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c++: Detect long double -> double narrowing [PR94590]

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This PR points out that we don't detect long double -> double narrowing
when long double happens to have the same precision as double; on x86_64
this can be achieved by -mlong-double-64.

[dcl.init.list]#7.2 specifically says "from long double to double or float,
or from double to float", but check_narrowing only checks

  TYPE_PRECISION (type) < TYPE_PRECISION (ftype)

so we need to handle the other cases too, e.g. by same_type_p as in
the following patch.

	PR c++/94590 - Detect long double -> double narrowing.
	* typeck2.c (check_narrowing): Detect long double -> double
	narrowing even when double and long double have the same
	precision.  Make it handle conversions to float too.

	* g++.dg/cpp0x/Wnarrowing18.C: New test.
This commit is contained in:
Marek Polacek 2020-04-19 18:46:40 -04:00
parent 21968d4ae0
commit 5d5dcc65aa
4 changed files with 35 additions and 1 deletions
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7
gcc/cp/ChangeLog
View File

@ -1,3 +1,10 @@
2020-05-07 Marek Polacek <polacek@redhat.com>
PR c++/94590 - Detect long double -> double narrowing.
* typeck2.c (check_narrowing): Detect long double -> double
narrowing even when double and long double have the same
precision. Make it handle conversions to float too.
2020-05-07 Marek Polacek <polacek@redhat.com>
PR c++/94255

9
gcc/cp/typeck2.c
View File

@ -1008,10 +1008,17 @@ check_narrowing (tree type, tree init, tsubst_flags_t complain,
|| !int_fits_type_p (init, type)))
ok = false;
}
/* [dcl.init.list]#7.2: "from long double to double or float, or from
double to float". */
else if (TREE_CODE (ftype) == REAL_TYPE
&& TREE_CODE (type) == REAL_TYPE)
{
if (TYPE_PRECISION (type) < TYPE_PRECISION (ftype))
if ((same_type_p (ftype, long_double_type_node)
&& (same_type_p (type, double_type_node)
|| same_type_p (type, float_type_node)))
|| (same_type_p (ftype, double_type_node)
&& same_type_p (type, float_type_node))
|| (TYPE_PRECISION (type) < TYPE_PRECISION (ftype)))
{
if (TREE_CODE (init) == REAL_CST)
{

5
gcc/testsuite/ChangeLog
View File

@ -1,3 +1,8 @@
2020-05-07 Marek Polacek <polacek@redhat.com>
PR c++/94590 - Detect long double -> double narrowing.
* g++.dg/cpp0x/Wnarrowing18.C: New test.
2020-05-07 Marek Polacek <polacek@redhat.com>
PR c++/94255

15
gcc/testsuite/g++.dg/cpp0x/Wnarrowing18.C Normal file
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@ -0,0 +1,15 @@
// PR c++/94590 - Detect long double -> double narrowing.
// { dg-do compile { target c++11 } }
// { dg-additional-options "-mlong-double-64" { target x86_64-*-* i?86-*-* } }
int
main ()
{
using T = long double;
extern long double ld;
extern T ld2;
extern const T ld3;
double d{ld}; // { dg-error "narrowing conversion" }
double d2{ld2}; // { dg-error "narrowing conversion" }
double d3{ld3}; // { dg-error "narrowing conversion" }
}