On Fri, 28 Jun 2024, Simon Martin wrote:
> We currently ICE when emitting the error message for this invalid code:
>
> === cut here ===
> struct foo {
> template<int> void not_const() {}
> };
> void fn(const foo& obj) {
> obj.not_const<5>();
> }
> === cut here ===
>
> The problem is that get_fndecl_argument_location assumes that it has a
> FUNCTION_DECL in its hands to find the location of the bad argument. It might
> however have a TEMPLATE_DECL if there's a single candidate that cannot be
> instantiated, like here.
>
> This patch simply defaults to using the FNDECL's location in this case, which
> fixes this PR.
>
> Successfully tested on x86_64-pc-linux-gnu.
>
> PR c++/115364
>
> gcc/cp/ChangeLog:
>
> * call.cc (get_fndecl_argument_location): Use FNDECL's location for
> TEMPLATE_DECLs.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/overload/template7.C: New test.
>
> ---
> gcc/cp/call.cc | 4 ++++
> gcc/testsuite/g++.dg/overload/template7.C | 9 +++++++++
> 2 files changed, 13 insertions(+)
> create mode 100644 gcc/testsuite/g++.dg/overload/template7.C
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 7bbc1fb0c78..d5ff2311e63 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -8347,6 +8347,10 @@ get_fndecl_argument_location (tree fndecl, int argnum)
> if (DECL_ARTIFICIAL (fndecl))
> return DECL_SOURCE_LOCATION (fndecl);
>
> + /* Use FNDECL's location for TEMPLATE_DECLs. */
> + if (TREE_CODE (fndecl) == TEMPLATE_DECL)
> + return DECL_SOURCE_LOCATION (fndecl);
> +
For TEMPLATE_DECL fndecl, it'd be more natural to return the
corresponding argument location of its DECL_TEMPLATE_RESULT (which
should be a FUNCTION_DECL). The STRIP_TEMPLATE macro would be
convenient to use here.
It seems this doesn't fix the regression completely however because
in GCC 11 the code was rejected with a "permerror" (which can be
downgraded to a warning with -fpermissive):
115364.C: In function ‘void fn(const foo&)’:
115364.C:5:43: error: passing ‘const foo’ as ‘this’ argument discards
qualifiers [-fpermissive]
5 | void fn(const foo& obj) { obj.not_const<5>(); }
| ~~~~~~~~~~~~~~~~^~
115364.C:3:24: note: in call to ‘void foo::not_const() [with int
<anonymous> = 5]’
3 | template<int> void not_const() {}
| ^~~~~~~~~
and we now reject with an ordinary error:
115364.C: In function ‘void fn(const foo&)’:
115364.C:5:27: error: cannot convert ‘const foo*’ to ‘foo*’
5 | void fn(const foo& obj) { obj.not_const<5>(); }
| ^~~
| |
| const foo*
115364.C:3:24: note: initializing argument 'this' of ‘template<int
<anonymous> > void foo::not_const()’
3 | template<int> void not_const() {}
| ^~~~~~~~~
To restore the error into a permerror, we need to figure out why we're
unexpectedly hitting this code path with a TEMPLATE_DECL, and why it's
necessary that the member function needs to take no arguments. It turns
out I looked into this and submitted a patch for PR106760 (of which this
PR115364 is a dup) last year:
https://gcc.gnu.org/pipermail/gcc-patches/2023-June/620514.html
The patch was approved, but I lost track of it and never pushed it :/
I'm going to go ahead and push that fix shortly, sorry for not doing so
earlier. Thanks for looking into this issue!
> int i;
> tree param;
>
> diff --git a/gcc/testsuite/g++.dg/overload/template7.C
> b/gcc/testsuite/g++.dg/overload/template7.C
> new file mode 100644
> index 00000000000..67191c4ff62
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/overload/template7.C
> @@ -0,0 +1,9 @@
> +// PR c++/115364
> +// { dg-do compile }
> +
> +struct foo {
> + template<int> void not_const() {} // { dg-note "initializing" }
> +};
> +void fn(const foo& obj) {
> + obj.not_const<5>(); // { dg-error "cannot convert" }
> +}
> --
> 2.44.0
>
>
>
>
