What's a modern term for "array/pointer equivalence"?
- The "array subscripting" operator [] works just as well for pointers as it does for arrays.
No, in fact it only works for pointers. Whenever you type [] in an expression, you always get a pointer to the first element. This is guaranteed to happen since arr[i] must be equivalent to *(arr + i). The former is "syntactic sugar" for the latter.
- A function parameter that seems to be an array actually declares a pointer.
This is actually a special case, referred to as "array adjustment", where the compiler implicitly changes the declaration of a function parameter of array type into a pointer to the first element. The rationale is surely to make functions compatible with the "array decay" of expressions, but the C standard keeps the terms separate.
Both cases, expressions and function parameters, are often referred to informally as "array decay". Though sometimes this is only used for expressions and not for function parameters. I don't think there exists a single, consistent use of the term. "Array decay" is the best one I think, although the C standard does not use that term anywhere.
(I dislike the term "equivalence", because an array can turn into a pointer, but not the other way around. Indeed there's always countless beginners coming up with confused beliefs such as "arrays and pointers are the same thing". Calling them "equivalent" doesn't exactly help.)
The C standard doesn't have a single word for this. It uses the word "conversion" when defining behavior (1) in 6.3.2.1p3, "equivalent" when defining behavior (2) in 6.5.2.1p2, and "adjustment" when defining behavior (3) in 6.7.6.3p7.
I am old-fashioned, and don't think there's anything wrong with calling this "array/pointer equivalence", provided it is clear in context that you are talking about expressions where (1) happens or function declarations where (3) happens. However, a more palatable term for the people who don't like "equivalence" would perhaps be "array-to-pointer conversion", since this confuses people most often when it's (1), I think.
I would go with the term of array decay. This term goes well with what it suggests. The C standard doesn't say about it in this context and yes the first day I heard the term I went for searching it in the standard but I couldn't find it (So it's a bit confusing regarding who coined the term etc). Also alternatively one can write due to "most scenarios array is converted into pointer"... - No this is not a single Noun. But this is not letting any misinterpretation to take place. Standard itself says it the "conversion".
Most of the time I try to say it the long way and then put the word ("array decaying") in bracket. In fact there are answers where I didn't even mention it and just went with the standard's words of conversion into pointer.