[Bug tree-optimization/83657] New: detect invalid calls to built-ins declared without prototype

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83657

            Bug ID: 83657
           Summary: detect invalid calls to built-ins declared without
                    prototype
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

As pointed out in bug 83656, GCC (in C mode) fails to issue
-Wbuiltin-declaration-mismatch for the not-entirely-compatible memcpy
declaration with no prototype in the test case below.  This makes it possible
to not just overlook such declarations in source code but also miss invalid
calls to them as in the following test case.

Independent of the fix for bug 83656, this is a request to also detect and
diagnose such invalid calls, similarly to how Clang diagnoses them.

$ cat d.c && gcc -O2 -S -Wall -Wextra -Wpedantic d.c
#if __cplusplus
extern "C" void* memcpy (...);
#else
void* memcpy ();
#endif

void f (char *d)
{
  memcpy (3, d, "123");   // invalid, should be diagnosed
}


For reference, Clang issues the following warnings:
d.c:5:11: warning: incompatible integer to pointer conversion passing 'int' to
parameter of type 'void *' [-Wint-conversion]
  memcpy (3, d, "123");
          ^
d.c:5:17: warning: incompatible pointer to integer conversion passing 'char
[4]' to parameter of type 'unsigned long' [-Wint-conversion]
  memcpy (3, d, "123");
                ^~~~~
2 warnings generated.