[Bug c++/71821] New: ICE on invalid C++11 code (incorrect argument for alignas): unexpected expression ‘f’ of kind template_id_expr

[su at cs dot ucdavis.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71821

            Bug ID: 71821
           Summary: ICE on invalid C++11 code (incorrect argument for
                    alignas): unexpected expression ‘f<int>’ of kind
                    template_id_expr
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu
  Target Milestone: ---

The following C++11 code causes an ICE when compiled with the current GCC trunk
on x86_64-linux-gnu in both 32-bit and 64-bit modes.  

It affects all versions since 4.8.x, and is rejected by 4.7.x and before (as
alignas wasn't yet unsupported). 


$ g++-trunk -v  
Using built-in specs.
COLLECT_GCC=g++-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/7.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-source-trunk/configure --enable-languages=c,c++,lto
--prefix=/usr/local/gcc-trunk --disable-bootstrap
Thread model: posix
gcc version 7.0.0 20160707 (experimental) [trunk revision 238117] (GCC) 
$ 
$ g++-trunk -c small.cpp
small.cpp:4:10: internal compiler error: unexpected expression ‘f<int>’ of kind
template_id_expr
 alignas (f < int >) char c;
          ^~~~~~~~~
0x8acd4c cxx_eval_constant_expression
        ../../gcc-source-trunk/gcc/cp/constexpr.c:4153
0x8b1ca9 cxx_eval_outermost_constant_expr
        ../../gcc-source-trunk/gcc/cp/constexpr.c:4211
0x79f4cd cp_parser_std_attribute_spec
        ../../gcc-source-trunk/gcc/cp/parser.c:24164
0x79f4cd cp_parser_std_attribute_spec_seq
        ../../gcc-source-trunk/gcc/cp/parser.c:24204
0x7a450f cp_parser_decl_specifier_seq
        ../../gcc-source-trunk/gcc/cp/parser.c:12575
0x7adc01 cp_parser_simple_declaration
        ../../gcc-source-trunk/gcc/cp/parser.c:12304
0x7ae051 cp_parser_block_declaration
        ../../gcc-source-trunk/gcc/cp/parser.c:12251
0x7b6cc0 cp_parser_declaration
        ../../gcc-source-trunk/gcc/cp/parser.c:12148
0x7b5794 cp_parser_declaration_seq_opt
        ../../gcc-source-trunk/gcc/cp/parser.c:12027
0x7b5ac8 cp_parser_translation_unit
        ../../gcc-source-trunk/gcc/cp/parser.c:4329
0x7b5ac8 c_parse_file()
        ../../gcc-source-trunk/gcc/cp/parser.c:37505
0x919b92 c_common_parse_file()
        ../../gcc-source-trunk/gcc/c-family/c-opts.c:1070
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <http://gcc.gnu.org/bugs.html> for instructions.
$ 


-----------------------------------------------------------


template < typename > constexpr int f () {  return 4; }

// should be: alignas (f < int >()) char c;
alignas (f < int >) char c;