On 3/3/23 12:51, Marek Polacek wrote:
Similarly to PR107938, this also started with r11-557, whereby cp_finish_decl can call check_initializer even in a template for a constexpr initializer.Here we are rejecting extern const Q q; template<int> constexpr auto p = q(0); even though q has a constexpr operator(). It's deemed non-const by decl_maybe_constant_var_p because even though 'q' is const it is not of integral/enum type. I think the fix is for p_c_e to treat q(0) as potentially-constant, as below. Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/12? PR c++/107939 gcc/cp/ChangeLog: * constexpr.cc (is_constexpr_function_object): New. (potential_constant_expression_1): Treat an object with constexpr operator() as potentially-constant. gcc/testsuite/ChangeLog: * g++.dg/cpp1y/var-templ74.C: Remove dg-error. * g++.dg/cpp1y/var-templ77.C: New test. --- gcc/cp/constexpr.cc | 23 ++++++++++++++++++++++- gcc/testsuite/g++.dg/cpp1y/var-templ74.C | 2 +- gcc/testsuite/g++.dg/cpp1y/var-templ77.C | 14 ++++++++++++++ 3 files changed, 37 insertions(+), 2 deletions(-) create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ77.C diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc index acf9847a4d1..7d786f332b4 100644 --- a/gcc/cp/constexpr.cc +++ b/gcc/cp/constexpr.cc @@ -8929,6 +8929,24 @@ check_for_return_continue (tree *tp, int *walk_subtrees, void *data) return NULL_TREE; }+/* Return true iff TYPE is a class with constexpr operator(). */+ +static bool +is_constexpr_function_object (tree type) +{ + if (!CLASS_TYPE_P (type)) + return false; + + for (tree f = TYPE_FIELDS (type); f; f = DECL_CHAIN (f)) + if (TREE_CODE (f) == FUNCTION_DECL + && DECL_OVERLOADED_OPERATOR_P (f) + && DECL_OVERLOADED_OPERATOR_IS (f, CALL_EXPR) + && DECL_DECLARED_CONSTEXPR_P (f)) + return true; + + return false; +} + /* Return true if T denotes a potentially constant expression. Issue diagnostic as appropriate under control of FLAGS. If WANT_RVAL is true, an lvalue-rvalue conversion is implied. If NOW is true, we want to @@ -9160,7 +9178,10 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now, } else if (fun) { - if (RECUR (fun, rval)) + if (VAR_P (fun) + && is_constexpr_function_object (TREE_TYPE (fun))) + /* Could be an object with constexpr operator(). */;
I guess if fun is not a function pointer, we don't know if we're using it as an lvalue or rvalue, so we want to pass 'any' for want_rval, which should make this work; I don't think we need to be specific about constexpr op(), as a constexpr conversion operator to fn* could also do the trick.
+ else if (RECUR (fun, rval)) /* Might end up being a constant function pointer. */; else return false; diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ74.C b/gcc/testsuite/g++.dg/cpp1y/var-templ74.C index 4e2e800a6eb..c76a7d949ac 100644 --- a/gcc/testsuite/g++.dg/cpp1y/var-templ74.C +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ74.C @@ -9,7 +9,7 @@ struct Q { extern const Q q;template<int>-constexpr const Q* p = q(0); // { dg-bogus "not usable" "PR107939" { xfail *-*-* } } +constexpr const Q* p = q(0);voidg () diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ77.C b/gcc/testsuite/g++.dg/cpp1y/var-templ77.C new file mode 100644 index 00000000000..b480f54b001 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ77.C @@ -0,0 +1,14 @@ +// PR c++/107939 +// { dg-do compile { target c++14 } } + +struct Q { + struct P { + const Q* p; + }; + int n; + constexpr P operator()(int) const { return {this}; } +}; + +extern const Q q; +template<int> +constexpr auto p = q(0); base-commit: 9056d0df830c5a295d7594d517d409d10476990d
