[flang][mlir] MLIRConfig.cmake overrides MLIR_TABLEGEN_EXE and breaks external/flang builds

[Acture]

In downstream (e.g., Nix-based) builds of Flang, explicitly setting MLIR_TABLEGEN_EXE is ignored due to MLIRConfig.cmake redefining the mlir-tblgen target.

This causes build failures such as:

ninja: error: 'include/flang/Optimizer/Dialect/CUF/Attributes/mlir-tblgen', needed by '.../CUFEnumAttr.h.inc', missing and no known rule to make it

Root cause:

TableGen.cmake expects mlir-tblgen to already be defined. But MLIRConfig overwrites the target, causing CMake to misresolve the executable.

Proposed fix:

Define a dummy imported target (mlir-tblgen and MLIR-TBLGen) early in flang/CMakeLists.txt if not already defined. This prevents accidental overwriting.
As shown in #150987

See attached PR for the patch.

Related:
• Previous Nixpkgs PR encountering this
• My downstream patch discussion

Would appreciate feedback on whether this is an appropriate fix or if a deeper MLIR-side solution is needed.

[Acture]

FAILED: lib/Evaluate/CMakeFiles/FortranEvaluate.dir/check-expression.cpp.o
/nix/store/0r34311s7lka3iisjvhdaqpblqyk6g9f-gcc-wrapper-14.3.0/bin/g++ -DFLANG_LITTLE_ENDIAN=1 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/build/flang-src-20.1.6/flang/build/lib/Evaluate -I/build/flang-src-20.1.6/flang/lib/Evaluate -I/build/flang-src-20.1.6/flang/include -I/build/flang-src-20.1.6/flang/build/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -fno-lifetime-dse -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-maybe-uninitialized -Wno-nonnull -Wno-class-memaccess -Wno-redundant-move -Wno-pessimizing-move -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wno-misleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -Wno-deprecated-copy -Wno-ctad-maybe-unsupported -fno-strict-aliasing -fno-semantic-interposition -O3 -DNDEBUG -std=c++17 -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -fno-exceptions -funwind-tables -MD -MT lib/Evaluate/CMakeFiles/FortranEvaluate.dir/check-expression.cpp.o -MF lib/Evaluate/CMakeFiles/FortranEvaluate.dir/check-expression.cpp.o.d -o lib/Evaluate/CMakeFiles/FortranEvaluate.dir/check-expression.cpp.o -c /build/flang-src-20.1.6/flang/lib/Evaluate/check-expression.cpp
g++: fatal error: Killed signal terminated program cc1plus
compilation terminated.
[91/477] Building CXX object lib/Evaluate/CMakeFiles/FortranEvaluate.dir/fold.cpp.o
[92/477] Building CXX object lib/Evaluate/CMakeFiles/FortranEvaluate.dir/fold-designator.cpp.o
[93/477] Building CXX object lib/Evaluate/CMakeFiles/FortranEvaluate.dir/fold-character.cpp.o
[94/477] Building CXX object lib/Evaluate/CMakeFiles/FortranEvaluate.dir/fold-complex.cpp.o
[95/477] Building CXX object lib/Evaluate/CMakeFiles/FortranEvaluate.dir/fold-logical.cpp.o
[96/477] Building CXX object lib/Evaluate/CMakeFiles/FortranEvaluate.dir/fold-real.cpp.o
[97/477] Building CXX object lib/Evaluate/CMakeFiles/FortranEvaluate.dir/fold-integer.cpp.o
In file included from /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/array:43,
from /build/flang-src-20.1.6/flang/include/flang/Common/enum-class.h:20,
from /build/flang-src-20.1.6/flang/include/flang/Common/idioms.h:26,
from /build/flang-src-20.1.6/flang/include/flang/Common/enum-set.h:18,
from /build/flang-src-20.1.6/flang/include/flang/Common/Fortran.h:15,
from /build/flang-src-20.1.6/flang/include/flang/Common/Fortran-features.h:12,
from /build/flang-src-20.1.6/flang/include/flang/Evaluate/common.h:12,
from /build/flang-src-20.1.6/flang/include/flang/Evaluate/type.h:19,
from /build/flang-src-20.1.6/flang/lib/Evaluate/character.h:12,
from /build/flang-src-20.1.6/flang/lib/Evaluate/fold-implementation.h:12,
from /build/flang-src-20.1.6/flang/lib/Evaluate/fold-integer.cpp:9:
In static member function 'static _Up* std::__copy_move<_IsMove, true, std::random_access_iterator_tag>::__copy_m(_Tp*, _Tp*, _Up*) [with _Tp = const long int; _Up = long int; bool _IsMove = false]',
inlined from '_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = false; _II = const long int*; _OI = long int*]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_algobase.h:521:30,
inlined from '_OI std::__copy_move_a1(_II, _II, _OI) [with bool _IsMove = false; _II = const long int*; _OI = long int*]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_algobase.h:548:42,
inlined from '_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = false; _II = const long int*; _OI = long int*]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_algobase.h:555:31,
inlined from '_OI std::copy(_II, _II, _OI) [with _II = const long int*; _OI = long int*]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_algobase.h:651:7,
inlined from 'static _ForwardIterator std::__uninitialized_copy::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const long int*; _ForwardIterator = long int*]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_uninitialized.h:147:27,
inlined from '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const long int*; _ForwardIterator = long int*]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_uninitialized.h:185:15,
inlined from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, allocator<_Tp>&) [with _InputIterator = const long int*; _ForwardIterator = long int*; _Tp = long int]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_uninitialized.h:373:37,
inlined from 'void std::vector<_Tp, _Alloc>::_M_range_initialize_n(_Iterator, _Iterator, size_type) [with _Iterator = const long int*; _Tp = long int; _Alloc = std::allocator]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:1719:37:
/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_algobase.h:452:23: warning: 'void* __builtin_memcpy(void*, const void*, long unsigned int)' writing between 9 and 9223372036854775807 bytes into a region of size 8 [-Wstringop-overflow=]
In file included from /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/aarch64-unknown-linux-gnu/bits/c++allocator.h:33,
from /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/allocator.h:46,
from /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/string:43,
from /build/flang-src-20.1.6/flang/include/flang/Common/enum-class.h:21:
In member function '_Tp* std::__new_allocator<_Tp>::allocate(size_type, const void*) [with _Tp = long int]',
inlined from 'static _Tp* std::allocator_traits<std::allocator<_CharT> >::allocate(allocator_type&, size_type) [with _Tp = long int]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/alloc_traits.h:515:28,
inlined from 'std::_Vector_base<_Tp, _Alloc>::pointer std::_Vector_base<_Tp, _Alloc>::_M_allocate(std::size_t) [with _Tp = long int; _Alloc = std::allocator]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:383:33,
inlined from 'void std::vector<_Tp, _Alloc>::_M_range_initialize_n(_Iterator, _Iterator, size_type) [with _Iterator = const long int*; _Tp = long int; _Alloc = std::allocator]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:1716:23:
/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/new_allocator.h:151:48: note: destination object of size 8 allocated by 'operator new'
ninja: build stopped: subcommand failed.
error: builder for '/nix/store/ymk2hpj5p3xwia00qw22qkzwa7zlj8b6-flang-20.1.6.drv' failed with exit code 1;
last 25 log lines:
> from /build/flang-src-20.1.6/flang/include/flang/Evaluate/common.h:12,
> from /build/flang-src-20.1.6/flang/include/flang/Evaluate/type.h:19,
> from /build/flang-src-20.1.6/flang/lib/Evaluate/character.h:12,
> from /build/flang-src-20.1.6/flang/lib/Evaluate/fold-implementation.h:12,
> from /build/flang-src-20.1.6/flang/lib/Evaluate/fold-integer.cpp:9:
> In static member function 'static _Up* std::__copy_move<_IsMove, true, std::random_access_iterator_tag>::__copy_m(_Tp*, _Tp*, _Up*) [with _Tp = const long int; _Up = long int; bool _IsMove = false]',
> inlined from '_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = false; _II = const long int*; _OI = long int*]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_algobase.h:521:30,
> inlined from '_OI std::__copy_move_a1(_II, _II, _OI) [with bool _IsMove = false; _II = const long int*; _OI = long int*]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_algobase.h:548:42,
> inlined from '_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = false; _II = const long int*; _OI = long int*]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_algobase.h:555:31,
> inlined from '_OI std::copy(_II, _II, _OI) [with _II = const long int*; _OI = long int*]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_algobase.h:651:7,
> inlined from 'static _ForwardIterator std::__uninitialized_copy::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const long int*; _ForwardIterator = long int*]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_uninitialized.h:147:27,
> inlined from '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const long int*; _ForwardIterator = long int*]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_uninitialized.h:185:15,
> inlined from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, allocator<_Tp>&) [with _InputIterator = const long int*; _ForwardIterator = long int*; _Tp = long int]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_uninitialized.h:373:37,
> inlined from 'void std::vector<_Tp, _Alloc>::_M_range_initialize_n(_Iterator, _Iterator, size_type) [with _Iterator = const long int*; _Tp = long int; _Alloc = std::allocator]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:1719:37:
> /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_algobase.h:452:23: warning: 'void* __builtin_memcpy(void*, const void*, long unsigned int)' writing between 9 and 9223372036854775807 bytes into a region of size 8 [-Wstringop-overflow=]
> In file included from /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/aarch64-unknown-linux-gnu/bits/c++allocator.h:33,
> from /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/allocator.h:46,
> from /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/string:43,
> from /build/flang-src-20.1.6/flang/include/flang/Common/enum-class.h:21:
> In member function '_Tp* std::__new_allocator<_Tp>::allocate(size_type, const void*) [with _Tp = long int]',
> inlined from 'static _Tp* std::allocator_traits<std::allocator<_CharT> >::allocate(allocator_type&, size_type) [with _Tp = long int]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/alloc_traits.h:515:28,
> inlined from 'std::_Vector_base<_Tp, _Alloc>::pointer std::_Vector_base<_Tp, _Alloc>::_M_allocate(std::size_t) [with _Tp = long int; _Alloc = std::allocator]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:383:33,
> inlined from 'void std::vector<_Tp, _Alloc>::_M_range_initialize_n(_Iterator, _Iterator, size_type) [with _Iterator = const long int*; _Tp = long int; _Alloc = std::allocator]' at /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/stl_vector.h:1716:23:
> /nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gcc-14.3.0/include/c++/14.3.0/bits/new_allocator.h:151:48: note: destination object of size 8 allocated by 'operator new'
> ninja: build stopped: subcommand failed.
For full logs, run:
nix log /nix/store/ymk2hpj5p3xwia00qw22qkzwa7zlj8b6-flang-20.1.6.drv