Fix issue 19070 - some octal literals are actually allowed

Author: schveiguy

spec/lex.dd

@@ -660,10 +660,15 @@ $(GNAME IntegerSuffix):
     $(B UL)
 
 $(GNAME DecimalInteger):
-    $(B 0)
+    $(GLINK ZeroDigits)
+    $(GLINK ZeroDigits) $(GLINK OctalDigit)
     $(GLINK NonZeroDigit)
     $(GLINK NonZeroDigit) $(GLINK DecimalDigitsUS)
 
+$(GNAME ZeroDigits):
+    $(B 0)
+    $(B 0) $(GSELF ZeroDigits)
+
 $(GNAME BinaryInteger):
     $(GLINK BinPrefix) $(GLINK BinaryDigitsNoSingleUS)
 
@@ -787,7 +792,8 @@ $(GNAME HexLetter):
         )
 
         $(P C-style octal integer notation was deemed too easy to mix up with decimal notation;
-        it is only fully supported in string literals.
+        it is only fully supported in string literals or when the decimal interpretation would be equivalent
+        (e.g. $(D 007) is accepted)
         D still supports octal integer literals interpreted at compile time through the $(REF octal, std,conv)
         template, as in $(D octal!167).)
         $(P Hexadecimal integers are a sequence of hexadecimal digits preceded