[Issue 22202] New: Wrong error message for implicit call to @system copy constructor in @safe code

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22202

          Issue ID: 22202
           Summary: Wrong error message for implicit call to @system copy
                    constructor in @safe code
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: snarwin+bugzilla at gmail.com

Example program:

---
struct SystemCopy
{
    this(ref inout SystemCopy other) inout {}
}

void fun(SystemCopy) @safe {}

void main() @safe
{
    SystemCopy s;
    fun(s);
}
---

As of DMD 2.097.0, attempting to compile this program yields the following
error message:

---
onlineapp.d(11): Error: function `onlineapp.fun(SystemCopy _param_0)` is not
callable using argument types `(SystemCopy)`
onlineapp.d(11):        `struct SystemCopy` does not define a copy constructor
for `SystemCopy` to `SystemCopy` copies
---

This message is incorrect. fun is callable with an argument of type SystemCopy,
and SystemCopy does define a copy constructor.

The actual error is that the function call `fun(s)` requires an implicit call
to SystemCopy's copy constructor, which is @system and therefore cannot be
called from main, which is @safe.

The same incorrect error message is also given if @safe is replaced with either
pure or @nogc.

Replacing @safe with nothrow, however, results in a correct error message:

---
onlineapp.d(11): Error: copy constructor `onlineapp.SystemCopy.this` is not
`nothrow`
onlineapp.d(8): Error: `nothrow` function `D main` may throw
---

--