I believe this is mostly covered by §3.5/6.
In particular:
The name of a function declared in block scope and the name of a variable declared by a block scope extern declaration have linkage. If there is a visible declaration of an entity with linkage having the same name and type, ignoring entities declared outside the innermost enclosing namespace scope, the block scope declaration declares that same entity and receives the linkage of the previous declaration. If there is more than one such matching entity, the program is ill-formed. Otherwise, if no matching entity is found, the block scope entity receives external linkage.
So, the extern int w;
declares a w
that has linkage (external linkage, in this case, since no matching entity is visible at that point).
Then you attempt to define a local w
which has no linkage (by §3.5/8).
That gives two declarations of the same name at the same scope, but with different linkages. That's prohibited by §3.3.1/4:
Given a set of declarations in a single declarative region, each of which specifies the same unqualified name,
- they shall all refer to the same entity, or all refer to functions and function templates; or
- exactly one declaration shall declare a class name or enumeration name that is not a typedef name and the other declarations shall all refer to the same variable or enumerator, or all refer to functions and function templates; in this case the class name or enumeration name is hidden (3.3.10).
Neither refers to a function, function template, class name, or enumeration name, so none of these "escape clauses" applies. The two declarations must refer to the same entity, which must have both external linkage and no linkage. Since that's impossible, the code is ill-formed.