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The compiler produces a segfault during gimple_build_switch_nlabels at gimple.cc807 when compiling the provided code with the specified options. 
The issue can also be reproduced on Compiler Explorer.

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OS and Platform:
# uname -a
Linux ubuntu 4.15.0-213-generic #224-Ubuntu SMP Mon Jun 19 13:30:12 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
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# gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/root/gdbtest/gcc/gcc-15/libexec/gcc/x86_64-pc-linux-gnu/15.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /root/gdbtest/gcc/obj/../gcc/configure --prefix=/root/gdbtest/gcc/gcc-15 --enable-languages=c,c++,fortran,go --disable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 15.0.0 20240509 (experimental) (GCC) 
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Program:
# cat test035.c
__GIMPLE a(argc) {
  switch (argc_20) {
  case 1:
    b;
  }


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Command Lines:
# gcc test035.c -O2 -fdump-tree-gimple -fdump-tree-switchconv -Wall -c -o test035.o
test035.c:1:1: error: ‘__GIMPLE’ only valid with ‘-fgimple’
    1 | __GIMPLE a(argc) {
      | ^~~~~~~~
test035.c:1:10: error: return type defaults to ‘int’ [-Wimplicit-int]
    1 | __GIMPLE a(argc) {
      |          ^
test035.c: In function ‘a’:
test035.c:1:10: error: type of ‘argc’ defaults to ‘int’ [-Wimplicit-int]
test035.c:5:3: internal compiler error: in gimple_build_switch_nlabels, at gimple.cc:807
    5 |   }
      |   ^
0x7e5713 gimple_build_switch_nlabels(unsigned int, tree_node*, tree_node*)
        /root/gdbtest/gcc/obj/../gcc/gcc/gimple.cc:807
0x7e5713 gimple_build_switch_nlabels(unsigned int, tree_node*, tree_node*)
        /root/gdbtest/gcc/obj/../gcc/gcc/gimple.cc:804
0xd09040 gimple_build_switch(tree_node*, tree_node*, vec<tree_node*, va_heap, vl_ptr> const&)
        /root/gdbtest/gcc/obj/../gcc/gcc/gimple.cc:827
0xa64ac1 c_parser_gimple_switch_stmt
        /root/gdbtest/gcc/obj/../gcc/gcc/c/gimple-parser.cc:2552
0xa64ac1 c_parser_gimple_compound_statement
        /root/gdbtest/gcc/obj/../gcc/gcc/c/gimple-parser.cc:440
0xa64d61 c_parser_parse_gimple_body(c_parser*, char*, c_declspec_il, profile_count)
        /root/gdbtest/gcc/obj/../gcc/gcc/c/gimple-parser.cc:253
0xa505a4 c_parser_declaration_or_fndef
        /root/gdbtest/gcc/obj/../gcc/gcc/c/c-parser.cc:3011
0xa5af4b c_parser_external_declaration
        /root/gdbtest/gcc/obj/../gcc/gcc/c/c-parser.cc:2046
0xa5b935 c_parser_translation_unit
        /root/gdbtest/gcc/obj/../gcc/gcc/c/c-parser.cc:1900
0xa5b935 c_parse_file()
        /root/gdbtest/gcc/obj/../gcc/gcc/c/c-parser.cc:26889
0xad3a51 c_common_parse_file()
        /root/gdbtest/gcc/obj/../gcc/gcc/c-family/c-opts.cc:1311
Please submit a full bug report, with preprocessed source (by using -freport-bug).
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.

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Also ICE on trunk, compiler explorer:https://godbolt.org/z/5sYY7a47c

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