Revert "SCM is either a union or scm_t_bits"

This reverts commit 80125469ef.

Conflicts:

	libguile/__scm.h

libguile/__scm.h

@@ -273,6 +273,31 @@
 #define SCM_DEBUG_REST_ARGUMENT SCM_DEBUG
 #endif
 
+/* The macro SCM_DEBUG_TYPING_STRICTNESS indicates what level of type checking
+ * shall be performed with respect to the use of the SCM datatype.  The macro
+ * may be defined to one of the values 0, 1 and 2.
+ *
+ * A value of 0 means that there will be no compile time type checking, since
+ * the SCM datatype will be declared as an integral type.  This setting should
+ * only be used on systems, where casting from integral types to pointers may
+ * lead to loss of bit information.
+ *
+ * A value of 1 means that there will an intermediate level of compile time
+ * type checking, since the SCM datatype will be declared as a pointer to an
+ * undefined struct.  This setting is the default, since it does not cost
+ * anything in terms of performance or code size.
+ *
+ * A value of 2 provides a maximum level of compile time type checking since
+ * the SCM datatype will be declared as a struct.  This setting should be used
+ * for _compile time_ type checking only, since the compiled result is likely
+ * to be quite inefficient.  The right way to make use of this option is to do
+ * a 'make clean; make CFLAGS=-DSCM_DEBUG_TYPING_STRICTNESS=2', fix your
+ * errors, and then do 'make clean; make'.
+ */
+#ifndef SCM_DEBUG_TYPING_STRICTNESS
+#define SCM_DEBUG_TYPING_STRICTNESS 1
+#endif
+
 
 
 /* {Feature Options}

libguile/tags.h

@@ -61,6 +61,7 @@
 /* For dealing with the bit level representation of scheme objects we define
  * scm_t_bits:
  */
+
 typedef scm_t_intptr  scm_t_signed_bits;
 typedef scm_t_uintptr scm_t_bits;
 
@@ -69,28 +70,47 @@ typedef scm_t_uintptr scm_t_bits;
 #define SCM_T_BITS_MAX        SCM_T_UINTPTR_MAX
 
 
-/* But as external interface, we pack the bits in a union.  This makes
- * the compiler treat SCM values as a disjoint type, allowing the
- * detection of many common errors.
+/* But as external interface, we define SCM, which may, according to the
+ * desired level of type checking, be defined in several ways:
  */
-union SCM
-{
-  scm_t_bits n;
-};
+#if (SCM_DEBUG_TYPING_STRICTNESS == 2)
+typedef union SCM { scm_t_bits n; } SCM;
+#   define SCM_UNPACK(x) ((x).n)
+#   define SCM_PACK(x) ((SCM) { (scm_t_bits) (x) })
+#elif (SCM_DEBUG_TYPING_STRICTNESS == 1)
+/* This is the default, which provides an intermediate level of compile time
+ * type checking while still resulting in very efficient code.
+ */
+    typedef struct scm_unused_struct { char scm_unused_field; } *SCM;
 
-#ifndef SCM_USING_PREHISTORIC_COMPILER
-/* With GCC at least, wrapping the bits in a union provides no
- * performance penalty.
- */
-typedef union SCM SCM;
-#define SCM_UNPACK(x) ((x).n)
-#define SCM_PACK(x) ((SCM) { (scm_t_bits) (x) })
+/*
+  The 0?: constructions makes sure that the code is never executed,
+  and that there is no performance hit.  However, the alternative is
+  compiled, and does generate a warning when used with the wrong
+  pointer type.
+
+  The Tru64 and ia64-hp-hpux11.23 compilers fail on `case (0?0=0:x)'
+  statements, so for them type-checking is disabled.  */
+#if defined __DECC || defined __HP_cc
+#   define SCM_UNPACK(x) ((scm_t_bits) (x))
 #else
-/* But we do provide an escape valve for less capable compilers.
+#   define SCM_UNPACK(x) ((scm_t_bits) (0? (*(SCM*)0=(x)): x))
+#endif
+
+/*
+  There is no typechecking on SCM_PACK, since all kinds of types
+  (unsigned long, void*) go in SCM_PACK
  */
-typedef scm_t_bits SCM;
-#define SCM_UNPACK(x) (x)
-#define SCM_PACK(x) ((SCM) (x))
+#   define SCM_PACK(x) ((SCM) (x))
+
+#else
+/* This should be used as a fall back solution for machines on which casting
+ * to a pointer may lead to loss of bit information, e. g. in the three least
+ * significant bits.
+ */
+    typedef scm_t_bits SCM;
+#   define SCM_UNPACK(x) (x)
+#   define SCM_PACK(x) ((SCM) (x))
 #endif