lang: generic struct head aliases bind the template (fix 0120) — alias-follow from each author's source in head selection; loud unknown-type on the .call type tail

BoxAlias :: Box; / Box :: r.Box; now resolve instantiation, methods,
annotations, and chains through the aliased template, and re-export one
flat-import level as ordinary own decls (the facade shape the std.sx
restructure needs). selectGenericStructHead consults aliasedStructTemplate
(nominal.zig) before the global template map — own-wins/single-flat alias
author, each hop pinned to the alias author's source, ns.X RHS through
namespaceAliasVerdictFrom, depth-capped. resolveTypeCallWithBindings'
silent .unresolved tail (panicked in LLVM emission) now diagnoses
"unknown type". Also aligns the stale pre-existing calls.test.zig UFCS
plan test with the opt-in model (a47ea14). Regression: examples/0211
(+rich/+facade). Gates: zig build test 426/426, suite 587/587.

examples/0211-generics-struct-alias-head-facade.sx

@@ -0,0 +1,9 @@
+// Companion of 0211: the re-export facade — own alias decls over another
+// module's members. Flat importers of THIS file see the aliases bare.
+#import "modules/std.sx";
+
+r :: #import "0211-generics-struct-alias-head-rich.sx";
+
+helper :: r.helper;
+Thing :: r.Thing;
+Box :: r.Box;

examples/0211-generics-struct-alias-head-rich.sx

@@ -0,0 +1,15 @@
+// Companion of 0211: the authoring module — a plain fn, a plain struct,
+// and a generic struct, all re-exported by -facade.sx via alias decls.
+#import "modules/std.sx";
+
+helper :: () -> s64 { 7 }
+
+Thing :: struct {
+    v: s64;
+    init :: () -> Thing { Thing.{ v = 42 } }
+}
+
+Box :: struct ($T: Type) {
+    item: T;
+    get :: (b: *Box(T)) -> T { b.item }
+}

examples/0211-generics-struct-alias-head.sx

@@ -0,0 +1,43 @@
+// Generic-struct head aliases: `BoxAlias :: Box;` binds the alias to the
+// SAME template — instantiation, methods, annotations, and alias chains all
+// resolve through it. Cross-module, a facade's `Box :: r.Box;` re-export is
+// the facade's OWN declaration, so it carries one flat-import level exactly
+// like a plain-struct alias (companion files: -rich.sx authors the decls,
+// -facade.sx re-exports them through a namespace alias).
+// Regression (issue 0120): the alias head used to lower silently to an
+// unresolved type and panic in the LLVM backend at instantiation.
+#import "modules/std.sx";
+#import "0211-generics-struct-alias-head-facade.sx";
+
+LocalBox :: struct ($T: Type) {
+    item: T;
+    get :: (b: *LocalBox(T)) -> T { b.item }
+}
+
+LocalAlias :: LocalBox;
+ChainAlias :: LocalAlias;
+
+main :: () {
+    // Same-file alias: instantiation + field + method.
+    b := LocalAlias(s64).{ item = 3 };
+    print("field: {}\n", b.item);
+    print("method: {}\n", b.get());
+
+    // Alias chain terminates at the template.
+    c := ChainAlias(s64).{ item = 11 };
+    print("chain: {}\n", c.item);
+
+    // Alias as a type annotation head.
+    a : LocalAlias(string) = .{ item = "ann" };
+    print("annot: {}\n", a.item);
+
+    // Cross-module re-exports carried one flat hop from the facade:
+    // plain fn, plain struct (static method), and the generic head.
+    print("helper: {}\n", helper());
+    t := Thing.init();
+    print("thing: {}\n", t.v);
+    f := Box(s64).{ item = 7 };
+    print("facade: {}\n", f.get());
+    x : Box(string) = .{ item = "qq" };
+    print("facade-annot: {}\n", x.item);
+}

examples/expected/0211-generics-struct-alias-head.exit

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

examples/expected/0211-generics-struct-alias-head.stderr

@@ -0,0 +1 @@
+

examples/expected/0211-generics-struct-alias-head.stdout

@@ -0,0 +1,8 @@
+field: 3
+method: 3
+chain: 11
+annot: ann
+helper: 7
+thing: 42
+facade: 7
+facade-annot: qq

issues/0120-generic-struct-alias-head-unresolved-panic.md

@@ -0,0 +1,161 @@
+# 0120 — aliasing a GENERIC struct head: silent `.unresolved`, backend panic
+
+> **RESOLVED** (2026-06-11, same session — Agra-directed fix). Root
+> cause: a const alias of a generic struct head was registered nowhere
+> (`type_alias_map` holds TypeIds, `struct_template_map` only direct
+> struct decls), and the head selector's miss fell through as
+> `.not_generic`; the `.call`-node type resolver then returned
+> `.unresolved` SILENTLY (its parameterized sibling diagnosed; it
+> didn't). Fix, option 1 (support): `selectGenericStructHead` now
+> follows const-alias decls (`aliasedStructTemplate` in
+> `src/ir/lower/nominal.zig`) — own-wins / single-flat author, each hop
+> resolved from the ALIAS AUTHOR's source (`namespaceAliasVerdictFrom`
+> for `ns.X` RHS), depth-capped against cycles, checked BEFORE the
+> template map so a facade's same-name re-export beats an invisible
+> global template. Plus the missing diagnostic: an unknown `.call` type
+> head now errors "unknown type 'X'" instead of silently poisoning
+> (`resolveTypeCallWithBindings`). Alias-vs-alias flat collisions stay
+> loud (not-visible diagnostic). Still unsupported, by scope:
+> `ns.AliasName(..)` qualified heads (namespace member that is itself
+> an alias). Regression test:
+> `examples/0211-generics-struct-alias-head.sx` (+ `-rich.sx` /
+> `-facade.sx` companions; pins same-file alias, method, chain,
+> annotation, and the cross-module facade re-export). Gates: zig build
+> test 426/426 (incl. fixing the PRE-EXISTING stale
+> `calls.test.zig` UFCS plan test that predated 0119's opt-in model),
+> suite 587/587.
+
+## Symptom
+
+`Alias :: Box;` where `Box` is a generic struct (`struct ($T: Type)`)
+lowers without any diagnostic, and instantiating through the alias
+(`Alias(s64).{ ... }`) reaches LLVM emission with an `.unresolved`
+type — the backend tripwire panics:
+
+```
+panic: unresolved type reached LLVM emission — a type resolution
+failure was not diagnosed/aborted
+  src/backend/llvm/types.zig:175 toLLVMTypeInfo
+  src/backend/llvm/ops.zig:1204 emitStructInit
+```
+
+Observed (one probe family, three manifestations of the same root):
+
+- field access through the aliased instantiation → **backend panic**
+  (no front-end diagnostic at all);
+- method call through the aliased instantiation (`b.get()`) →
+  misleading `unresolved 'get'` (the receiver's type never resolved);
+- cross-module re-export (`facade.sx`: `Box :: r.Box;`, consumer
+  flat-imports facade) → consumer gets `type 'Box' is not visible;
+  #import the module that declares it` even though the alias is the
+  facade's OWN declaration.
+
+Expected: one of the two, decided explicitly —
+
+1. **Support it** (desirable): a const decl whose RHS names a generic
+   struct head (bare `Box` or qualified `r.Box`) binds the alias to the
+   SAME template; instantiation, methods, and one-level flat-import
+   carry behave exactly as the non-generic struct alias already does.
+2. **Reject it loudly**: a decl-site diagnostic ("cannot alias a
+   generic struct head" or similar) at `Alias :: Box;`.
+
+Silently lowering and panicking in the backend is neither — it is the
+REJECTED-PATTERNS "silent unresolved" shape.
+
+For contrast, both of these alias re-exports already WORK across one
+flat-import hop (own-decl visibility): `helper :: r.helper;` (plain
+fn) and `Thing :: r.Thing;` (non-generic struct, including its static
+`init`). Only the generic head breaks. A fix must not regress these.
+
+## Reproduction
+
+Backend panic (primary):
+
+```sx
+#import "modules/std.sx";
+
+Box :: struct ($T: Type) {
+    item: T;
+}
+
+BoxAlias :: Box;
+
+main :: () {
+    b := BoxAlias(s64).{ item = 3 };
+    print("{}\n", b.item);
+}
+```
+
+Method-call variant (front-end `unresolved 'get'`, same root):
+
+```sx
+#import "modules/std.sx";
+
+Box :: struct ($T: Type) {
+    item: T;
+    get :: (b: *Box(T)) -> T { b.item }
+}
+
+BoxAlias :: Box;
+
+main :: () {
+    b := BoxAlias(s64).{ item = 3 };
+    print("{}\n", b.get());
+}
+```
+
+Cross-module variant (`rich.sx` declares `Box`; `facade.sx` has
+`r :: #import "rich.sx"; Box :: r.Box;`; a consumer flat-importing
+facade.sx gets `type 'Box' is not visible` at `Box(s64).{ ... }`).
+
+## Investigation prompt
+
+Generic structs live as TEMPLATES in
+`src/ir/program_index.zig` — `struct_template_map`
+(`StringHashMap(StructTemplate)`, registered by `registerStructDecl`;
+a parallel `struct_template_by_decl` exists but isn't read for
+selection yet). Instantiation resolves the head name against that map
+in `src/ir/lower/nominal.zig` (see the qualified-head comments around
+nominal.zig:357–382) and monomorphizes via
+`lower_generic.instantiateGenericStruct` (re-exported at
+`src/ir/lower.zig:1820`).
+
+`BoxAlias :: Box;` is a const decl whose RHS identifier names a
+template, not a value or a concrete Type — const-decl lowering neither
+registers `BoxAlias` as a template alias nor rejects the decl. The
+instantiation head lookup for `BoxAlias` then misses, and the
+`Name(args).{ ... }` path continues with an `.unresolved` struct type
+instead of diagnosing the miss — that silent continuation is the bug
+underneath all three manifestations, and fixing it is step one
+regardless of the language decision: a struct_init whose head fails to
+resolve must produce a hard diagnostic, never reach emission.
+
+Then the language decision (confirm with Agra if option 2 is ever
+preferred; the motivating use case wants option 1): when a const
+decl's RHS resolves to a generic struct head — bare identifier or
+`ns.X` through a namespace alias — register the alias name in the
+template registry bound to the same `StructTemplate`, scoped to the
+declaring module with ordinary own-decl visibility so one-level
+flat-import carry works (mirror whatever makes `Thing :: r.Thing;`
+re-export correctly today). Mind collision semantics (own-wins /
+ambiguity) and that the alias must also work as a plain type head in
+annotations (`x: BoxAlias(s64)`), nested generics
+(`List(BoxAlias(s64))` if applicable), and method/UFCS dispatch on
+instantiations through the alias.
+
+Motivating context: the std.sx-as-pure-re-exports restructure wants
+`List :: list.List;` in `modules/std.sx` (with `list :: #import
+"modules/std/list.sx";`) so `List` stays bare-visible to std.sx's flat
+importers. Plain fns and plain structs already re-export this way;
+generic heads are the missing piece.
+
+Verification:
+
+1. Primary repro: prints `3`, exit 0 (option 1) — or a clean decl-site
+   diagnostic, no panic (option 2).
+2. Matrix: method-call variant runs (`b.get()` → 3); cross-module
+   variant runs through the facade; `helper :: r.helper;` and
+   `Thing :: r.Thing;` re-exports unchanged; two facades carrying the
+   same alias name still diagnose ambiguity.
+3. `bash tests/run_examples.sh` — full suite ok, zero failures.
+4. Pin the repro as a regression example per CLAUDE.md.

readme.md

@@ -483,6 +483,25 @@ Carried aliases follow declaration rules: an own declaration shadows a carried
 alias, two flat imports carrying the same alias make its use ambiguous, and
 carry does not chain through a second flat hop.
 
+**Re-exporting through alias declarations.** Since visibility never chains,
+a facade re-exports another module's members as its OWN declarations —
+ordinary aliases, which its direct flat importers then see bare. This works
+for functions, plain types, and generic struct heads alike (the generic
+alias binds the same template, so instantiation and methods resolve
+through it):
+
+```sx
+// facade.sx
+r :: #import "rich.sx";
+helper :: r.helper;       // fn re-export
+Thing  :: r.Thing;        // struct re-export
+Box    :: r.Box;          // generic head re-export — same template
+
+// consumer.sx
+#import "facade.sx";
+b := Box(s64).{ item = 3 };   // rich.sx's Box, via the facade
+```
+
 ### Implicit Context
 
 Every program gets an implicit `context` with a default allocator:

specs.md

@@ -1251,6 +1251,25 @@ A name bound to an existing type.
 SOME_TYPE :: f64;
 ```
 
+A generic struct HEAD can be aliased too — the alias binds to the same
+template, so instantiation, methods, annotations, and alias chains resolve
+through it:
+
+```sx
+Box :: struct ($T: Type) { item: T; }
+BoxAlias :: Box;                          // same template
+b := BoxAlias(s64).{ item = 3 };
+b2 : BoxAlias(string) = .{ item = "x" };  // annotation head too
+```
+
+The RHS may be a namespace member (`Box :: r.Box;`) — the alias is an
+ordinary OWN declaration of the aliasing file, so it is visible to that
+file's direct flat importers like any other declaration (this is how a
+facade re-exports another module's generic struct). Each hop of an alias
+chain resolves with the visibility of the file that declares THAT hop,
+not the use site's. Not yet supported: a qualified head whose namespace
+member is itself an alias (`ns.BoxAlias(..)`).
+
 ### Generic Functions (Monomorphization)
 Functions can be parameterized over types using `$T` syntax. The `$` prefix introduces a type parameter; subsequent uses of the name reference it.
 ```sx

src/ir/calls.test.zig

@@ -429,14 +429,17 @@ test "plan: free-function UFCS prepends receiver, distinct from namespace_fn" {
     var l = Lowering.init(&module);
     const cr = CallResolver{ .l = &l };
 
-    // struct Counter, and a FREE function `bump :: (c: Counter) -> s32` — NOT
-    // registered as `Counter.bump`, so it can only be reached via UFCS.
+    // struct Counter, and a FREE ufcs function `bump :: ufcs (c: Counter) ->
+    // s32` — NOT registered as `Counter.bump`, so it can only be reached via
+    // UFCS. Dot-dispatch is OPT-IN: the fn carries `is_ufcs` and is
+    // registered in `fn_ast_map`, where the plan's opt-in gate reads it.
     const counter = module.types.intern(.{ .@"struct" = .{ .name = module.types.internString("Counter"), .fields = &.{} } });
     const c_param = ast.Param{ .name = "c", .name_span = .{ .start = 0, .end = 0 }, .type_expr = typeExpr(alloc, "Counter") };
     const params = [_]ast.Param{c_param};
     const ret_stmt = mk(alloc, .{ .return_stmt = .{ .value = intLit(alloc, 7) } });
     const body = mk(alloc, .{ .block = .{ .stmts = &[_]*Node{ret_stmt} } });
-    const fd = ast.FnDecl{ .name = "bump", .params = &params, .return_type = typeExpr(alloc, "s32"), .body = body };
+    const fd = ast.FnDecl{ .name = "bump", .params = &params, .return_type = typeExpr(alloc, "s32"), .body = body, .is_ufcs = true };
+    l.program_index.fn_ast_map.put("bump", &fd) catch unreachable;
     l.lowerFunction(&fd, "bump", false);
     const fid = l.resolveFuncByName("bump").?;
     module.functions.items[@intFromEnum(fid)].has_implicit_ctx = true;

src/ir/lower.zig

@@ -1186,8 +1186,16 @@ pub const Lowering = struct {
     /// edges of flat edges do not chain). Two distinct carried targets for
     /// the same alias are ambiguous.
     pub fn namespaceAliasVerdict(self: *Lowering, alias: []const u8) AliasVerdict {
-        const edges = self.program_index.namespace_edges orelse return .none;
         const from = self.current_source_file orelse return .none;
+        return self.namespaceAliasVerdictFrom(alias, from);
+    }
+
+    /// `namespaceAliasVerdict` with an explicit querying source — for callers
+    /// resolving an alias on behalf of ANOTHER module (e.g. following a const
+    /// alias decl whose RHS is `ns.X`: `ns` binds in the alias author's file,
+    /// not the use site's).
+    pub fn namespaceAliasVerdictFrom(self: *Lowering, alias: []const u8, from: []const u8) AliasVerdict {
+        const edges = self.program_index.namespace_edges orelse return .none;
         if (edges.getPtr(from)) |own| {
             if (own.get(alias)) |t| return .{ .target = t };
         }
@@ -1679,6 +1687,7 @@ pub const Lowering = struct {
     pub const rawNamedTypePtr = lower_nominal.rawNamedTypePtr;
     pub const buildGenericStructTemplate = lower_nominal.buildGenericStructTemplate;
     pub const qualifiedStructTemplate = lower_nominal.qualifiedStructTemplate;
+    pub const aliasedStructTemplate = lower_nominal.aliasedStructTemplate;
     pub const qualifiedMemberMissing = lower_nominal.qualifiedMemberMissing;
     pub const bareVisibleStructDecl = lower_nominal.bareVisibleStructDecl;
     pub const bareVisibleStructTemplate = lower_nominal.bareVisibleStructTemplate;

src/ir/lower/generic.zig

@@ -974,6 +974,17 @@ pub fn selectGenericStructHead(self: *Lowering, name: []const u8, alias: ?[]cons
         if (self.program_index.struct_template_map.getPtr(name)) |tmpl| return .{ .template = tmpl.* };
         return .not_generic;
     }
+    // Const-alias head (`BoxAlias :: Box;` / `Box :: r.Box;`, issue 0120):
+    // follow the alias decl hop-by-hop to its authoring template, each hop
+    // resolved from that alias author's own source. Checked BEFORE the map:
+    // the alias may share its name with a same-name template that is NOT
+    // visible from here (a facade's `Box :: r.Box;` re-export of rich's
+    // `Box`), and the map branch would poison on that invisible author.
+    // Only fires when the single visible author (own-wins / single-flat)
+    // IS an alias-shaped const decl, so real template heads are untouched.
+    if (self.current_source_file) |from| {
+        if (self.aliasedStructTemplate(name, from)) |t| return .{ .template = t };
+    }
     if (self.program_index.struct_template_map.getPtr(name)) |tmpl| {
         if (self.headTypeLeak(name, span)) return .poisoned;
         if (self.bareVisibleStructTemplate(name)) |vt| return .{ .template = vt };
@@ -1230,7 +1241,14 @@ pub fn resolveTypeCallWithBindings(self: *Lowering, cl: *const ast.Call) TypeId
     }
     // Try as a named type
     const name_id = self.module.types.internString(callee_name);
-    return self.module.types.findByName(name_id) orelse .unresolved;
+    if (self.module.types.findByName(name_id)) |t| return t;
+    // The callee names no known type constructor — not Vector, not a generic
+    // struct template (or alias), not a type-returning function, not a named
+    // type. A silent `.unresolved` here reaches LLVM emission as a panic;
+    // diagnose and poison (the parameterized sibling below already does).
+    if (self.diagnostics) |d|
+        d.addFmt(.err, cl.callee.span, "unknown type '{s}'", .{callee_name});
+    return .unresolved;
 }
 
 /// Resolve a parameterized type expr, substituting bindings for type/value params.

src/ir/lower/nominal.zig

@@ -394,6 +394,77 @@ pub fn qualifiedMemberMissing(self: *Lowering, alias: []const u8, member: []cons
     return true;
 }
 
+/// The `*ConstDecl` a raw author wraps when it is a const ALIAS of another
+/// name — `BoxAlias :: Box;` (identifier RHS) or `Box :: r.Box;` (namespace-
+/// member RHS). Null for every other shape, including const-wrapped struct /
+/// fn DEFINITIONS, which are authors in their own right.
+fn constAliasOfRaw(ref: resolver_mod.RawDeclRef) ?*const ast.ConstDecl {
+    return switch (ref) {
+        .const_decl => |cd| switch (cd.value.data) {
+            .identifier, .field_access => cd,
+            else => null,
+        },
+        else => null,
+    };
+}
+
+/// The single author of `name` as seen from `from` — own wins, else exactly
+/// one flat-import author. Null when absent or when ≥2 flat authors compete
+/// (the use site then diagnoses the unresolved head; no silent pick).
+fn singleVisibleAuthor(self: *Lowering, name: []const u8, from: []const u8) ?resolver_mod.RawAuthor {
+    var res = self.resolver();
+    const set = res.collectVisibleAuthors(name, from, .user_bare_flat);
+    defer if (set.flat.len > 0) self.alloc.free(set.flat);
+    if (set.own) |o| return o;
+    if (set.flat.len == 1) return set.flat[0];
+    return null;
+}
+
+/// Resolve `name`, as seen from `from`, to a generic-struct template by
+/// following const ALIAS declarations (issue 0120). Entry for the head
+/// selector's bare tail: the FIRST hop must be alias-shaped — a direct
+/// struct author is the template map's business, never this path's. Each
+/// hop resolves from the ALIAS AUTHOR's source, so visibility is the
+/// author's, not the use site's (a consumer one flat hop from a facade
+/// reaches the facade's `Box :: r.Box;` without seeing `r` itself).
+pub fn aliasedStructTemplate(self: *Lowering, name: []const u8, from: []const u8) ?StructTemplate {
+    const author = singleVisibleAuthor(self, name, from) orelse return null;
+    if (constAliasOfRaw(author.raw) == null) return null;
+    return followToTemplate(self, author, 8);
+}
+
+/// One alias hop: a generic-struct author terminates the chain with its
+/// rebuilt source-pinned template; an alias author recurses on its RHS —
+/// bare identifier from the author's own source, `ns.X` through the
+/// author's namespace edge into the target module's own member. The depth
+/// cap breaks alias cycles (`A :: B; B :: A;`).
+fn followToTemplate(self: *Lowering, author: resolver_mod.RawAuthor, depth: u8) ?StructTemplate {
+    if (depth == 0) return null;
+    if (structDeclOfRaw(author.raw)) |sd| {
+        if (sd.type_params.len == 0) return null;
+        return self.buildGenericStructTemplate(sd, author.source);
+    }
+    const cd = constAliasOfRaw(author.raw) orelse return null;
+    switch (cd.value.data) {
+        .identifier => |id| {
+            const next = singleVisibleAuthor(self, id.name, author.source) orelse return null;
+            return followToTemplate(self, next, depth - 1);
+        },
+        .field_access => |fa| {
+            if (fa.object.data != .identifier) return null;
+            const target = switch (self.namespaceAliasVerdictFrom(fa.object.data.identifier.name, author.source)) {
+                .target => |t| t,
+                .none, .ambiguous => return null,
+            };
+            var res = self.resolver();
+            const member_set = res.collectNamespaceAuthors(target, fa.field);
+            const member = member_set.own orelse return null;
+            return followToTemplate(self, member, depth - 1);
+        },
+        else => return null,
+    }
+}
+
 /// The bare-VISIBLE single generic-struct author of `name` (its `StructDecl` +
 /// defining source) when that author is NOT the one the global last-wins
 /// `struct_template_map` already holds — the E4 non-transitive selection for a