Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
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++.
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
