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* [gcc r14-3718] c++: improve verify_constant diagnostic [PR91483]
@ 2023-09-05 18:30 Marek Polacek
0 siblings, 0 replies; only message in thread
From: Marek Polacek @ 2023-09-05 18:30 UTC (permalink / raw)
To: gcc-cvs
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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