On 8/6/24 2:00 PM, Patrick Palka wrote:
On Tue, 6 Aug 2024, Jason Merrill wrote:
On 8/5/24 6:09 PM, Patrick Palka wrote:
On Mon, 5 Aug 2024, Jason Merrill wrote:
On 8/5/24 3:47 PM, Patrick Palka wrote:
On Mon, 5 Aug 2024, Jason Merrill wrote:
On 8/5/24 1:14 PM, Patrick Palka wrote:
On Mon, 5 Aug 2024, Jason Merrill wrote:
On 8/2/24 4:18 PM, Patrick Palka wrote:
On Fri, 2 Aug 2024, Patrick Palka wrote:
On Fri, 2 Aug 2024, Jason Merrill wrote:
On 8/1/24 2:52 PM, Patrick Palka wrote:
In recent versions of GCC we've been diagnosing more and
more
kinds of
errors inside a template ahead of time. This is a largely
good
thing
because it catches bugs, typos, dead code etc sooner.
But if the template never gets instantiated then such
errors
are
harmless, and can be inconvenient to work around if say
the
code
in
question is third party and in maintenence mode. So it'd
be
useful to
"maintenance"
Fixed
diff --git a/gcc/cp/error.cc b/gcc/cp/error.cc
index d80bac822ba..0bb0a482e28 100644
--- a/gcc/cp/error.cc
+++ b/gcc/cp/error.cc
@@ -165,6 +165,58 @@ class cxx_format_postprocessor :
public
format_postprocessor
deferred_printed_type m_type_b;
};
+/* A map from TEMPLATE_DECL to the location of the
first
error (if
any)
+ within the template that we permissivly downgraded to
a
warning.
*/
"permissively"
Fixed
+relaxed_template_errors_t *relaxed_template_errors;
+
+/* Callback function
diagnostic_context::m_adjust_diagnostic_info.
+
+ In -fpermissive mode we downgrade errors within a
template
to
+ warnings, and only issue an error if we later need to
instantiate
+ the template. */
+
+static void
+cp_adjust_diagnostic_info (diagnostic_context *context,
+ diagnostic_info *diagnostic)
+{
+ tree ti;
+ if (diagnostic->kind == DK_ERROR
+ && context->m_permissive
+ && !current_instantiation ()
+ && in_template_context
+ && (ti = get_template_info (current_scope ())))
+ {
+ if (!relaxed_template_errors)
+ relaxed_template_errors = new
relaxed_template_errors_t;
+
+ tree tmpl = TI_TEMPLATE (ti);
+ if (!relaxed_template_errors->get (tmpl))
+ relaxed_template_errors->put (tmpl,
diagnostic->richloc->get_loc ());
+ diagnostic->kind = DK_WARNING;
Rather than check m_permissive directly and downgrade to
DK_WARNING,
how
about
downgrading to DK_PERMERROR? That way people will get the
[-fpermissive]
clue.
...though I suppose DK_PERMERROR doesn't work where you call
this
hook
in
report_diagnostic, at which point we've already reassigned
it
into
DK_WARNING
or DK_ERROR in diagnostic_impl.
But we could still set diagnostic->option_index even for
DK_ERROR,
whether to
context->m_opt_permissive or to its own warning flag,
perhaps
-Wno-template-body?
Fixed by adding an enabled-by-default -Wtemplate-body flag and
setting
option_index to it for each downgraded error. Thus
-permissive
-Wno-template-body would suppress the downgraded warnings
entirely,
and
only issue a generic error upon instantiation of the erroneous
template.
... or did you have in mind to set option_index even when not
using
-fpermissive so that eligible non-downgraded errors get the
[-fpermissive] or [-Wtemplate-body] hint as well?
Yes.
IMHO I'm not sure that'd be worth the extra noise since the vast
majority of users appreciate and expect errors to get diagnosed
inside
templates.
But people trying to build legacy code should appreciate the
pointer
for
how
to make it compile, as with other permerrors.
And on second thought I'm not sure what extra value a new
warning
flag
adds either. I can't think of a good reason why one would use
-fpermissive -Wno-template-body?
One would use -Wno-template-body (or -Wno-error=template-body)
without
-fpermissive, like with the various permerror_opt cases.
Since compiling legacy/unmaintained code is the only plausible use
case,
why have a dedicated warning flag instead of just recommending
-fpermissive
when compiling legacy code? I don't quite understand the motivation
for
adding a new permerror_opt flag for this class of errors.
It seems to me an interesting class of errors, but I don't mind
leaving it
under just -fpermissive if you prefer.
-Wnarrowing is an existing permerror_opt flag, but I can imagine
it's
useful to pass -Wno-error=narrowing etc when incrementally migrating
C / C++98 code to modern C++ where you don't want any conformance
errors
allowed by -fpermissive to sneak in. So being able to narrowly
control
this class of errors seems useful, so a dedicated flag makes sense.
But there's no parallel for -Wtemplate-body here, since by
assumption
the code base is unmaintained / immutable. Otherwise the more
proper
fix would be to just fix and/or delete the uninstantiated erroneous
template. If say you're #including a legacy header that has such
errors, then doing #pragma GCC diagnostic "-fpermissive -w" around
the #include should be totally fine too.
I just realized #pragma GCC diagnostic warning "-fpermissive" etc
doesn't actually work since -fpermissive isn't a warning flag. So
having a dedicated flag for the class of errors has at least one clear
benefit -- we can use #pragmas to selectively disable the errors now.
I just don't see the use case for being able to narrowly control
this
class of errors that justifies the extra implementation complexity
(specifically for properly detecting -Wno-error=template-body in the
callback hook)?
The hook shouldn't need to do anything special; report_diagnostic
handles
-Wno-error=whatever.
The issue was that the callback has to know in advance whether
-Wno-error=template-body is active so that it can flag the template as
having a relaxed error. Checking !warning_enabled_at wasn't enough
because it will return true even for -Wno-error=template-body. I
think we just need to check diagnostic_enabled directly, like so?
Why not always flag it? It doesn't seem harmful to give the "instatiating
erroneous template" error later even if we gave a hard error during
parsing.
Hmm, it just seems redundant to me I guess? The reason we need the
"instantiating erroneous template" error is to ensure that _some_ error
was issued rendering the TU ill-formed when an erroneous template gets
instantiated. If parse-time errors are hard errors then this is
guaranteed, but it's not if we're downgrading such errors.
Yes, it's redundant, but avoiding it doesn't seem worth duplicating all the
logic to determine whether it will be an error or not.
Simpler might be to skip the "instantiating" error if seen_error()?
Sure. This makes the expected diagnostics in the -Wno-template-body
testcase a bit lacking since there's three instantiated templates with
one logical error each, but only the first error is diagnosed. But I
don't particularly mind that.
On that note it just occurred to me that we don't need to abort
instantiation after the "instantiating erroneous template" error --
for sake of error recovery we can proceed to instantiate as if we
issued a hard error at parse time.
Agreed.
Jason
-- >8 --
Subject: [PATCH] c++: permit errors inside uninstantiated templates [PR116064]
In recent versions of GCC we've been diagnosing more and more kinds of
errors inside a template ahead of time. This is a largely good thing
because it catches bugs, typos, dead code etc sooner.
But if the template never gets instantiated then such errors are harmless
and can be inconvenient to work around if say the code in question is
third party and in maintenance mode. So it'd be handy to be able to
prevent these template errors from rendering the entire TU uncompilable.
(Note that such code is "ill-formed no diagnostic required" according
the standard.)
To that end this patch turns any errors issued within a template into
premerrors associated with a new -Wtemplate-body flag so that they can
perm
be downgraded via e.g. -fpermissive or -Wno-template=template-body. If
the template containing a downgraded error later needs to be instantiated,
we'll issue an error then. But if the template never gets instantiated
then the downgraded error won't affect validity of the rest of the TU.
This is implemented via a diagnostic hook that gets called for each
diagnostic, and which adjusts an error diagnostic if we detect it's
occurring from a template context and additionally flags this template
context as erroneous.
As an example, permissive-error1a.C gives:
gcc/testsuite/g++.dg/template/permissive-error1a.C: In function 'void f()':
gcc/testsuite/g++.dg/template/permissive-error1a.C:7:5: warning: increment of
read-only variable 'n' [-Wtemplate-body]
7 | ++n;
| ^
...
gcc/testsuite/g++.dg/template/permissive-error1a.C: In instantiation of 'void
f() [with T = int]':
gcc/testsuite/g++.dg/template/permissive-error1a.C:26:9: required from here
26 | f<int>();
| ~~~~~~^~
gcc/testsuite/g++.dg/template/permissive-error1a.C:5:6: error: instantiating
erroneous template pattern
Drop "pattern" here too.
5 | void f() {
| ^
gcc/testsuite/g++.dg/template/permissive-error1a.C:7:5: note: first error
appeared here
7 | ++n; // {
| ^
...
What happens if we compile an interface/header unit with an erroneous
uninstantiated template? I'd probably reject such a module rather than
write out the erroneity. Possibly just if the template isn't discarded,
not sure if that distinction is too much trouble to bother with.
PR c++/116064
gcc/c-family/ChangeLog:
* c.opt (Wtemplate-body): New warning.
gcc/cp/ChangeLog:
* cp-tree.h (erroneous_templates_t): Declare.
(erroneous_templates): Declare.
(cp_seen_error): Declare.
(basic_seen_error): Define as alias to existing seen_error.
Or we could use (seen_error)() where we mean the basic one?
(seen_error): #define to cp_seen_error.
* error.cc (get_current_template): Define.
(relaxed_template_errors): Define.
(cp_adjust_diagnostic_info): Define.
(cp_seen_error): Define.
(cxx_initialize_diagnostics): Set
diagnostic_context::m_adjust_diagnostic_info.
* pt.cc (instantiate_class_template): Issue a hard error
when trying to instantiate a template pattern containing
a permissively downgraded error.
(instantiate_decl): Likewise.
gcc/ChangeLog:
* diagnostic.cc (diagnostic_context::initialize): Set
m_adjust_diagnostic_info.
(diagnostic_context::report_diagnostic): Call
m_adjust_diagnostic_info.
* diagnostic.h (diagnostic_context::diagnostic_enabled): Make
public.
You shouldn't need this change now?
+static void
+cp_adjust_diagnostic_info (diagnostic_context *context,
+ diagnostic_info *diagnostic)
+{
+ if (diagnostic->kind == DK_ERROR)
+ if (tree tmpl = get_current_template ())
+ {
+ diagnostic->option_index = OPT_Wtemplate_body;
+
+ if (context->m_permissive)
+ diagnostic->kind = DK_WARNING;
+
+ if (!erroneous_templates)
+ erroneous_templates = new erroneous_templates_t;
+ if (!erroneous_templates->get (tmpl))
+ {
+ /* Remember that this template had a parse-time error so
+ that we'll ensure a hard error has been issued upon
+ its instantiation. */
+ location_t error_loc = diagnostic->richloc->get_loc ();
+ erroneous_templates->put (tmpl, error_loc);
Instead of get+put you could use the get return value to store error_loc?
+@opindex Wtemplate-body
+@opindex Wno-template-body
+@item -Wno-template-body @r{(C++ and Objective-C++ only)}
+Disable diagnosing errors when parsing a template, and instead issue an
+error only upon instantiation of the template. This flag can also be
+used to downgrade such errors into warnings with @option{Wno-error=} or
+@option{-fpermissive}.
Please also refer to this flag in the -fpermissive documentation.
Jason
