Re: [PATCH] c++: FIX_TRUNC_EXPR in tsubst [PR102990]

[Marek Polacek <polacek@redhat.com>]

On Wed, Mar 23, 2022 at 04:35:32PM -0400, Jason Merrill wrote:
> On 3/22/22 19:55, Marek Polacek wrote:
> > This is a crash where a FIX_TRUNC_EXPR gets into tsubst_copy_and_build
> > where it hits gcc_unreachable ().
> > 
> > The history of tsubst_copy_and_build/FIX_TRUNC_EXPR is such that it
> > was introduced in r181478, but it did the wrong thing, whereupon it
> > was turned into gcc_unreachable () in r258821 (see this thread:
> > <https://gcc.gnu.org/pipermail/gcc-patches/2018-March/495853.html>).
> > 
> > In a template, we should never create a FIX_TRUNC_EXPR (that's what
> > conv_unsafe_in_template_p is for).  But in this test we are NOT in
> > a template when we call digest_nsdmi_init which ends up calling
> > convert_like, converting 1.0e+0 to int, so convert_to_integer_1
> > gives us a FIX_TRUNC_EXPR.
> > 
> > But then when we get to parsing f's parameters, we are in a template
> > when processing decltype(Helpers{}), and since r268321, when the
> > compound literal isn't instantiation-dependent and the type isn't
> > type-dependent, finish_compound_literal falls back to the normal
> > processing, so it calls digest_init, which does fold_non_dependent_init
> > and since the FIX_TRUNC_EXPR isn't dependent, we instantiate and
> > therefore crash in tsubst_copy_and_build.
> 
> Hmm, we shouldn't be doing fold_non_dependent_init on the result of
> get_nsdmi.  Why does that happen?

OK, so we have decltype(Helpers{}), finish_compound_literal gets
Helpers{}, it's not type-dependent, so:

- call digest_init (type=Helpers, init={})
  init is BRACE_ENCLOSED_INITIALIZER_P, type is !TYPE_NON_AGGREGATE_CLASS
  so go down to...
- process_init_constructor_record (type=Helpers, init={})
   - we walk the fields of Helpers, there's 'inputs' of type knob_t
     type_build_ctor_call (knob_t) is true, we want to default-init this
     field
   - create a {} as the init for default-initialization
   - call massage_init_elt (type=knob_t, init={}) to adjust the init
     - we do so by calling digest_init_r (type=knob_t, init={})
       - here we again call process_init_constructor_record, walk knob_t's
         fields, see the field 'value', it has a DECL_INITIAL, so call
	 get_nsdmi, that returns the FIX_TRUNC_EXPR we've created before
       - so digesting {} for knob_t produced
         init = {.value=(int) NON_LVALUE_EXPR <1.0e+0>}
     - then we call fold_non_dependent_init on this init, and die

Do we have a mechanism to avoid folding get_nsdmi's trees that I've missed?
 
> > Either I can tweak p_c_e to say that a FIX_TRUNC_EXPR in a template
> > is not potentially constant, or I can just remove the whole F_T_E
> > case, since:
> > a) we could not have created an IMPLICIT_CONV_EXPR here, and
> > b) similar code, FLOAT_EXPR, is not handled here, either.
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/11?
> > 
> > 	PR c++/102990
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* pt.cc (tsubst_copy_and_build) <case FIX_TRUNC_EXPR>: Remove.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/cpp0x/nsdmi-template22.C: New test.
> > 	* g++.dg/cpp0x/nsdmi-template23.C: New test.
> > ---
> >   gcc/cp/pt.cc                                  |  4 ----
> >   gcc/testsuite/g++.dg/cpp0x/nsdmi-template22.C | 13 +++++++++++++
> >   gcc/testsuite/g++.dg/cpp0x/nsdmi-template23.C | 13 +++++++++++++
> >   3 files changed, 26 insertions(+), 4 deletions(-)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp0x/nsdmi-template22.C
> >   create mode 100644 gcc/testsuite/g++.dg/cpp0x/nsdmi-template23.C
> > 
> > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > index 715eea27577..a3becc19290 100644
> > --- a/gcc/cp/pt.cc
> > +++ b/gcc/cp/pt.cc
> > @@ -20184,10 +20184,6 @@ tsubst_copy_and_build (tree t,
> >   				templated_operator_saved_lookups (t),
> >   				complain|decltype_flag));
> > -    case FIX_TRUNC_EXPR:
> > -      /* convert_like should have created an IMPLICIT_CONV_EXPR.  */
> > -      gcc_unreachable ();
> > -
> >       case ADDR_EXPR:
> >         op1 = TREE_OPERAND (t, 0);
> >         if (TREE_CODE (op1) == LABEL_DECL)
> > diff --git a/gcc/testsuite/g++.dg/cpp0x/nsdmi-template22.C b/gcc/testsuite/g++.dg/cpp0x/nsdmi-template22.C
> > new file mode 100644
> > index 00000000000..4ed2501035c
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp0x/nsdmi-template22.C
> > @@ -0,0 +1,13 @@
> > +// PR c++/102990
> > +// { dg-do compile { target c++11 } }
> > +
> > +struct knob_t {
> > +  /* Let's create a FIX_TRUNC_EXPR.  */
> > +  int value = 1.0;
> > +};
> > +
> > +struct Helpers {
> > +  knob_t inputs;
> > +};
> > +
> > +template<class> void f(decltype(Helpers{}));
> > diff --git a/gcc/testsuite/g++.dg/cpp0x/nsdmi-template23.C b/gcc/testsuite/g++.dg/cpp0x/nsdmi-template23.C
> > new file mode 100644
> > index 00000000000..240cab4347a
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp0x/nsdmi-template23.C
> > @@ -0,0 +1,13 @@
> > +// PR c++/102990
> > +// { dg-do compile { target c++11 } }
> > +
> > +struct knob_t {
> > +  /* Let's create a FLOAT_EXPR.  */
> > +  double value = 1UL;
> > +};
> > +
> > +struct Helpers {
> > +  knob_t inputs;
> > +};
> > +
> > +template<class> void f(decltype(Helpers{}));
> > 
> > base-commit: 5d2233f4033dfa37ad88dc2eab138524fe64242e
> 

Marek