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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c++
Subject: Re: I think references should have been const by default
Date: Mon, 25 Oct 2021 10:56:24 -0700
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James Kuyper <jameskuyper@alumni.caltech.edu> writes:
> On 10/25/21 5:47 AM, Juha Nieminen wrote:
> ...
>> Most modern C compilers will give you a warning if you try to assign
>> a const char* (eg. a string literal) to a non-const char* (or give
>> one to a function taking a non-const char*),
>
> They must do so on assignment; 6.5.16p2 occurs in a "Constraints" section:
> "An assignment operator shall have a modifiable lvalue as its left operand."
>
> And if a function prototype is in scope "... the arguments are
> implicitly converted, as if by assignment, to the types of the
> corresponding parameters ..." (6.5.2.2p7), so the same constraints apply
> there, too. For the same reason, they also apply to return statements.

I think the example being referred to was something like:
    char *s;
    s = "hello";
which does not require a diagnostic in C (because C string literals are
not const).  The following is recommended in C and required in C++:
    const char *s;
    s = "hello";
but here s is still a modifiable lvalue because the "const" applies to
what s points to, not to s itself.

A case that would invoke the constraint in 6.5.16p2 is:
    char *const s;
    s = "hello";
because s itself is read-only; you can't assign *anything* to it.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips
void Void(void) { Void(); } /* The recursive call of the void */