The comment for DECL_SIZE makes it clear it may be non-constant but not that it may be null. The comment for TYPE_SIZE mentions neither.
The attached update adds a few sentences to make these caveats clear. If no one has any suggestions I'll commit it as obvious today or tomorrow. Thanks Martin
gcc/ChangeLog: * tree.def (TYPE_SIZE): Clarify. * tree.h (TYPE_SIZE, TYPE_SIZE_UNIT, DECL_SIZE): Add comments. Index: gcc/tree.def =================================================================== --- gcc/tree.def (revision 274541) +++ gcc/tree.def (working copy) @@ -77,7 +77,10 @@ DEFTREECODE (BLOCK, "block", tcc_exceptional, 0) /* Each data type is represented by a tree node whose code is one of the following: */ /* Each node that represents a data type has a component TYPE_SIZE - containing a tree that is an expression for the size in bits. + that evaluates either to a tree that is a (potentially non-constant) + expression representing the type size in bits, or to a null pointer + when the size of the type is unknown (for example, for incomplete + types such as arrays of unspecified bound). The TYPE_MODE contains the machine mode for values of this type. The TYPE_POINTER_TO field contains a type for a pointer to this type, or zero if no such has been created yet. Index: gcc/tree.h =================================================================== --- gcc/tree.h (revision 274541) +++ gcc/tree.h (working copy) @@ -1952,7 +1952,10 @@ class auto_suppress_location_wrappers so they must be checked as well. */ #define TYPE_UID(NODE) (TYPE_CHECK (NODE)->type_common.uid) +/* Type size in bits as a tree expression. Need not be constant + and may be null. */ #define TYPE_SIZE(NODE) (TYPE_CHECK (NODE)->type_common.size) +/* Likewise, type size in bytes. */ #define TYPE_SIZE_UNIT(NODE) (TYPE_CHECK (NODE)->type_common.size_unit) #define TYPE_POINTER_TO(NODE) (TYPE_CHECK (NODE)->type_common.pointer_to) #define TYPE_REFERENCE_TO(NODE) (TYPE_CHECK (NODE)->type_common.reference_to) @@ -2480,7 +2483,7 @@ extern machine_mode vector_type_mode (const_tree); #define DECL_INITIAL(NODE) (DECL_COMMON_CHECK (NODE)->decl_common.initial) /* Holds the size of the datum, in bits, as a tree expression. - Need not be constant. */ + Need not be constant and may be null. */ #define DECL_SIZE(NODE) (DECL_COMMON_CHECK (NODE)->decl_common.size) /* Likewise for the size in bytes. */ #define DECL_SIZE_UNIT(NODE) (DECL_COMMON_CHECK (NODE)->decl_common.size_unit)