Vscode-cpptools: Linter provides no warnings for invalid C code

[joonoro]

See title, create a fresh test.c file with the contents:

#include <stdio.h>

int main()
{
    asdfd;
    return 0;
}

(If) Linting is working properly, asdfd will be underlined with squigglies as you would expect. What you would not expect is that turning it into a function call makes the squigglies go away:

#include <stdio.h>

int main()
{
    asdfd();
    return 0;
}

Furthermore, hovering over it produces the popup int asdfd(), as if it was defined to return int somewhere. This is just silly.

Related to #746

[bobbrow]

Indeed. That is silly. We'll investigate.

[sean-mcmanus]

This only repros in C files, not C++. It looks like the IntelliSense compiler is using the old default int function return value C89 behavior, so asdfd() is being interpreted as a declaration for "int asdfd()", even though we're passing in the C11 flag.

[joonoro]

Ok so it's valid C89, but surely this should be an error even in C89?

#include <stdio.h>
#include <stdlib.h>

int main()
{
    char *line = maloc(100);
    return 0;
}

The misspelled malloc line shows squigglies only when placed outside the main function, but no squigglies inside. Still shows the int hint.

[sean-mcmanus]

Visual Studio 2017 also shows maloc (or any other undefined function call identifier) as being a function returning an int. I'll ask the VS-side people if they think our shared compiler has a bug.

[sean-mcmanus]

While it's not valid in the C99 standard, it looks like all the C compilers we support don't support this rule, I assume because it would be a breaking change with C89. So this is currently "by design", but if you can locate us a compiler (or linter) that enforces the non-implicit int behavior, we theoretically could add a new flag to trigger the enforcement behavior.

[joonoro]

Sorry I'm not sure I understand, the last example I posted doesn't compile with either GCC or Clang even with -std=c89, yet VS Code doesn't show any error squigglies for it. Is it not an error? Something is definitely wrong here. There should at least be warnings.

test.c:8:11: warning: incompatible integer to pointer conversion initializing 'char *' with an expression of type 'int' [-Wint-conversion]
    char *line = maloc(100);
          ^      ~~~~~~~~~~
1 warning generated.
/tmp/test-3f53c3.o: In function `main':
test.c:(.text+0x17): undefined reference to `maloc'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[sean-mcmanus]

Thanks for the info. The compiler is not giving you an error -- it successfully compiles and fails during linking. IntelliSense currently only gives compiler errors. This sounds like a feature request for warnings and/or linker errors.

[joonoro]

Warnings + linker errors sounds good to me.

[sean-mcmanus]

FYI, you can enable the warnings "experimental/hidden" feature via modifiying the msvc.json file next to the binary by removing the --no_warnings flag. Not sure when we'll get around to adding a setting for that. VS 2017 doesn't have these warnings enabled so it hasn't been tested as much. A previous issue reported was https://github.com/Microsoft/vscode-cpptools/issues/2091 .

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