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var-op-expand.mk revision 1.3 
      1 # $NetBSD: var-op-expand.mk,v 1.3 2020/11/08 13:46:15 rillig Exp $
      2 #
      3 # Tests for the := variable assignment operator, which expands its
      4 # right-hand side.
      5 
      6 # TODO: Implementation
      7 
      8 # XXX: edge case: When a variable name refers to an undefined variable, the
      9 # behavior differs between the '=' and the ':=' assignment operators.
     10 # This bug exists since at least 1993-03-21.
     11 #
     12 # The '=' operator expands the undefined variable to an empty string, thus
     13 # assigning to VAR_ASSIGN_.  In the name of variables to be set, it should
     14 # really be forbidden to refer to undefined variables.
     15 #
     16 # The ':=' operator expands the variable name twice.  In one of these
     17 # expansions, the undefined variable expression is preserved (controlled by
     18 # preserveUndefined in VarAssign_EvalSubst), in the other expansion it expands
     19 # to an empty string.  This way, 2 variables are created using a single
     20 # variable assignment.  It's magic. :-/
     21 .MAKEFLAGS: -dv
     22 VAR_ASSIGN_${UNDEF}=	undef value
     23 VAR_SUBST_${UNDEF}:=	undef value
     24 .MAKEFLAGS: -d0
     25 
     26 all:
     27 	@:;
     28