[BUG] Function expression can't be the left argument to `is`/`as`

[JohelEGP]

Title: Function expression can't be the left argument to is/as.

Minimal reproducer (https://cpp2.godbolt.org/z/K53erj1fh):

main: () = {
  :() = 0; is int;
}

Commands:

cppfront -clean-cpp1 main.cpp2

Expected result:

Same as (https://cpp2.godbolt.org/z/8nszP1Wo4):

main: () = {
  cp := :(x) -> _ = x;
  cp(:() = 0) is int;
}

Actual result and error:

main.cpp2(1,12): error: ill-formed initializer (at '{')
main.cpp2(1,1): error: unexpected text at end of Cpp2 code section (at 'main')
main.cpp2(1,0): error: parse failed for section starting here

[msadeqhe]

I think your example code is wrong, because:

• If the return type of an unnamed function is omitted, it will be void.
• Use parenthesis instead of ;, otherwise the statement will be ended before is.

This is the correct example code (it needs parenthesis around the unnamed function):

main: () = {
    x: = (: () -> int = 0) is int; // true
}

Or this one (it doesn't need parenthesis around the unnamed function):

main: () = {
    x: = : () = {} is int; // false
}

[filipsajdak]

I have a fix for that. @JohelEGP found an actual bug.

At the same time, I agree with you, @msadeqhe, that this code should rise an error as it discards the result of the is expression.

[filipsajdak]

I need to correct my build system to handle self-compilation by the previous version of cppfront. I will deliver the patch soon.

[filipsajdak]

By writing above messages I just realize that is and as functions has discardable results - probably they should be all marked with [[nodiscard]]

[filipsajdak]

Just in case you would like to test the fix by yourself, or if I am not able to deliver fast, I am attaching a diff:

diff --git a/source/parse.h b/source/parse.h
index f716925..65d2fb0 100644
--- a/source/parse.h
+++ b/source/parse.h
@@ -3606,6 +3606,10 @@ private:
                     && curr().type() != lexeme::LeftParen               // not imediatelly called
                     && curr().type() != lexeme::RightParen              // not as a last argument to function
                     && curr().type() != lexeme::Comma                   // not as first or in-the-middle, function argument
+                    && curr() != "is"
+                    && curr() != "as"
+                    && curr() != "do"
+                    && curr() != "next"
                 ) {
                     // this is a fix for a short function syntax that should have double semicolon used
                     // (check comment in expression_statement(bool semicolon_required))

[JohelEGP]

If the return type of an unnamed function is omitted, it will be void.

It's still valid code. Not practically useful, but slide-ware/example-ware useful. Like for reporting bugs.

Use parenthesis instead of ;, otherwise the statement will be ended before is.

I think this should work to disambiguate when you want the is outside the function expression's statement.
Seems like a natural fit, considering the other cases you can add ;.
So I think it's a bug that it doesn't work.

I need to correct my build system to handle self-compilation by the previous version of cppfront. I will deliver the patch soon.

I didn't notice because the generated Cpp1 source files are checked in.
I just launched my usual command to compile cppfront and it worked.

[filipsajdak]

I didn't notice because the generated Cpp1 source files are checked in.
I just launched my usual command to compile cppfront and it worked.

My usual command worked as well but after I have added reflect.h2 to dependencies I have created recurrence in dependencies.

Nevertheless, I want my cmake to be able to build cppfront from scratch.

[filipsajdak]

The original code is not working, but the following is:

main: () = {
  (:() = 0;) is int;
}