[Bug c/122115] Add warning to catch implicit type conversions

[jg at jguk dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122115

--- Comment #3 from Jonny Grant <jg at jguk dot org> ---
// Not real code, just example
#include <stdbool.h>
#include <stddef.h>
#include <stdlib.h>

size_t f()
{
    return (size_t)rand();
}

size_t f_arg(size_t val)
{
    return (size_t)rand() * val;
}

enum
{
    myenum,
    otherenum = -1
};

int main()
{
    // Implicit conversion of return
    bool foo = f();
    __builtin_printf("%d\n", foo);

    // implicit conversion: int -> bool (nonzero becomes true)
    int n = 5;
    bool b = n;

    __builtin_printf("n=%d, b=%d\n", n, b);

    // Implicit conversion of argument
    size_t ret = f_arg(foo);
    __builtin_printf("%zu\n", ret);

    // Implicit conversion in arithmetic
    size_t sum = b*foo;
    __builtin_printf("sum %zu\n", sum);

    // Implicit conversion of enum
    size_t enum_copy = myenum;
    __builtin_printf("enum_copy %zu\n", enum_copy);

    // Implicit conversion of *negative* enum to bool
    bool enum_bool = otherenum;
    __builtin_printf("enum_bool %d\n", enum_bool);

    // Implicit conversion in conditions
    if(f()) __builtin_printf("f() true\n");
}


Warnings such as:

test.c:25: warning: implicit conversion of return 'size_t' -> 'bool'
[implicit-bool-conversion]
test.c:30: warning: implicit conversion in assignment 'int' -> 'bool'
[implicit-bool-conversion]
test.c:35: warning: implicit conversion in argument 'size_t' -> 'bool'
[implicit-bool-conversion]
test.c:39: warning: implicit conversion in arithmetic 'bool' and 'bool' ->
'size_t' [implicit-conversion]
test.c:44: warning: implicit conversion in assignment 'enum' -> 'size_t'
[implicit-enum-conversion]
test.c:47: warning: implicit conversion in assignment 'enum' -> 'bool'
[implicit-enum-conversion]
test.c:51: warning: implicit conversion in condition 'size_t' -> 'bool'
[implicit-bool-conversion]


I'm mostly interested in the implicit conversion to bool in return (assignment
to size_t) in this example.
Yes, I appreciate the C language allows all these implicit conversions, and
they convert to int. clang-tidy has some checkers for these sort of things.

I recall C99 added _Bool so it should be a real type.

I couldn't get warnings from this program using -Wconversion and -std=c99
https://godbolt.org/z/Wo6qGEMT3