stage2: fix extern fn decl parsing and astgen

[mattbork]

AstGen's fnDecl skips the call to detectLocalShadowing for parameter names of external functions. However, LocalVal scopes are still added for each parameter, so shadowing can change the type of later parameters. For example, in

const X = u32;
extern fn f(X: *X, b: *X) void;

the emitted param block looks like

%7 = param("X", {
  %4 = decl_val("X") token_offset:2:17
  %5 = ptr_type_simple(%4, One)
  %6 = break_inline(%7, %5)
}) token_offset:2:13
%10 = param("b", {
  %8 = ptr_type_simple(%7, One)
  %9 = break_inline(%10, %8)
}

which makes the type of b invalid.

These changes preserve the permissive shadowing by not adding any extern function parameter names to the current scope. This makes a test for extern functions with bodies in compile_errors.zig fail because parse.zig does not check for this like parser.cpp does, so I added that as well.

Note that this change prevents any reference to earlier parameters in an extern function, so extern fn f(a: u32, b: @TypeOf(a)) void becomes invalid. I think this is probably the right choice when shadowing is allowed, but one alternative would be to add just the parameter names that don't shadow anything to the current scope. Or perhaps the best choice is to simply apply the usual rules for shadowing to extern functions as well. Both are easy changes.

[andrewrk]

Thanks!

I think this is probably the right choice when shadowing is allowed

I agree with your conclusion.