[cfe-users] Get AST of uncomplete C++ code

[Jan Korous via cfe-users]

> On Apr 24, 2020, at 2:44 PM, Richard Smith via cfe-users <cfe-users at lists.llvm.org> wrote:
> 
> On Tue, 21 Apr 2020 at 05:41, Dr S3curity via cfe-users <cfe-users at lists.llvm.org <mailto:cfe-users at lists.llvm.org>> wrote:
> Hi,
> 
> Imagine the very basic code below, it has some missing elements, we dont have the foo() function and MyClass class,
> 
> ```test.cpp
> int main(int argc, char *argv[])
> {
>     MyClass* mc = new MyClass();
>     mc.get_count();
>     foo(1, 2, 3);
>     std::cout << "this is a test" << endl;
> }
> ```
> 
> How can I get AST for that code?
> I tried `clang -Xclang -ast-dump -fsyntax-only test.cpp`, I can get the AST tree but without MyClass and foo() function, it doesn't even say foo is a function with 3 integer parameters.

I am not really familiar with -frecovery-ast but since even such relatively simple C++ snippet is ambivalent I am not sure clang can actually answer your questions completely :(
It would have to "know" things like the following before it could tell you what "foo" or "MyClass" are.

- Can't foo be a global object of class with call operator accepting 3 int-s?
- Do the parameters actually need to be int-s or just any types that have implicit conversions from int?
- Can't MyClass be a typedef?
- Can't macros be involved?
- Can't the code actually be semantically incorrect?

> This can be problematic for different cases for example IDE's where you dont have all the project dependencies but yet you need to know what are the elements in your code.
> 
> Is that possible to do with libclang? Any other suggestions are welcome.
> 
> You can request that more invalid portions of the AST are retained by using `-Xclang -frecovery-ast`. In this case, that will preserve the function call expression as a RecoveryExpr. For now at least, we don't have the ability to preserve AST information for the invalid "new MyClass" nor the expression using of undeclared "std::cout" and "endl", but -frecovery-ast is expected to improve over time to retain more such AST nodes. (And perhaps eventually it will be enabled by default, at which point the flag for it might disappear. We don't guarantee any forward compatibility with -Xclang flags.)

IIUC you'd still have to decide what you think the RecoveryExpr AST node actually is - it could be a CXXMemberCallExpr if it's a call operator or just a CallExpr if foo is a free function.

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