[libstc++] Don't throw in std::assoc_legendre for m > l

[André Brand <andre dot j dot brand at gmx dot de>]

The return value specified in "8.1.2 associated Legendre polynomials"
of ISO/IEC JTC 1/SC 22/WG 21 N3060 (which is identical to the
expression in the doxygen comment of the patched function) is well-
defined for m>l: it is always zero because $ P_l(x) $ is a polynomial
of degree l.

The standard does not enforce an exception in this case because none of
the requirements in 8.1 (5) on page 11 of ISO/IEC JTC 1/SC 22/WG 21
N3060 are met.

Note: the implementation of st::assoc_legendre in Visual Studio 2017
(tested with Visual Studio 15.9.7) silently returns zero.

Index: libstdc++-v3/include/tr1/legendre_function.tcc
===================================================================
--- libstdc++-v3/include/tr1/legendre_function.tcc	(revision 269352)
+++ libstdc++-v3/include/tr1/legendre_function.tcc	(working copy)
@@ -67,13 +67,13 @@
     /**
      *   @brief  Return the Legendre polynomial by recursion on degree
      *           @f$ l @f$.
-     * 
+     *
      *   The Legendre function of @f$ l @f$ and @f$ x @f$,
      *   @f$ P_l(x) @f$, is defined by:
      *   @f[
      *     P_l(x) = \frac{1}{2^l l!}\frac{d^l}{dx^l}(x^2 - 1)^{l}
      *   @f]
-     * 
+     *
      *   @param  l  The degree of the Legendre polynomial.  @f$l >= 0@f$.
      *   @param  x  The argument of the Legendre polynomial.  @f$|x| <= 1@f$.
      */
@@ -120,17 +120,17 @@
     /**
      *   @brief  Return the associated Legendre function by recursion
      *           on @f$ l @f$.
-     * 
+     *
      *   The associated Legendre function is derived from the Legendre function
      *   @f$ P_l(x) @f$ by the Rodrigues formula:
      *   @f[
      *     P_l^m(x) = (1 - x^2)^{m/2}\frac{d^m}{dx^m}P_l(x)
      *   @f]
-     * 
+     *
      *   @param  l  The degree of the associated Legendre function.
      *              @f$ l >= 0 @f$.
      *   @param  m  The order of the associated Legendre function.
-     *              @f$ m <= l @f$.
+     *              @f$ m >= 0 @f$.
      *   @param  x  The argument of the associated Legendre function.
      *              @f$ |x| <= 1 @f$.
      *   @param  phase  The phase of the associated Legendre function.
@@ -146,8 +146,7 @@
         std::__throw_domain_error(__N("Argument out of range"
                                       " in __assoc_legendre_p."));
       else if (__m > __l)
-        std::__throw_domain_error(__N("Degree out of range"
-                                      " in __assoc_legendre_p."));
+        return _Tp(0);
       else if (__isnan(__x))
         return std::numeric_limits<_Tp>::quiet_NaN();
       else if (__m == 0)
@@ -192,7 +191,7 @@
 
     /**
      *   @brief  Return the spherical associated Legendre function.
-     * 
+     *
      *   The spherical associated Legendre function of @f$ l @f$, @f$ m @f$,
      *   and @f$ \theta @f$ is defined as @f$ Y_l^m(\theta,0) @f$ where
      *   @f[
@@ -202,7 +201,7 @@
      *   @f]
      *   is the spherical harmonic function and @f$ P_l^m(x) @f$ is the
      *   associated Legendre function.
-     * 
+     *
      *   This function differs from the associated Legendre function by
      *   argument (@f$x = \cos(\theta)@f$) and by a normalization factor
      *   but this factor is rather large for large @f$ l @f$ and @f$ m @f$
@@ -210,7 +209,7 @@
      *   and @f$ m @f$.
      *   @note Unlike the case for __assoc_legendre_p the Condon-Shortley
      *   phase factor @f$ (-1)^m @f$ is present here.
-     * 
+     *
      *   @param  l  The degree of the spherical associated Legendre function.
      *              @f$ l >= 0 @f$.
      *   @param  m  The order of the spherical associated Legendre function.