On Fri, Mar 25, 2022 at 12:07:31PM -0400, Patrick Palka via Gcc-patches wrote:
> When an if-stmt is deemed non-constant because both of its branches are
> non-constant, we issue a rather generic error which, given that it points
> to the 'if' token, misleadingly suggests the condition is at fault:
>
> constexpr-105050.C:8:3: error: expression ‘<statement>’ is not a constant
> expression
> 8 | if (p != q && *p < 0)
> | ^~
>
> This patch clarifies the error message to read:
>
> constexpr-105050.C:8:3: error: neither branch of ‘if’ is a constant
> expression
> 8 | if (p != q && *p < 0)
> | ^~
>
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
LGTM.
> PR c++/105050
>
> gcc/cp/ChangeLog:
>
> * constexpr.cc (potential_constant_expression_1) <case IF_STMT>:
> Clarify error message when a if-stmt is non-constant because its
> branches are non-constant.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp1y/constexpr-105050.C: New test.
> ---
> gcc/cp/constexpr.cc | 7 ++++++-
> gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C | 12 ++++++++++++
> 2 files changed, 18 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C
>
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 778680b8270..9c40b051574 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -9439,7 +9439,12 @@ potential_constant_expression_1 (tree t, bool
> want_rval, bool strict, bool now,
> }
> }
> if (flags & tf_error)
> - error_at (loc, "expression %qE is not a constant expression", t);
> + {
> + if (TREE_CODE (t) == IF_STMT)
> + error_at (loc, "neither branch of %<if%> is a constant expression");
> + else
> + error_at (loc, "expression %qE is not a constant expression", t);
> + }
> return false;
>
> case VEC_INIT_EXPR:
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C
> b/gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C
> new file mode 100644
> index 00000000000..99d5c9960ac
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C
> @@ -0,0 +1,12 @@
> +// PR c++/105050
> +// { dg-do compile { target c++14 } }
> +
> +void g();
> +void h();
> +
> +constexpr void f(int* p, int *q) {
> + if (p != q && *p < 0) // { dg-error "neither branch of 'if' is a constant
> expression" }
> + g();
> + else
> + h();
> +}
> --
> 2.35.1.655.ga68dfadae5
>
Marek
