Created attachment 47998 [details]
Source file to be compiled.

The attached file elicits an ICE in the GCC "trunk" version used by Compiler Explorer (https://godbolt.org/z/iPK_JV):
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<source>: In function 'int main()':
<source>:20:24: internal compiler error: tree check: accessed elt 1 of 'tree_vec' with 0 elts in tsubst_pack_expansion, at cp/pt.c:12765
   20 |   Foo<bool>::foo<true>();
      |                        ^
Please submit a full bug report,
with preprocessed source if appropriate.
See <https://gcc.gnu.org/bugs/> for instructions.
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When using GCC 9.2, no ICE happens, but the file fails to be parsed:
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<source>: In function 'int main()':
<source>:20:24: error: no matching function for call to 'Foo<bool>::foo<true>()'
   20 |   Foo<bool>::foo<true>();
      |                        ^
<source>:11:3: note: candidate: 'template<ARGS ...args> static void Foo<ARGS>::foo() [with ARGS ...args = {args ...}; ARGS = {bool}]'
   11 |   foo(void)
      |   ^~~
<source>:11:3: note:   template argument deduction/substitution failed:
<source>:21:31: error: no matching function for call to 'Foo<int, long int>::foo<42, 23>()'
   21 |   Foo<int, long>::foo<42, 23>();
      |                               ^
<source>:11:3: note: candidate: 'template<ARGS ...args> static void Foo<ARGS>::foo() [with ARGS ...args = {args ...}; ARGS = {int, long int}]'
   11 |   foo(void)
      |   ^~~
<source>:11:3: note:   template argument deduction/substitution failed:
<source>:21:31: error: wrong number of template arguments (2, should be 1)
   21 |   Foo<int, long>::foo<42, 23>();
      |                               ^
<source>:22:34: error: no matching function for call to 'Foo<char, char>::foo<'h', 'i'>()'
   22 |   Foo<char, char>::foo<'h', 'i'>();
      |                                  ^
<source>:11:3: note: candidate: 'template<ARGS ...args> static void Foo<ARGS>::foo() [with ARGS ...args = {args ...}; ARGS = {char, char}]'
   11 |   foo(void)
      |   ^~~
<source>:11:3: note:   template argument deduction/substitution failed:
<source>:22:34: error: wrong number of template arguments (2, should be 1)
   22 |   Foo<char, char>::foo<'h', 'i'>();
      |                                  ^
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Clang correctly compiles the file.