[Bug c++/66572] [6 Regression] Bogus Wlogical-op warning for operands coming from template instantiations

[miyuki at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66572

--- Comment #3 from Mikhail Maltsev <miyuki at gcc dot gnu.org> ---
Started with r222408. The commit message says:

PR c/63357
* c-common.c (warn_logical_operator): Warn if the operands have the same
expressions.
* doc/invoke.texi: Update description of -Wlogical-op.

Apperntly, the warning about same expressions in a logical expression was added
to Wlogical-op after GCC 5.1 release (I didn't know that) and it had this
problem from the very beginning.

In warn_logical_operator we have a check:

  /* We do not warn for constants because they are typical of macro
     expansions that test for features.  */
  if (CONSTANT_CLASS_P (op_left) || CONSTANT_CLASS_P (op_right))
    return;

But in this case op_left and op_right are:
<var_decl 0x7ffff6118bd0 value
    type <boolean_type 0x7ffff6263888 bool readonly public unsigned QI
        size <integer_cst 0x7ffff610bf48 constant 8>
        unit size <integer_cst 0x7ffff610bf60 constant 1>
        align 8 symtab 0 alias set -1 canonical type 0x7ffff6263888 precision 1
min <integer_cst 0x7ffff612e1b0 0> max <integer_cst 0x7ffff612e1e0 1>>

I think it's still possible to fix this check to handle C++ boolean constants
properly (as if they were literal constants). But for general case we probably
must somehow know (e.g. add a new flag to enum tsubst_flags and pass it to
warn_logical_operator from build_new_op_1), that current expression depends on
template parameter.