c++: Deprecate arithmetic convs on different enums [PR97573]

c++: Deprecate arithmetic convs on different enums [PR97573]

I noticed that C++20 P1120R0 deprecated certain arithmetic conversions
as outlined in [depr.arith.conv.enum], but we don't warn about them.  In
particular, "If one operand is of enumeration type and the other operand
is of a different enumeration type or a floating-point type, this
behavior is deprecated."  These will likely become ill-formed in C++23,
so we should warn by default in C++20.  To this effect, this patch adds
two new warnings (like clang++): -Wdeprecated-enum-enum-conversion and
-Wdeprecated-enum-float-conversion.  They are enabled by default in
C++20.  In older dialects, to enable these warnings you can now use
-Wenum-conversion which I made available in C++ too.  Note that unlike
C, in C++ it is not enabled by -Wextra, because that breaks bootstrap.

We already warn about comparisons of two different enumeration types via
-Wenum-compare, the rest is handled in this patch: we're performing the
usual arithmetic conversions in these contexts:
  - an arithmetic operation,
  - a bitwise operation,
  - a comparison,
  - a conditional operator,
  - a compound assign operator.

Using the spaceship operator as enum <=> real_type is ill-formed but we
don't reject it yet.  We should also address [depr.array.comp] too, but
it's not handled in this patch.

gcc/c-family/ChangeLog:

	PR c++/97573
	* c-opts.c (c_common_post_options): In C++20, turn on
	-Wdeprecated-enum-enum-conversion and
	-Wdeprecated-enum-float-conversion.
	* c.opt (Wdeprecated-enum-enum-conversion,
	Wdeprecated-enum-float-conversion): New options.
	(Wenum-conversion): Allow for C++ too.

gcc/cp/ChangeLog:

	PR c++/97573
	* call.c (build_conditional_expr_1): Warn about the deprecated
	enum/real type conversion in C++20.  Also warn about a non-enumerated
	and enumerated type in ?: when -Wenum-conversion is on.
	* typeck.c (do_warn_enum_conversions): New function.
	(cp_build_binary_op): Call it.

gcc/ChangeLog:

	PR c++/97573
	* doc/invoke.texi: Document -Wdeprecated-enum-enum-conversion
	and -Wdeprecated-enum-float-conversion.  -Wenum-conversion is
	no longer C/ObjC only.

gcc/testsuite/ChangeLog:

	PR c++/97573
	* g++.dg/cpp0x/linkage2.C: Add dg-warning.
	* g++.dg/parse/attr3.C: Likewise.
	* g++.dg/cpp2a/enum-conv1.C: New test.
	* g++.dg/cpp2a/enum-conv2.C: New test.
	* g++.dg/cpp2a/enum-conv3.C: New test.