Hi Marek!
On 2023-09-01T20:00:01-0400, Marek Polacek <[email protected]> wrote:
> When verify_constant complains, it's pretty terse. Consider
>
> void test ()
> {
> constexpr int i = 42;
> constexpr const int *p = &i;
> }
>
> where it says "'& i' is not a constant expression". OK, but why?
>
> With this patch, we say:
>
> b.C:5:28: error: '& i' is not a constant expression
> 5 | constexpr const int *p = &i;
> | ^~
> b.C:5:28: note: pointer to 'i' is not a constant expression
> b.C:4:17: note: address of non-static constexpr variable 'i' may differ on
> each invocation of the enclosing function; add 'static' to give it a constant
> address
> 4 | constexpr int i = 42;
> | ^
> | static
>
> which brings g++ on par with clang++.
I noticed that for the following variant code of your
'g++.dg/diagnostic/constexpr3.C':
struct A {
A *ap = this;
};
constexpr A *
foo (A *a)
{
return a;
}
void
test ()
{
A a_;
constexpr A *a = foo (&a_); // { dg-error "not a constant expression" }
// { dg-message "pointer to .a_. is not a constant expression|may differ"
"" { xfail *-*-* } .-1 }
}
..., GCC still only diagnoses:
constexpr3_.C:18:25: error: '& a_' is not a constant expression
..., but doesn't provide any further details (XFAILed 'dg-message').
Compare to Clang:
constexpr3_.C:18:16: error: constexpr variable 'a' must be initialized by a
constant expression
18 | constexpr A *a = foo (&a_);
| ^ ~~~~~~~~~
constexpr3_.C:18:16: note: pointer to 'a_' is not a constant expression
constexpr3_.C:17:5: note: declared here
17 | A a_;
| ^
Is (a) my reasoning/expectation correct, and (b) this worth addressing?
Grüße
Thomas
> gcc/cp/ChangeLog:
>
> * constexpr.cc (verify_constant_explain_r): New.
> (verify_constant): Call it.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/diagnostic/constexpr3.C: New test.
> ---
> gcc/cp/constexpr.cc | 56 +++++++++++++++++++-
> gcc/testsuite/g++.dg/diagnostic/constexpr3.C | 32 +++++++++++
> 2 files changed, 87 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/g++.dg/diagnostic/constexpr3.C
>
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 8bd5c4a47f8..6d5aed82377 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -3381,6 +3381,54 @@ ok:
> }
> }
>
> +/* *TP was not deemed constant by reduced_constant_expression_p. Explain
> + why and suggest what could be done about it. */
> +
> +static tree
> +verify_constant_explain_r (tree *tp, int *, void *)
> +{
> + bool ref_p = false;
> +
> + switch (TREE_CODE (*tp))
> + {
> + CASE_CONVERT:
> + if (TREE_CODE (TREE_OPERAND (*tp, 0)) != ADDR_EXPR)
> + break;
> + ref_p = TYPE_REF_P (TREE_TYPE (*tp));
> + *tp = TREE_OPERAND (*tp, 0);
> + gcc_fallthrough ();
> + case ADDR_EXPR:
> + {
> + tree op = TREE_OPERAND (*tp, 0);
> + if (VAR_P (op)
> + && DECL_DECLARED_CONSTEXPR_P (op)
> + && !TREE_STATIC (op)
> + /* ??? We should also say something about temporaries. */
> + && !DECL_ARTIFICIAL (op))
> + {
> + if (ref_p)
> + inform (location_of (*tp), "reference to %qD is not a constant "
> + "expression", op);
> + else
> + inform (location_of (*tp), "pointer to %qD is not a constant "
> + "expression", op);
> + const location_t op_loc = DECL_SOURCE_LOCATION (op);
> + rich_location richloc (line_table, op_loc);
> + richloc.add_fixit_insert_before (op_loc, "static ");
> + inform (&richloc,
> + "address of non-static constexpr variable %qD may differ on
> "
> + "each invocation of the enclosing function; add %<static%> "
> + "to give it a constant address", op);
> + }
> + break;
> + }
> + default:
> + break;
> + }
> +
> + return NULL_TREE;
> +}
> +
> /* Some expressions may have constant operands but are not constant
> themselves, such as 1/0. Call this function to check for that
> condition.
> @@ -3398,7 +3446,13 @@ verify_constant (tree t, bool allow_non_constant, bool
> *non_constant_p,
> && t != void_node)
> {
> if (!allow_non_constant)
> - error ("%q+E is not a constant expression", t);
> + {
> + auto_diagnostic_group d;
> + error_at (cp_expr_loc_or_input_loc (t),
> + "%q+E is not a constant expression", t);
> + cp_walk_tree_without_duplicates (&t, verify_constant_explain_r,
> + nullptr);
> + }
> *non_constant_p = true;
> }
> if (TREE_OVERFLOW_P (t))
> diff --git a/gcc/testsuite/g++.dg/diagnostic/constexpr3.C
> b/gcc/testsuite/g++.dg/diagnostic/constexpr3.C
> new file mode 100644
> index 00000000000..b6e43a93664
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/diagnostic/constexpr3.C
> @@ -0,0 +1,32 @@
> +// { dg-do compile { target c++14 } }
> +
> +struct X {
> + int const& var;
> +};
> +
> +struct A {
> + A *ap = this;
> +};
> +
> +constexpr A
> +foo ()
> +{
> + return {};
> +}
> +
> +void
> +test ()
> +{
> + constexpr int i = 42; // { dg-message "may differ on each invocation" }
> +
> + constexpr X x{i}; // { dg-error "not a constant expression" }
> + // { dg-message "reference to .i. is not a constant expression" "" {
> target *-*-* } .-1 }
> + constexpr const int *p = &i; // { dg-error "not a constant expression" }
> + // { dg-message "pointer to .i. is not a constant expression" "" { target
> *-*-* } .-1 }
> +
> + constexpr A a = foo (); // { dg-error "not a constant expression" }
> + // { dg-message "pointer to .a. is not a constant expression|may differ"
> "" { target *-*-* } .-1 }
> +
> + constexpr const int *q = __builtin_launder (&i); // { dg-error "not a
> constant expression" }
> + // { dg-message "pointer to .i. is not a constant expression" "" { target
> *-*-* } .-1 }
> +}
>
> base-commit: 6f06152541d62ae7c8579b7d7bf552be19e15b05
> --
> 2.41.0
