libstdc++: Add compile-time checks to__glibcxx_assert [PR 71960]

This change evaluates __glibcxx_assert checks unconditionally when a
function is being constant evaluated (when std::is_constant_evaluated()
is true). If the check fails, compilation will fail with an error.

If the function isn't being constant evaluated, the normal runtime check
will be done if enabled by _GLIBCXX_ASSERTIONS or _GLIBCXX_DEBUG, the
same as before.

Tangentially, the __glibcxx_assert and _GLIBCXX_PARALLEL_ASSERT macros
are changed to expand to 'do { } while (false)' when assertions are
disabled, instead of expanding to nothing. This avoids -Wempty-body
warnings when a disabled assertion is used in an 'if' or 'else'
statement e.g.

  if constexpr (/* precondition is testable */)
    __glibcxx_assert(precondition);

a.C:9:27: warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]
    9 |     __glibcxx_assert(precondition);
      |                                  ^

libstdc++-v3/ChangeLog:

	PR libstdc++/71960
	* include/bits/c++config (__glibcxx_assert_impl): Remove
	do-while so that uses of the macro need to add it.
	(__glibcxx_assert): Rename macro for runtime assertions
	to __glibcxx_assert_2.
	(__glibcxx_assert_1): Define macro for constexpr assertions.
	(__glibcxx_assert): Define macro for constexpr and runtime
	assertions.
	* include/bits/range_access.h (ranges::advance): Remove
	redundant precondition checks during constant evaluation.
	* include/parallel/base.h (_GLIBCXX_PARALLEL_ASSERT): Always
	use do-while in macro expansion.
	* include/std/ranges (iota_view::iota_view(W, B)): Remove
	redundant braces.

libstdc++-v3/include/bits/c++config

@@ -477,19 +477,16 @@ namespace std
     __builtin_abort();
   }
 }
-#define __glibcxx_assert_impl(_Condition)				 \
-  do 									 \
-  {							      		 \
-    if (! (_Condition))                                                  \
-      std::__replacement_assert(__FILE__, __LINE__, __PRETTY_FUNCTION__, \
-				#_Condition);				 \
-  } while (false)
+#define __glibcxx_assert_impl(_Condition)			       \
+  if (!bool(_Condition))					       \
+    std::__replacement_assert(__FILE__, __LINE__, __PRETTY_FUNCTION__, \
+			      #_Condition)
 #endif
 
 #if defined(_GLIBCXX_ASSERTIONS)
-# define __glibcxx_assert(_Condition) __glibcxx_assert_impl(_Condition)
+# define __glibcxx_assert_2(_Condition) __glibcxx_assert_impl(_Condition)
 #else
-# define __glibcxx_assert(_Condition)
+# define __glibcxx_assert_2(_Condition)
 #endif
 
 // Macros for race detectors.
@@ -683,6 +680,26 @@ namespace std
 # endif
 #endif // GCC
 
+#if _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
+# define __glibcxx_assert_1(_Condition)		\
+    if (__builtin_is_constant_evaluated())	\
+     {						\
+       void __failed_assertion();	\
+       if (!bool(_Condition))			\
+	 __failed_assertion();	\
+     }						\
+    else
+#else
+# define __glibcxx_assert_1(_Condition)
+#endif
+
+# define __glibcxx_assert(_Condition)	  \
+  do {					  \
+    __glibcxx_assert_1(_Condition)	  \
+    { __glibcxx_assert_2(_Condition); }   \
+  } while (false)
+
+
 // PSTL configuration
 
 #if __cplusplus >= 201703L

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

@@ -987,10 +987,7 @@ namespace ranges
 	}
       else
 	{
-#ifdef __cpp_lib_is_constant_evaluated
-	  if (std::is_constant_evaluated() && __n < 0)
-	    throw "attempt to decrement a non-bidirectional iterator";
-#endif
+	  // cannot decrement a non-bidirectional iterator
 	  __glibcxx_assert(__n >= 0);
 	  while (__n-- > 0)
 	    ++__it;
@@ -1065,10 +1062,7 @@ namespace ranges
 	}
       else
 	{
-#ifdef __cpp_lib_is_constant_evaluated
-	  if (std::is_constant_evaluated() && __n < 0)
-	    throw "attempt to decrement a non-bidirectional iterator";
-#endif
+	  // cannot decrement a non-bidirectional iterator
 	  __glibcxx_assert(__n >= 0);
 	  return __n;
 	}

libstdc++-v3/include/parallel/base.h

@@ -420,9 +420,10 @@ namespace __gnu_parallel
     }
 
 #if _GLIBCXX_PARALLEL_ASSERTIONS && defined(__glibcxx_assert_impl)
-#define _GLIBCXX_PARALLEL_ASSERT(_Condition) __glibcxx_assert_impl(_Condition)
+# define _GLIBCXX_PARALLEL_ASSERT(_Condition) \
+  do { __glibcxx_assert_impl(_Condition); } while (false)
 #else
-#define _GLIBCXX_PARALLEL_ASSERT(_Condition)
+# define _GLIBCXX_PARALLEL_ASSERT(_Condition) do { } while (false)
 #endif
 
 } //namespace __gnu_parallel

libstdc++-v3/include/std/ranges

@@ -860,9 +860,7 @@ namespace ranges
       : _M_value(__value), _M_bound(__bound)
       {
 	if constexpr (totally_ordered_with<_Winc, _Bound>)
-	  {
-	    __glibcxx_assert( bool(__value <= __bound) );
-	  }
+	  __glibcxx_assert( bool(__value <= __bound) );
       }
 
       constexpr _Iterator