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c++: Fix constexpr vs. reference parameter.

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[expr.const] specifically rules out mentioning a reference even if its
address is never used, because it implies indirection that is similarly
non-constant for a pointer variable.

	PR c++/66477
	* constexpr.c (cxx_eval_constant_expression) [PARM_DECL]: Don't
	defer loading the value of a reference.
This commit is contained in:
Jason Merrill 2020-02-03 16:03:45 -05:00
parent 8fda2c274a
commit 87fbd5347b
5 changed files with 54 additions and 4 deletions
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6
gcc/cp/ChangeLog
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@ -1,3 +1,9 @@
2020-02-03 Jason Merrill <jason@redhat.com>
PR c++/66477
* constexpr.c (cxx_eval_constant_expression) [PARM_DECL]: Don't
defer loading the value of a reference.
2020-02-03 Jason Merrill <jason@redhat.com>
PR c++/91953

2
gcc/cp/constexpr.c
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@ -5322,8 +5322,6 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
r = *p;
else if (lval)
/* Defer in case this is only used for its type. */;
else if (TYPE_REF_P (TREE_TYPE (t)))
/* Defer, there's no lvalue->rvalue conversion. */;
else if (COMPLETE_TYPE_P (TREE_TYPE (t))
&& is_really_empty_class (TREE_TYPE (t), /*ignore_vptr*/false))
{

2
gcc/testsuite/g++.dg/cpp0x/constexpr-array-ptr6.C
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@ -12,7 +12,7 @@ constexpr auto sz_d = size(array_double);
static_assert(sz_d == 3, "Array size failure");
void f(bool (&param)[2]) {
static_assert(size(param) == 2, "Array size failure"); // Line 13
static_assert(size(param) == 2, "Array size failure"); // { dg-error "" }
short data[] = {-1, 2, -45, 6, 88, 99, -345};
static_assert(size(data) == 7, "Array size failure");
}

46
gcc/testsuite/g++.dg/cpp0x/constexpr-ref12.C Normal file
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@ -0,0 +1,46 @@
// PR c++/66477
// { dg-do compile { target c++11 } }
struct a { constexpr bool g() const { return true; } };
constexpr bool g(a&) { return true;}
constexpr bool h(a) { return true;}
a a1;
a& ar = a1;
void f(a ap, a& arp)
{
a a2;
a& ar2 = a2;
// Most of these are OK because no data is actually loaded.
static_assert (a1.g(),"");
static_assert (g(a1),"");
static_assert (h(a1),"");
static_assert (a2.g(),"");
static_assert (g(a2),"");
static_assert (h(a2),"");
static_assert (ap.g(),"");
static_assert (g(ap),"");
static_assert (h(ap),"");
static_assert (ar.g(),"");
static_assert (g(ar),"");
static_assert (h(ar),"");
// But these are specifically prohibited in [expr.const]/4.12:
// * an id-expression that refers to a variable or data member of reference
// type unless the reference has a preceding initialization and either
// ** it is usable in constant expressions or
// ** its lifetime began within the evaluation of e;
static_assert (ar2.g(),""); // { dg-error "constant" }
static_assert (g(ar2),""); // { dg-error "constant" }
static_assert (h(ar2),""); // { dg-error "constant" }
static_assert (arp.g(),""); // { dg-error "constant" }
static_assert (g(arp),""); // { dg-error "constant" }
static_assert (h(arp),""); // { dg-error "constant" }
}

2
gcc/testsuite/g++.dg/cpp1y/lambda-generic-const10.C
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@ -11,7 +11,7 @@ int main()
constexpr auto x = f(); //ok, call constexpr const non-static method
[](auto const &f) {
constexpr auto x = f(); /*ok*/
constexpr auto x = f(); // { dg-error "" }
}(f);
[&]() {