Add STATIC_ASSERT_EXPR

This new type of a STATIC_ASSERT is used where an expression can go;
hence potentially in macro expansions.  Since it is compile-time, it
can't be used on arguments that are expressions with side effects, so
can safely be used in macros.

It has limitations however, only reaching full parity when gcc brace
groups are enabled, turning into an ASSUME which is a no-op in the worst
case.

However, most code will get compiled on a platform with gcc brace
groups, and will pass or fail at compile time; so the chances of
something bad actually getting into the field are minimal.

Author: khwilliamson

perl.h

@@ -4252,6 +4252,7 @@ hint to the compiler that this condition is likely to be false.
 
 /*
 =for apidoc   Am||STATIC_ASSERT_DECL|const_expr
+=for apidoc_item  STATIC_ASSERT_EXPR
 =for apidoc_item  STATIC_ASSERT_STMT
 
 These are like assert(), but for compile time invariants. That is, their
@@ -4269,9 +4270,22 @@ function.
 C<STATIC_ASSERT_DECL> expands to a declaration and is suitable for use inside a
 function or at file scope (outside of any function).
 
-=cut
+C<STATIC_ASSERT_EXPR> expands to an expression and is suitable anywhere an
+expression is usable.  It has some limitations compared to the other two when
+used on a platform without L</C<PERL_USE_GCC_BRACE_GROUPS>>.  On those
+platforms it expands to an ASSUME().  When called with a compile-time
+expression, the compiler should optimize out the expression, so that the use of
+this is "free", but constness is not enforced.  On non-DEBUGGING builds, this
+will expand to a no-op, so again it is "free", but no checking is done.
+
+Thus code that uses this macro can be ported to all platforms without needing
+to change, and it will work as well as is possible on that platform.
+Presumably it will get compiled at some point before release on a platform
+where it has parity with the other two forms.
 
+=cut
 */
+
 #if (! defined(__IBMC__) || __IBMC__ >= 1210)                               \
  && ((   defined(static_assert) && (   defined(_ISOC11_SOURCE)              \
                                     || (__STDC_VERSION__ - 0) >= 201101L))  \
@@ -4335,6 +4349,12 @@ function or at file scope (outside of any function).
 #define STATIC_ASSERT_STMT(COND)                                            \
                         STMT_START { STATIC_ASSERT_DECL(COND); } STMT_END
 
+#ifdef PERL_USE_GCC_BRACE_GROUPS
+#  define STATIC_ASSERT_EXPR(COND) ({ STATIC_ASSERT_DECL(COND); })
+#else
+#  define STATIC_ASSERT_EXPR(COND)  ASSUME(COND)
+#endif
+
 #ifndef __has_builtin
 #  define __has_builtin(x) 0 /* not a clang style compiler */
 #endif