Inconsistent and misleading error messages for non-assignable operands of [] and []= #64561

When an index, or a value assigned through operator []=, isn't assignable to the corresponding parameter type, the CFE and analyzer report it with one of two messages:

• A: "The argument type 'num' can't be assigned to the parameter type 'int'." (CFE ArgumentTypeNotAssignable, analyzer argument_type_not_assignable)
• B: "A value of type 'num' can't be assigned to a variable of type 'int'." (CFE InvalidAssignmentError, analyzer invalid_assignment)

Which message is used depends on the form of the expression, and differs between the front ends. Message B is also misleading here: there's no variable of type int, only a parameter of operator [] or operator []=.

Repro

class C {
  int operator [](int i) => 0;
  void operator []=(int i, int v) {}
}

extension E on String {
  int operator [](int i) => 0;
  void operator []=(int i, int v) {}
}

class D extends C {
  void m(num n, C c, C? nc, String s) {
    super[n];
    c[n];
    s[n];
    E(s)[n];

    super[n] = 0;
    super[0] = n;
    c[n] = 0;
    c[0] = n;
    E(s)[0] = n;
    nc?[0] = n;
    c..[0] = n;

    var x1 = super[0] = n;
    var x2 = c[0] = n;

    super[n] += 0;
    c[n] += 0;
    c[0] ??= n;
  }
}

Results (SDK built from main on 2026-10-09):

• Analyzer: consistent. Indices get A, assigned values get B.
• CFE: indices get A only for super[i] and statement-context super[i] = v; every other index gets B. Assigned values get B, except statement-context super[i] = v, which gets A. That exception arises because the CFE lowers statement-context super[i] = v to an invocation of operator []=, which is type-checked like an ordinary method call. CFE and analyzer disagree on flow analysis of super[() {...}] = v #64510 is about the same lowering; the pending fix for it keeps the current messages, leaving them to this issue.

Suggested fix

Report all of these with a single, accurate message in both front ends. Since the index and the assigned value are both arguments to operator [] / operator []=, message A is already accurate for all of them. Alternatively, a new message could name the operator, e.g. "The argument type 'num' can't be assigned to the parameter type 'int' of 'operator []='."