My reading of the C++ standard is that this behavior is effectively mandated and that one can write a program which can tell if an ABI observed the proposed optimization.

[expr.call]: "The lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the argument expression."

[conv.lval]: "... if T has a class type, the conversion copy-initializes the result object from the glvalue."

The way a program can tell if a compiler is compliant to the standard is like so:

  struct S { int large[100]; };

  int compliant(struct S a, const struct S *b);

  int escape(const void *x);

  int bad() {
    struct S s;
    escape(&s);
    return compliant(s, &s);
  }

  int compliant(struct S a, const struct S *b) {
    int r = &a != b;
    escape(&a);
    escape(b);
    return r;
  }

There are three calls to 'escape'. A programmer may assume that the first and third call to escape observes a different object than the second call to escape and they may assume that 'compliant' returns '1'.