[cfe-users] False positive -Wcomma with static_cast of dependent expression

[Mat Sutcliffe via cfe-users]

Hello list,

The following code generates one false positive of the -Wcomma
warning: (tested with clang trunk via Matt Godbolt's Compiler
Explorer)

template 
void foo()
{
(void)42, 0;// ok
static_cast(42), 0;   // ok

(void)T{}, 0;   // ok
static_cast(T{}), 0;  // false positive
}

The warning ignores comma expressions where the LHS is a CastExpr with
a CastKind of CK_ToVoid, but the expression static_cast(T{}) has
a CastKind of CK_Dependent (confirmed with Compiler Explorer's AST
view).

I came across this when trying to do the "expander trick" without a
fold expression or C-style cast, as in
https://stackoverflow.com/a/30563282/1639256

Thanks
Mat
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Re: [cfe-users] dynmaic_cast in uninstantiated function templates

[Mat Sutcliffe via cfe-users]

On Wed, 24 Oct 2018 at 02:06, Olaf wrote:
>
> the code below compiles on gcc and icc, but not on clang with the error:
>
> source_file.cpp:14:16: error: 'B' is an incomplete type
>   if (auto b = dynamic_cast(a))
>
> However fn is an uninstantiated function template.
> Is this a glitch in clang or is clang the only one right here?
>
> class B;
>
> template
> void fn(A* a, T& x)
> {
>   dynamic_cast(a);
> }

All three compilers are correct here, as far as the standard is
concerned. The code is ill-formed, but implementations are not
required to diagnose this kind of error. There is no type T that could
produce a well-formed instantiation of fn.
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