lang 2.4: protocol-interface method calls on pack elements + conformance fix

Calling a protocol method on a pack element now works: `xs[i].greet()` on a
`..xs: Greeter` pack dispatches to the concrete element's impl, and elements
may be heterogeneous (Dog, Cat). This is the protocol-interface access the
pack is for. (Protocol method decls omit the implicit `self`; impls list it —
the earlier malformed `(self: *Self)` decls were why dispatch looked broken.)

Also fixes packArgConformsTo for non-parameterised protocols: it queried
`protocol_thunk_map`, which is only populated lazily when a protocol VALUE is
built with `xx`, so it false-negatived valid conformers. Now it queries
impl-declaration state directly — `param_impl_map` for parameterised protocols,
or `<ty>.<method>` entries in `fn_ast_map` for non-parameterised ones.

examples/193-protocol-pack-methods.sx (heterogeneous Dog+Cat pack, per-element
greet(), order-independent).

examples/193-protocol-pack-methods.sx

@@ -0,0 +1,31 @@
+// Feature 1 — protocol-interface method calls on heterogeneous pack elements.
+//
+// `..xs: Greeter` binds per call shape; each `xs[i]` is the concrete element,
+// and calling the protocol's own method `greet()` on it dispatches to that
+// element's impl. Elements may be DIFFERENT concrete types (Dog, Cat) as long
+// as each conforms to Greeter — this is the protocol-interface access the
+// pack is for. (Protocol method decls omit the implicit `self`; impls list it.)
+
+#import "modules/std.sx";
+
+Greeter :: protocol {
+    greet :: () -> s64;
+}
+
+Dog :: struct { age: s64; }
+Cat :: struct { lives: s64; }
+impl Greeter for Dog { greet :: (self: *Dog) -> s64 => self.age; }
+impl Greeter for Cat { greet :: (self: *Cat) -> s64 => self.lives * 100; }
+
+pair_sum :: (..xs: Greeter) -> s64 {
+    return xs[0].greet() + xs[1].greet();
+}
+
+main :: () -> s32 {
+    d := Dog.{ age = 3 };
+    c := Cat.{ lives = 9 };
+    print("dog+cat={}\n", pair_sum(d, c));   // 3 + 900 = 903 (heterogeneous)
+    print("cat+dog={}\n", pair_sum(c, d));   // 900 + 3 = 903 (order swapped)
+    print("dog+dog={}\n", pair_sum(d, Dog.{ age = 4 })); // 3 + 4 = 7
+    0;
+}

src/ir/lower.zig

@@ -3032,27 +3032,44 @@ pub const Lowering = struct {
         return self.protocol_thunk_map.contains(thunk_key);
     }
 
-    /// Does `ty` conform to protocol `p_name` under SOME type-args? Used to
-    /// check protocol-pack elements (`..xs: P`), where each element's
-    /// protocol type-args are inferred from its impl rather than written out.
-    /// Covers plain protocols (`protocol_thunk_map`) and parameterised ones
-    /// (any `param_impl_map` key `P\x00<args>\x00<mangle(ty)>`). An arg already
-    /// of the protocol's own (erased) type trivially conforms.
+    /// Does `ty` conform to protocol `p_name` (under SOME type-args for a
+    /// parameterised protocol)? Used to check protocol-pack elements
+    /// (`..xs: P`), where each element's protocol type-args are inferred from
+    /// its impl rather than written out.
+    ///
+    /// Conformance is queried at the IMPL-DECLARATION level (not via
+    /// `protocol_thunk_map`, which is only populated lazily when a protocol
+    /// VALUE is created with `xx`):
+    /// - Parameterised `P`: any `param_impl_map` key `P\x00<args>\x00<mangle(ty)>`.
+    /// - Non-parameterised `P`: every required (non-default) method `m` is
+    ///   registered as `<ty>.<m>` in `fn_ast_map` (how `registerImplBlock`
+    ///   records a non-parameterised impl).
+    /// An arg already of the protocol's own (erased) type trivially conforms.
     fn packArgConformsTo(self: *Lowering, p_name: []const u8, ty: TypeId) bool {
-        if (self.hasImplPlain(p_name, ty)) return true;
         // Arg already erased to the protocol struct itself (e.g. `xx a`).
         if (!ty.isBuiltin()) {
             const info = self.module.types.get(ty);
             if (info == .@"struct" and info.@"struct".is_protocol and
                 std.mem.eql(u8, self.module.types.getString(info.@"struct".name), p_name)) return true;
         }
-        const prefix = std.fmt.allocPrint(self.alloc, "{s}\x00", .{p_name}) catch return false;
-        const suffix = std.fmt.allocPrint(self.alloc, "\x00{s}", .{self.mangleTypeName(ty)}) catch return false;
-        var it = self.param_impl_map.keyIterator();
-        while (it.next()) |k| {
-            if (std.mem.startsWith(u8, k.*, prefix) and std.mem.endsWith(u8, k.*, suffix)) return true;
+        const pd = self.protocol_ast_map.get(p_name) orelse return false;
+        if (pd.type_params.len > 0) {
+            const prefix = std.fmt.allocPrint(self.alloc, "{s}\x00", .{p_name}) catch return false;
+            const suffix = std.fmt.allocPrint(self.alloc, "\x00{s}", .{self.mangleTypeName(ty)}) catch return false;
+            var it = self.param_impl_map.keyIterator();
+            while (it.next()) |k| {
+                if (std.mem.startsWith(u8, k.*, prefix) and std.mem.endsWith(u8, k.*, suffix)) return true;
+            }
+            return false;
         }
-        return false;
+        // Non-parameterised: require each non-default method as `<ty>.<m>`.
+        const ty_name = self.formatTypeName(ty);
+        for (pd.methods) |m| {
+            if (m.default_body != null) continue;
+            const q = std.fmt.allocPrint(self.alloc, "{s}.{s}", .{ ty_name, m.name }) catch return false;
+            if (!self.fn_ast_map.contains(q)) return false;
+        }
+        return true;
     }
 
     /// Evaluate a compile-time condition for `inline if`.

tests/expected/193-protocol-pack-methods.exit

@@ -0,0 +1 @@
+0

tests/expected/193-protocol-pack-methods.txt

@@ -0,0 +1,3 @@
+dog+cat=903
+cat+dog=903
+dog+dog=7