fix `sqrt' on numbers with negative real and inexact-zero imag

Closes PR 13028

collects/tests/racket/number.rktl

@@ -1671,6 +1671,10 @@
 (test (make-rectangular 0 (expt 5 13)) sqrt (- (expt 5 26)))
 (test (make-rectangular 0 545915034.0) z-round (sqrt (- (expt 5 25))))
 
+(test 0.0+1.0i sqrt -1.0+0.0i)
+(test 0.0f0+1.0f0i sqrt -1.0f0+0.0f0i)
+(test 0.0+0.0i sqrt 0.0+0.0i)
+
 (err/rt-test (sqrt "a"))
 (arity-test sqrt 1 1)
 

src/racket/src/complex.c

@@ -356,8 +356,20 @@ Scheme_Object *scheme_complex_sqrt(const Scheme_Object *o)
     r = scheme_sqrt(1, &r);
     if (!SCHEME_COMPLEXP(r))
       return scheme_make_complex(r, i);
-    else
-      return r;
+    else {
+      c = (Scheme_Complex *)r;
+      if (SAME_OBJ(c->r, zero)) {
+        /* need an inexact-zero real part: */
+#ifdef MZ_USE_SINGLE_FLOATS
+        if (SCHEME_FLTP(c->i))
+          r = scheme_make_float(0.0);
+        else
+#endif
+          r = scheme_make_double(0.0);
+        return scheme_make_complex(r, c->i);
+      } else
+        return r;
+    }
   }
 
   ssq = scheme_bin_plus(scheme_bin_mult(r, r),