Remove Support for Small Octals

DIPs/1NNN-GED.md

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+# Remove Support for Small Octals
+
+| Field           | Value                                                           |
+|-----------------|-----------------------------------------------------------------|
+| DIP:            | (number/id -- assigned by DIP Manager)                          |
+| Review Count:   | 0 (edited by DIP Manager)                                       |
+| Author:         | Garrett D'Amore [email protected]                              |
+| Implementation: | (links to implementation PR if any)                             |
+| Status:         | Draft                                                           |
+
+## Abstract
+
+We propose to remove support for octal integer literals less than 8.
+Support for octal literals 8 or larger has already been removed.
+
+## Contents
+* [Rationale](#rationale)
+* [Prior Work](#prior-work)
+* [Description](#description)
+* [Breaking Changes and Deprecations](#breaking-changes-and-deprecations)
+* [Reference](#reference)
+* [Copyright & License](#copyright--license)
+* [Reviews](#reviews)
+
+## Rationale
+
+It is perverse, and surprising, that octal values less than 8 are supported,
+while those that are larger than 8 are not.  The rationale for this is likely
+that there is no risk of confusion, since 07 and 7 are the same value.
+
+However, supporting octal constants of such a limited range (1 through 7) brings
+very little value, since the most common contemporary use of octals is for
+file permissions on POSIX systems (e.g. 0644).
+
+Expressing these constants in decimal is actually more compact (1 character
+instead of 2).
+
+Additionally support for only this limited range violates the principal of
+least surprise.
+
+It also requires explicit handling during the lex phase, to properly sort out,
+creating a very small bit of additional complexity with no tangible value.
+
+The formal grammar specification at https://dlang.org/spec/lex.html#integerliteral
+does not have any provision to allow for octal constants of any kind, but
+rather explicitly prohibits them with a rationale:
+
+> C-style octal integer notation was deemed too easy to mix up with decimal notation;
+> it is only fully supported in string literals. D still supports octal integer literals
+> interpreted at compile time through the std.conv.octal template, as in octal!167.
+
+(This DIP is not intended to change the handling of octal character constants nor
+of `std.conv.octal`.)
+
+Thus, this proposal intends to harmonize the behavior of DMD with the specified
+grammar.
+
+The author has a Tree-Sitter grammar for D at https://github.com/gdamore/tree-sitter-d,
+which was written following the online specification, and that grammar therefore does
+not include support for tiny octal constants.  The author would like to avoid adding
+support for them, since they aren't part of the official grammar.
+
+## Prior Work
+
+D has already removed support for ordinary octal constants larger than 7,
+but a solution for users who need it using `std.conv.octal` is provided
+for the rare use cases where an octal value will be more readable.
+
+(In the author's 30 year career doing systems level work, the only cases
+where octals have been useful has been for the aforementioned file permissions.)
+
+## Description
+
+The parsing of a value like '03' should be treated in the same way as a literal
+like '033'.
+
+There are no grammar changes required, but the DMD compiler behavior should
+be changed to match the specified grammar.
+
+## Breaking Changes and Deprecations
+
+This is of course a breaking change, although it's hard to imagine (for the
+author of this DIP at least) an actual use of tiny octal values in the field.
+
+Support for handling these can permitted through a deprecation period
+with an associated compiler switch.
+
+## Reference
+
+https://dlang.org/spec/lex.html#integerliteral
+
+## Copyright & License
+Copyright (c) 2022 by the D Language Foundation
+
+Licensed under [Creative Commons Zero 1.0](https://creativecommons.org/publicdomain/zero/1.0/legalcode.txt)
+
+## Reviews
+The DIP Manager will supplement this section with a summary of each review stage
+of the DIP process beyond the Draft Review.