c++ - -O1/2/3 with -std=c++1y/11/98 - If <cmath> is included i'm getting error: '_hypot' was not declared in this scope

Question

I've just updated MinGW using mingw-get-setup and i'm unable to build anyting that contains <cmath> header if I use anything larger than -O0 with -std=c++1y. (I also tried c++11 and c++98) I'm getting errors like this one:

g++.exe -pedantic-errors -pedantic -Wextra -Wall -std=c++1y -O3  -c Z:ProjectsC++L6srcevents.cpp -o objsrcevents.o
In file included from z:landermingwlibgccmingw324.8.1includec++cmath:44:0,
                 from Z:ProjectsC++L6srcutils.h:4,
                 from Z:ProjectsC++L6srcevents.cpp:10:
z:landermingwincludemath.h: In function 'float hypotf(float, float)':
z:landermingwincludemath.h:635:30: error: '_hypot' was not declared in this scope
 { return (float)(_hypot (x, y)); }

Is something wrong on my side?
Or version at mingw repo is bugged? And if so, is there any quick fix for this header?

Answer 1

To avoid any further speculation, and downright bad suggestions such as using #if 0, let me give an authoritative answer, from the perspective of a MinGW project contributor.

Yes, the MinGW.org implementation of include/math.h does have a bug in its inline implementation of hypotf (float, float); the bug is triggered when compiling C++, with the affected header included (as it is when cmath is included), and any compiler option which causes __STRICT_ANSI__ to become defined is specified, (as is the case for those -std=c... options noted by the OP). The appropriate solution is not to occlude part of the math.h file, with #if 0 or otherwise, but to correct the broken inline implementation of hypotf (float, float); simply removing the spurious leading underscore from the inline reference to _hypot (float, float), where its return value is cast to the float return type should suffice.

Alternatively, substituting an equivalent -std=gnu... for -std=c... in the compiler options should circumvent the bug, and may offer a suitable workaround.

FWIW, I'm not entirely happy with MinGW.org's current implementation of hypotl (long double, long double) either; correcting both issues is on my punch list for the next release of the MinGW runtime, but ATM, I have little time to devote to preparing this.

Update

This bug is no longer present in the current release of the MinGW.org runtime library (currently mingwrt-3.22.4, but fixed since release 3.22). If you are using anything older than this, (including any of the critically broken 4.x releases), you should upgrade.