Implict casts disappeared from syntactic init list expressions in C++

[Abramo Bagnara via cfe-commits]

Il 28/08/2015 23:27, Richard Smith ha scritto:
> On Tue, Aug 25, 2015 at 10:27 AM, Abramo Bagnara
> <abramo.bagnara at bugseng.com <mailto:abramo.bagnara at bugseng.com>> wrote:
> 
>     Comparing the result of InitListExpr::getSyntacticForm between r224986
>     and r245836 I've discovered that integer to char implicit cast for
>     integer literal 3 is no longer added to AST for C++ (while it is present
>     in C).
> 
>     This is the source used to test:
> 
>     char v[10] = { 3 };
> 
>     Taken in account that:
> 
>     - implicit cast (and other conversions, constructor calls, etc.) are
>     very important also for who need to visit the syntactic form (obvious in
>     *both* C and C++)
> 
>     - to generate that for the syntactic form permit to increase the
>     efficiency and the sharing when using designated range extensions (as
>     the conversion chain don't need to be replicated for each entry)
> 
>     I think it is a regression. Am I missing something?
> 
> 
> Why do you expect this semantic information to appear in the syntactic
> form of the initializer? 

Compare:

int x = 2.0;

with

struct s {
  int x;
} v = { .x = 2.0 };

For first declaration I have non-syntactic nodes (namely
ImplicitCastExpr) along with syntactic nodes, while for the second I
don't have that (for C++). This is an obstacle to write semi-syntactic
checkers that aims to find e.g. implicit cast from double to int in its
syntactic context.
Note that although we might visit the semantic form, we'll lose the
designators (not present in semantic form).

To resume, the reason why I would expect that are:

1) this is how it always has worked for C (and fortunately still works
this way)


2) this is how it always has worked (although partially, there was some
bugs) for C++. In past we have had patches to fix the areas where this
invariant was not respected (see commit 3146766
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20111212/050339.html
as an example).

This behavior has changed rather recently (if you think it is useful I
can find the commit that has removed the implicit casts from syntactic
form in C++)

Before such commit(s) the only bug I was aware where AST was missing
conversion chain was the following:

struct R2 {
  R2(int) {
  }
};
R2 v2[] = { 1.0 };


3) this way it would be congruent with other areas of AST where we have
non-syntactic nodes along with syntactic ones


4) it would permit to share more nodes in semantic form (and avoid to
rebuild many times the same conversion chain).

Looking at following typescript you can observe that ImplicitCastExpr is
shared only for C, but not for C++. I've initialized only two entries,
but it might be 1000 or 10000000.

$ cat p.c
int x[1000000] = { [0 ... 1] = 3.0 };
$ clang-3.8 -cc1 -ast-dump -x c p.c
TranslationUnitDecl 0x272de40 <<invalid sloc>> <invalid sloc>
|-TypedefDecl 0x272e338 <<invalid sloc>> <invalid sloc> implicit
__int128_t '__int128'
|-TypedefDecl 0x272e398 <<invalid sloc>> <invalid sloc> implicit
__uint128_t 'unsigned __int128'
|-TypedefDecl 0x272e648 <<invalid sloc>> <invalid sloc> implicit
__builtin_va_list 'struct __va_list_tag [1]'
`-VarDecl 0x272e718 <p.c:1:1, col:36> col:5 x 'int [1000000]' cinit
  `-InitListExpr 0x272e8b0 <col:18, col:36> 'int [1000000]'
    |-array filler
    | `-ImplicitValueInitExpr 0x272e918 <<invalid sloc>> 'int'
    |-ImplicitCastExpr 0x272e900 <col:32> 'int' <FloatingToIntegral>
    | `-FloatingLiteral 0x272e7f8 <col:32> 'double' 3.000000e+00
    `-ImplicitCastExpr 0x272e900 <col:32> 'int' <FloatingToIntegral>
      `-FloatingLiteral 0x272e7f8 <col:32> 'double' 3.000000e+00
$ clang-3.8 -cc1 -ast-dump -x c++ p.c
TranslationUnitDecl 0x3300e60 <<invalid sloc>> <invalid sloc>
|-TypedefDecl 0x3301398 <<invalid sloc>> <invalid sloc> implicit
__int128_t '__int128'
|-TypedefDecl 0x33013f8 <<invalid sloc>> <invalid sloc> implicit
__uint128_t 'unsigned __int128'
|-TypedefDecl 0x3301718 <<invalid sloc>> <invalid sloc> implicit
__builtin_va_list 'struct __va_list_tag [1]'
`-VarDecl 0x33017e8 <p.c:1:1, col:36> col:5 x 'int [1000000]' cinit
  `-InitListExpr 0x3301980 <col:18, col:36> 'int [1000000]'
    |-array filler
    | `-ImplicitValueInitExpr 0x3301a00 <<invalid sloc>> 'int'
    |-ImplicitCastExpr 0x33019d0 <col:32> 'int' <FloatingToIntegral>
    | `-FloatingLiteral 0x33018c8 <col:32> 'double' 3.000000e+00
    `-ImplicitCastExpr 0x33019e8 <col:32> 'int' <FloatingToIntegral>
      `-FloatingLiteral 0x33018c8 <col:32> 'double' 3.000000e+00


5) if we would visit semantic form in a checker searching for implicit
cast from float to int in the C source above we'll find *two* of them,
while syntactically we have only one. This means that we should be
forced do some dirty tricks to avoid double reporting.

-- 
Abramo Bagnara

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