Discard reference: use dedicated error instead of ENVAL

Author: lwaern-intel

doc/1.4/language.md

@@ -4064,6 +4064,7 @@ independent method callback(int i, void *aux) {
 ```
 
 ### The Discard Reference (`_`)
+<a id="discard-reference"/>
 ```
 _
 ```

py/dml/ast.py

@@ -55,6 +55,7 @@ def __setstate__(self, data):
     'default',
     'default_dml12',
     'delete',
+    'discard',
     'dml',
     'dml_typedef',
     'dowhile',

py/dml/codegen.py

@@ -1222,16 +1222,13 @@ def expr_variable(tree, location, scope):
         if in_dev_tree:
             e = in_dev_tree
     if e is None:
-        # TODO/HACK: The discard ref is exposed like this to allow it to be as
-        # keyword-like as possible while still allowing it to be shadowed.
-        # Once we remove support for discard_ref_shadowing the discard ref
-        # should become a proper keyword and its codegen be done via dedicated
-        # dispatch
-        if name == '_' and tree.site.dml_version() != (1, 2):
-            return mkDiscardRef(tree.site)
         raise EIDENT(tree.site, name)
     return e
 
+@expression_dispatcher
+def expr_discard(tree, location, scope):
+    return mkDiscardRef(tree.site)
+
 @expression_dispatcher
 def expr_objectref(tree, location, scope):
     [name] = tree.args

py/dml/ctree.py

@@ -3597,6 +3597,9 @@ def __init__(self, site, type):
     def __str__(self):
         return '_'
 
+    def exc(self):
+        return EDISCARDREF(self.site)
+
     def write(self, source):
         if self.explicit_type:
             return source.as_expr(self.type).discard(explicit=True)

py/dml/dmllex14.py

@@ -13,17 +13,21 @@
 
 tokens = (common_tokens
           + ('HASHCONDOP', 'HASHCOLON')
+          + ('DISCARD',)
           + tuple(hashids.values()))
 
 t_HASHCONDOP = r'\#\?'
 t_HASHCOLON = r'\#:'
+t_DISCARD = r'_'
 
 keywords_dml14 = dict(keywords_common)
 for kw in ['param', 'saved', 'async', 'await', 'with', 'shared', 'stringify',
            'export', 'as', 'independent', 'startup', 'memoized', 'hook']:
     keywords_dml14[kw] = kw.upper()
     tokens += (kw.upper(),)
 
+keywords_dml14['_'] = 'DISCARD'
+
 reserved_idents = reserved_idents_common + (
     'PARAM', 'SAVED', 'INDEPENDENT', 'STARTUP', 'MEMOIZED')
 

py/dml/dmlparse.py

@@ -1801,6 +1801,11 @@ def expression_ident(t):
                   | DEFAULT'''
     t[0] = ast.variable(site(t), t[1])
 
+@prod_dml14
+def expression_discardref(t):
+    '''expression : discard'''
+    t[0] = t[1]
+
 @prod_dml14
 def expression_this(t):
     '''expression : THIS'''
@@ -2660,6 +2665,11 @@ def objident(t):
                 | REGISTER'''
     t[0] = t[1]
 
+@prod_dml14
+def discard(t):
+    'discard : DISCARD'
+    t[0] = ast.discard(site(t, 1))
+
 def ident_rule(idents):
     return 'ident : ' +  "\n| ".join(idents)
 

py/dml/messages.py

@@ -1893,6 +1893,16 @@ class EPRAGMA(DMLError):
     """
     fmt = "Unknown pragma: %s"
 
+class EDISCARDREF(DMLError):
+    """
+    The expression *`_`* resolves to the [discard
+    reference](language.html#discard-reference), and can only be used as an
+    assignment target, in order to e.g. throw away return values of a function.
+    """
+    version = "1.4"
+    fmt = ("'_' can only be used as an assignment target "
+           + "(to discard some value)")
+
 #
 # WARNINGS (keep these as few as possible)
 #

test/1.4/errors/T_ENVAL.dml

@@ -46,6 +46,11 @@ method init() {
     b.r1;
     /// ERROR ENVAL
     b.r2.f;
+
+    /// ERROR EDISCARDREF
+    _;
+    // no error
+    _ = true;
 }
 
 // dev is not a value in 1.4.  Need to capture this with a rather unnatural