Overview:
An if-with-initializer with an empty init-statement followed by a semicolon is valid in C++17, but rejected by clang (trunk).

Steps to reproduce:
Compile this code:

int main()
{
   if (; true);
}

Actual Results:
The following errors are shown:

3 : <source>:3:6: error: expected expression
        if (; true);
            ^
3 : <source>:3:12: error: extraneous ')' before ';'
        if (; true);
                  ^

Expected results:
The code should compile.

Additional information:
According to the C++17 grammar, the init-statement in an if-with-initializer is allowed to be an empty expression followed by a semicolon.

GCC 7.2 and MSVC 19 2017 both accept this code.