Disallow duplicate declarations within the same scope

[jaehyun1ee]

This PR revises the specification to disallow duplicate declarations within the same scope. This has been discussed in P4 LDWG.

• It disallows duplicate declarations, e.g.,

control p() {
    const bit<8> x = 8;
    const bit<4> x = 4;   // Error: duplicate declaration of x
    apply {}
}

• But it still allows declaration shadowing (same identifier in an inner scope), e.g.,

const bit<4> x = 1;
control p() {
    const bit<8> x = 8;   // x declaration shadows global x
    apply {}
}

• And it still allows functions and methods with the same names (up to overload resolution). Recall that P4 allows overloading of functions and methods, when they can be differentiated by either the number of parameters or the names of parameters:

extern void f(in bit<8> x);
extern void f(in bit<8> y); // allowed: different parameter names
extern void f(in bit<16> x, in bit<8> y); // allowed: different number of parameters
extern void f(in bit<16> x, in bit<8> x); // Error: duplicate declaration of previous function

Some related past issues in p4-spec and p4c are:

• Name duplication and name shadowing #974
  • This is perhaps the first issue asking for clarification regarding name duplication. Was closed after it got stale.
• Is shadowing of functions in the same namespace intended? p4c#5096
  • The recent p4c issue, which led to a discussion in 2025-November P4 LDWG meeting. The consensus was to disallow duplicate declarations within the same scope.

In particular, this PR modifies sections 6.6.1. Scopes, 6.9. Name Resolution, and 7.2.11.2. Extern Objects.

• 6.6.1 add that duplicate declarations are disallowed.
• 6.9 adds a detailed explanation of duplicate declaration, and also explains that name shadowing (up to compiler warning) and function/method overloading are still allowed.
• The part explaining function/method overloading in 7.2.11.2 was moved to 6.9.