[Bug c++/80971] New: [7/8 Regression] ICE with 'if constexpr' in template function

[reichelt at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80971

            Bug ID: 80971
           Summary: [7/8 Regression] ICE with 'if constexpr' in template
                    function
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: ice-on-valid-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: reichelt at gcc dot gnu.org
  Target Milestone: ---

The following valid code snippet (compiled with -std=c++1z)
triggers an ICE since GCC 7.1.0:

======================================================
struct A
{
  constexpr operator bool() const { return true; }
};

template<int> bool foo()
{
  if constexpr (A())
    return false;
  return true;
}
======================================================

bug.cc: In function 'bool foo()':
bug.cc:8:20: internal compiler error: unexpected expression 'A()' of kind
cast_expr
   if constexpr (A())
                    ^
0x6461c7 cxx_eval_constant_expression
        ../../gcc/gcc/cp/constexpr.c:4592
0x649557 cxx_eval_outermost_constant_expr
        ../../gcc/gcc/cp/constexpr.c:4652
0x7fca49 finish_if_stmt_cond(tree_node*, tree_node*)
        ../../gcc/gcc/cp/semantics.c:736
0x74552c cp_parser_selection_statement
        ../../gcc/gcc/cp/parser.c:11172
0x74552c cp_parser_statement
        ../../gcc/gcc/cp/parser.c:10583
0x745fbd cp_parser_statement_seq_opt
        ../../gcc/gcc/cp/parser.c:11054
0x74608f cp_parser_compound_statement
        ../../gcc/gcc/cp/parser.c:11008
0x760853 cp_parser_function_body
        ../../gcc/gcc/cp/parser.c:21445
0x760853 cp_parser_ctor_initializer_opt_and_function_body
        ../../gcc/gcc/cp/parser.c:21483
0x765e61 cp_parser_function_definition_after_declarator
        ../../gcc/gcc/cp/parser.c:26290
0x74f8bd cp_parser_function_definition_from_specifiers_and_declarator
        ../../gcc/gcc/cp/parser.c:26202
0x74f8bd cp_parser_init_declarator
        ../../gcc/gcc/cp/parser.c:19168
0x74fe3a cp_parser_single_declaration
        ../../gcc/gcc/cp/parser.c:26748
0x76910c cp_parser_template_declaration_after_parameters
        ../../gcc/gcc/cp/parser.c:26352
0x768d94 cp_parser_explicit_template_declaration
        ../../gcc/gcc/cp/parser.c:26587
0x768d94 cp_parser_template_declaration_after_export
        ../../gcc/gcc/cp/parser.c:26606
0x741c79 cp_parser_declaration
        ../../gcc/gcc/cp/parser.c:12472
0x77753b cp_parser_declaration_seq_opt
        ../../gcc/gcc/cp/parser.c:12399
0x77781a cp_parser_translation_unit
        ../../gcc/gcc/cp/parser.c:4364
0x77781a c_parse_file()
        ../../gcc/gcc/cp/parser.c:38475
Please submit a full bug report, [etc.]