标准中哪里说下面的 typedef 是有效的？

Question

Both g++ and clang++ compile this snippet:

typedef int int3[];
int3 i { 0, 1, 2 };

But where in the standard does it say that the array typedef declaration above is valid?

How do you infer that int3[] is a "synonym" for int[] ?

Answer 1

But where in the standard does it say that the typedef declaration above is valid?

It's valid. As per the standard:

9.2.4 The typedef specifier [dcl.typedef]

1 - Declarations containing the decl-specifier typedef declare identifiers that can be used later for naming fundamental (6.8.2) or compound (6.8.3) types. [...]

Where:

6.8.3 Compound types [basic.compound]

1 - Compound types can be constructed in the following ways:

— (1.1) arrays of objects of a given type, 9.3.4.5;

[...]

---

How do you infer that int3[] is a "synonym" for int[] ?

The declaration is well defined, including for arrays with unknown bounds:

9.3.4.5 Arrays [dcl.array]

[...]

5 - Any type of the form “cv-qualifier-seq array of NU” is adjusted to “array of N cv-qualifier-seq U”, and similarly for “array of unknown bound of U”.

[...]