]`. That remains tracked by #19374.
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