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From: Marek Polacek <mpolacek@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org Subject: [gcc r14-3718] c++: improve verify_constant diagnostic [PR91483] Date: Tue, 5 Sep 2023 18:30:09 +0000 (GMT) [thread overview] Message-ID: <20230905183009.6C32E3858D32@sourceware.org> (raw) https://gcc.gnu.org/g:b78cedc6b5bc062717a3e0efb10da8e19af1c422 commit r14-3718-gb78cedc6b5bc062717a3e0efb10da8e19af1c422 Author: Marek Polacek <polacek@redhat.com> Date: Fri Sep 1 17:26:01 2023 -0400 c++: improve verify_constant diagnostic [PR91483] 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++. PR c++/91483 gcc/cp/ChangeLog: * constexpr.cc (verify_constant_explain_r): New. (verify_constant): Call it. gcc/testsuite/ChangeLog: * g++.dg/diagnostic/constexpr3.C: New test. Diff: --- gcc/cp/constexpr.cc | 63 +++++++++++++++++++++++++++- gcc/testsuite/g++.dg/diagnostic/constexpr3.C | 33 +++++++++++++++ 2 files changed, 95 insertions(+), 1 deletion(-) diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc index 8bd5c4a47f81..0ca4370deab7 100644 --- a/gcc/cp/constexpr.cc +++ b/gcc/cp/constexpr.cc @@ -3381,6 +3381,61 @@ 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 *walk_subtrees, void *) +{ + bool ref_p = false; + + /* No need to look into types or unevaluated operands. */ + if (TYPE_P (*tp) || unevaluated_p (TREE_CODE (*tp))) + { + *walk_subtrees = false; + return NULL_TREE; + } + + 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 +3453,13 @@ 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 000000000000..39ddf01a73c6 --- /dev/null +++ b/gcc/testsuite/g++.dg/diagnostic/constexpr3.C @@ -0,0 +1,33 @@ +// PR c++/91483 +// { 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 } +}
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