Index: configure.in
===================================================================
RCS file: /cvsroot/guile/guile/guile-core/configure.in,v
retrieving revision 1.268.2.37
diff -u -r1.268.2.37 configure.in
--- configure.in	6 Feb 2008 22:27:59 -0000	1.268.2.37
+++ configure.in	9 Feb 2008 18:43:21 -0000
@@ -715,6 +716,30 @@
   [AC_DEFINE(HAVE_CRYPT,1,
              [Define to 1 if you have the `crypt' function.])])
 
+# When compiling with GCC on some OSs (Solaris, AIX), _Complex_I doesn't work;
+# in the reported cases so far, 1.0fi works well instead.
+if test "$ac_cv_type_complex_double" = yes; then
+  AC_MSG_CHECKING([for i])
+  AC_TRY_COMPILE([
+#if HAVE_COMPLEX_H
+#include <complex.h>
+#endif
+complex double z;
+],[
+z = _Complex_I;
+],[AC_DEFINE(GUILE_I,_Complex_I,[The imaginary unit (positive square root of -1).])
+   AC_MSG_RESULT([_Complex_I])],[AC_TRY_COMPILE([
+#if HAVE_COMPLEX_H
+#include <complex.h>
+#endif
+complex double z;
+],[
+z = 1.0fi;
+],[AC_DEFINE(GUILE_I,1.0fi)
+   AC_MSG_RESULT([1.0fi])],[ac_cv_type_complex_double=no
+                            AC_MSG_RESULT([not available])])])
+fi
+
 # glibc 2.3.6 (circa 2006) and various prior versions had a bug where
 # csqrt(-i) returned a negative real part, when it should be positive
 # for the principal root.
Index: libguile/numbers.c
===================================================================
RCS file: /cvsroot/guile/guile/guile-core/libguile/numbers.c,v
retrieving revision 1.281.2.12
diff -u -r1.281.2.12 numbers.c
--- libguile/numbers.c	6 Feb 2008 13:17:49 -0000	1.281.2.12
+++ libguile/numbers.c	9 Feb 2008 18:43:23 -0000
@@ -162,11 +162,12 @@
 #endif
 }
 
-
+#if defined (GUILE_I)
 /* For an SCM object Z which is a complex number (ie. satisfies
    SCM_COMPLEXP), return its value as a C level "complex double". */
 #define SCM_COMPLEX_VALUE(z)                                    \
-  (SCM_COMPLEX_REAL (z) + _Complex_I * SCM_COMPLEX_IMAG (z))
+  (SCM_COMPLEX_REAL (z) + GUILE_I * SCM_COMPLEX_IMAG (z))
+#endif
 
 /* Convert a C "complex double" to an SCM value. */
 #if HAVE_COMPLEX_DOUBLE
@@ -6011,7 +6012,7 @@
 {
   if (SCM_COMPLEXP (z))
     {
-#if HAVE_COMPLEX_DOUBLE && HAVE_CLOG
+#if HAVE_COMPLEX_DOUBLE && HAVE_CLOG && defined (GUILE_I)
       return scm_from_complex_double (clog (SCM_COMPLEX_VALUE (z)));
 #else
       double re = SCM_COMPLEX_REAL (z);
@@ -6045,7 +6046,7 @@
       /* Mingw has clog() but not clog10().  (Maybe it'd be worth using
          clog() and a multiply by M_LOG10E, rather than the fallback
          log10+hypot+atan2.)  */
-#if HAVE_COMPLEX_DOUBLE && HAVE_CLOG10
+#if HAVE_COMPLEX_DOUBLE && HAVE_CLOG10 && defined (GUILE_I)
       return scm_from_complex_double (clog10 (SCM_COMPLEX_VALUE (z)));
 #else
       double re = SCM_COMPLEX_REAL (z);
@@ -6077,7 +6078,7 @@
 {
   if (SCM_COMPLEXP (z))
     {
-#if HAVE_COMPLEX_DOUBLE && HAVE_CEXP
+#if HAVE_COMPLEX_DOUBLE && HAVE_CEXP && defined (GUILE_I)
       return scm_from_complex_double (cexp (SCM_COMPLEX_VALUE (z)));
 #else
       return scm_c_make_polar (exp (SCM_COMPLEX_REAL (z)),
@@ -6111,7 +6112,7 @@
 {
   if (SCM_COMPLEXP (x))
     {
-#if HAVE_COMPLEX_DOUBLE && HAVE_USABLE_CSQRT
+#if HAVE_COMPLEX_DOUBLE && HAVE_USABLE_CSQRT && defined (GUILE_I)
       return scm_from_complex_double (csqrt (SCM_COMPLEX_VALUE (x)));
 #else
       double re = SCM_COMPLEX_REAL (x);