On Thu, 2015-12-03 at 15:38 -0500, Jason Merrill wrote:
On 12/03/2015 09:55 AM, David Malcolm wrote:
Testcase g++.dg/template/ref3.C:
1 // PR c++/28341
2
3 template<const int&> struct A {};
4
5 template<typename T> struct B
6 {
7 A<(T)0> b; // { dg-error "constant|not a valid" }
8 A<T(0)> a; // { dg-error "constant|not a valid" }
9 };
10
11 B<const int&> b;
The output of this test for both c++11 and c++14 is unaffected
by the patch kit:
g++.dg/template/ref3.C: In instantiation of 'struct B<const int&>':
g++.dg/template/ref3.C:11:15: required from here
g++.dg/template/ref3.C:7:11: error: '0' is not a valid template argument for type 'const int&' because it is not an lvalue
g++.dg/template/ref3.C:8:11: error: '0' is not a valid template argument for type 'const int&' because it is not an lvalue
However, the c++98 output is changed:
Status quo for c++98:
g++.dg/template/ref3.C: In instantiation of 'struct B<const int&>':
g++.dg/template/ref3.C:11:15: required from here
g++.dg/template/ref3.C:7:11: error: a cast to a type other than an integral or enumeration type cannot appear in a constant-expression
g++.dg/template/ref3.C:8:11: error: a cast to a type other than an integral or enumeration type cannot appear in a constant-expression
(line 7 and 8 are at the closing semicolon for fields b and a)
With the patchkit for c++98:
g++.dg/template/ref3.C: In instantiation of 'struct B<const int&>':
g++.dg/template/ref3.C:11:15: required from here
g++.dg/template/ref3.C:7:5: error: a cast to a type other than an integral or enumeration type cannot appear in a constant-expression
g++.dg/template/ref3.C:7:5: error: a cast to a type other than an integral or enumeration type cannot appear in a constant-expression
So the 2nd:
"error: a cast to a type other than an integral or enumeration type cannot appear in a constant-expression"
moves from line 8 to line 7 (and moves them to earlier, having ranges)
What's happening is that cp_parser_enclosed_template_argument_list
builds a CAST_EXPR, the first time from cp_parser_cast_expression,
the second time from cp_parser_functional_cast; these have locations
representing the correct respective caret&ranges, i.e.:
A<(T)0> b;
^~~~
and:
A<T(0)> a;
^~~~
Eventually finish_template_type is called for each, to build a RECORD_TYPE,
and we get a cache hit the 2nd time through here in pt.c:
8281 hash = spec_hasher::hash (&elt);
8282 entry = type_specializations->find_with_hash (&elt, hash);
8283
8284 if (entry)
8285 return entry->spec;
due to:
template_args_equal (ot=<cast_expr 0x7ffff19bc400>, nt=<cast_expr 0x7ffff19bc480>) at ../../src/gcc/cp/pt.c:7778
which calls:
cp_tree_equal (t1=<cast_expr 0x7ffff19bc400>, t2=<cast_expr 0x7ffff19bc480>) at ../../src/gcc/cp/tree.c:2833
and returns equality.
Hence we get a single RECORD_TYPE for the type A<(T)(0)>, and hence
when issuing the errors it uses the TREE_VEC for the first one,
using the location of the first line.
Why does the type sharing affect where the parser gives the error?
I believe what's happening is that the patchkit is setting location_t
values for more expressions than before, including the expression for
the template param. pt.c:tsubst_expr has this:
if (EXPR_HAS_LOCATION (t))
input_location = EXPR_LOCATION (t);
I believe that before (in the status quo), the substituted types didn't
have location_t values, and hence the above conditional didn't fire;
input_location was coming from a *token* where the expansion happened,
hence we got an error message on the relevant line for each expansion.
With the patch, the substituted types have location_t values within
their params, hence the conditional above fires: input_location is
updated to use the EXPR_LOCATION, which comes from that of the param
within the type - but with type-sharing it's using the first place where
the type is created.
Perhaps a better fix is for cp_parser_non_integral_constant_expression
to take a location_t, rather than have it rely on input_location?