[PATCH] Consider unsigned long for non-u/U decimal literals (C90/C++03)

[Richard Smith]

================
Comment at: include/clang/Basic/DiagnosticCommonKinds.td:119
@@ +118,3 @@
+  InGroup<ImplicitlyUnsignedLiteral>;
+def warn_old_implicitly_unsigned_long : Warning<
+  "integer literal is of type unsigned long only when %select{C99|C++11}0 "
----------------
hubert.reinterpretcast wrote:
> rsmith wrote:
> > In C++98, this should probably be an `ExtWarn` rather than a `Warning`, because this case has undefined behavior. Splitting this into two warnings would also allow you to put the warning into `-Wc99-compat` or `-Wc++11-compat` as appropriate, and to indicate that the C++98 case has undefined behavior.
> In C++98, the undefined behaviour is only on the case of a decimal integer literal with none of `u`/`U`/`l`/`L`. If the `l`/`L` is present, the choice of `unsigned long` is required to be considered after `long`.
> 
> I agree that an `ExtWarn` is appropriate for the undefined behaviour case, but I do not think that the `-Wc99-compat`/`-Wc++11-compat` split still follows.
OK, so:

 * For the cases where the code has undefined behavior, the warning should be an `ExtWarn`
 * For the cases where the code has defined behavior but changes meaning in a later standard, the warning should be in `-Wc*-compat`
 * For the cases where the code becomes invalid in a later standard, the warning should say so
 * We should split this warning up into enough separate diagnostics that we can cover all of the above cases with correct, clear, helpful diagnostics

I think the different cases are:

 1) C89, no suffix or L, LONG_MAX == LLONG_MAX (c99-compat), "will be ill-formed in C99 onwards"
 2) C89, no suffix or L, LONG_MAX < LLONG_MAX (c99-compat), "will have type 'long long' in C99 onwards"
 3) C++98, no suffix, LONG_MAX == LLONG_MAX (c++11-compat, ExtWarn), "will be ill-formed in C++11 onwards"
 4) C++98, L suffix, LONG_MAX == LLONG_MAX (c++11-compat), "will be ill-formed in C++11 onwards"
 5) C++98, no suffix, LONG_MAX < LLONG_MAX (c++11-compat, ExtWarn), "will have type 'long long' in C++11 onwards"
 6) C++98, L suffix, LONG_MAX < LLONG_MAX (c++11-compat), "will have type 'long long' in C++11 onwards"

I think you can cover the above with three different diagnostics, for 1+2, 3+5, and 4+6, with a %select in each one selecting on whether 'long long' is large enough for the literal.

Each of these should be in either `-Wc99-compat` or `-Wc++11-compat` because the meaning of the program would be different under `-std=c99` or `-std=c++11`, respectively.

http://reviews.llvm.org/D9794

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