libstdc++: Implement LWG 3150 for std::uniform_random_bit_generator

	* include/bits/random.h (uniform_random_bit_generator): Require min()
	and max() to be constant expressions and min() to be less than max().
	* testsuite/26_numerics/random/concept.cc: Check additional cases.
	* testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-error lineno.

libstdc++-v3/ChangeLog

+2020-02-15  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/bits/random.h (uniform_random_bit_generator): Require min()
+	and max() to be constant expressions and min() to be less than max().
+	* testsuite/26_numerics/random/concept.cc: Check additional cases.
+	* testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-error lineno.
+
 2020-02-13  Patrick Palka  <ppalka@redhat.com>
 
 	* include/Makefile.am: Add <bits/ranges_uninitialized.h>.

libstdc++-v3/include/bits/random.h

@@ -60,6 +60,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	{ _Gen::min() } -> same_as<invoke_result_t<_Gen&>>;
 	{ _Gen::max() } -> same_as<invoke_result_t<_Gen&>>;
+	requires bool_constant<(_Gen::min() < _Gen::max())>::value;
       };
 #endif
 

libstdc++-v3/testsuite/26_numerics/random/concept.cc

@@ -219,3 +219,30 @@ struct N11
 };
 
 static_assert( ! std::uniform_random_bit_generator<N11> );
+
+struct N12
+{
+  unsigned operator()();
+  static unsigned min() { return 0; } // not constexpr
+  static constexpr unsigned max() { return 1; }
+};
+
+static_assert( ! std::uniform_random_bit_generator<N12> ); // LWG 3150
+
+struct N13
+{
+  unsigned operator()();
+  static constexpr unsigned min() { return 0; }
+  static unsigned max() { return 1; } // not constexpr
+};
+
+static_assert( ! std::uniform_random_bit_generator<N13> ); // LWG 3150
+
+struct N14
+{
+  unsigned operator()();
+  static constexpr unsigned min() { return 1; }
+  static constexpr unsigned max() { return 0; } // max not greater than min
+};
+
+static_assert( ! std::uniform_random_bit_generator<N14> ); // LWG 3150

libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc

@@ -10,6 +10,6 @@ std::__detail::_Adaptor<std::mt19937, unsigned long> aurng(urng);
 auto x = std::generate_canonical<std::size_t,
 			std::numeric_limits<std::size_t>::digits>(urng);
 
-// { dg-error "static assertion failed: template argument must be a floating point type" "" { target *-*-* } 171 }
+// { dg-error "static assertion failed: template argument must be a floating point type" "" { target *-*-* } 172 }
 
 // { dg-error "static assertion failed: template argument must be a floating point type" "" { target *-*-* } 3281 }