feat(metatype): comptime-eval generic type-fn body locals

A generic ($T) -> Type type-fn comptime-evaluated only its return
EXPRESSION, so a local declared before the return ('vs := …; return
make_enum(…, vs)') was unresolved. Now a body with a prelude (statements
before the return) has its full body evaluated: createComptimeFunction-
WithPrelude lowers the pre-return statements into the comptime function's
scope before the return expr, so the locals resolve.

- comptime.zig: createComptimeFunctionWithPrelude (prelude stmts +
  expr); evalComptimeTypeBody (extract prelude + return expr, scan the
  whole body for declare() forward types); runComptimeTypeFunc factored
  out of evalComptimeType (shared bail/declare-never-defined handling).
- generic.zig: route a type-fn body WITH a prelude through
  evalComptimeTypeBody; no-prelude bodies stay on evalComptimeType (zero
  change for RecvResult/TryResult etc.).

Non-generic builders (whole body already evaluated) and the List-growth
path are unaffected. Suite green (684).

src/ir/lower.zig

@@ -1567,6 +1567,8 @@ pub const Lowering = struct {
     pub const evalComptimeInt = lower_comptime.evalComptimeInt;
     pub const evalComptimeString = lower_comptime.evalComptimeString;
     pub const evalComptimeType = lower_comptime.evalComptimeType;
+    pub const evalComptimeTypeBody = lower_comptime.evalComptimeTypeBody;
+    pub const runComptimeTypeFunc = lower_comptime.runComptimeTypeFunc;
     pub const renameNominalType = lower_comptime.renameNominalType;
     pub const lowerComptimeGlobal = lower_comptime.lowerComptimeGlobal;
     pub const lowerComptimeSideEffect = lower_comptime.lowerComptimeSideEffect;
@@ -1578,6 +1580,7 @@ pub const Lowering = struct {
     pub const substituteComptimeNodes = lower_comptime.substituteComptimeNodes;
     pub const fnBodyHasReturn = lower_comptime.fnBodyHasReturn;
     pub const createComptimeFunction = lower_comptime.createComptimeFunction;
+    pub const createComptimeFunctionWithPrelude = lower_comptime.createComptimeFunctionWithPrelude;
     pub const constExprValue = lower_comptime.constExprValue;
     pub const constArrayLiteral = lower_comptime.constArrayLiteral;
     pub const constStructLiteral = lower_comptime.constStructLiteral;

src/ir/lower/comptime.zig

@@ -448,7 +448,39 @@ pub fn evalComptimeType(self: *Lowering, expr: *const Node) ?TypeId {
     // body. The interp's `declare` returns this same slot; `define` completes it.
     preregisterForwardTypes(self, expr);
     const func_id = self.createComptimeFunction("__ctype", expr, .any);
+    return self.runComptimeTypeFunc(func_id, expr.span);
+}
 
+/// Comptime-evaluate a type-fn BODY that has local statements before its
+/// `return` (the plain `evalComptimeType` only sees the return expression, so a
+/// local declared before it is unresolved). Lowers the pre-return statements as
+/// a prelude, then the return expression. Only used for bodies that actually
+/// have a prelude — the no-prelude case stays on `evalComptimeType`.
+pub fn evalComptimeTypeBody(self: *Lowering, body: *const Node, ret_expr: *const Node) ?TypeId {
+    // Scan the WHOLE body for `declare("Name")` (a local `h := declare(…)` is in
+    // the prelude, not the return) so forward types register before lowering.
+    preregisterForwardTypes(self, body);
+    const prelude = preludeBeforeReturn(body);
+    const func_id = self.createComptimeFunctionWithPrelude("__ctype", prelude, ret_expr, .any);
+    return self.runComptimeTypeFunc(func_id, ret_expr.span);
+}
+
+/// The statements of a block body that PRECEDE its first `return` — the locals a
+/// type-fn binds before minting. Empty for a non-block (arrow) body.
+fn preludeBeforeReturn(body: *const Node) []const *const Node {
+    if (body.data != .block) return &.{};
+    const stmts = body.data.block.stmts;
+    for (stmts, 0..) |stmt, i| {
+        if (stmt.data == .return_stmt) return stmts[0..i];
+    }
+    return &.{};
+}
+
+/// Run a comptime type-construction function and post-process its result: render
+/// any interp bail as a build-gating diagnostic (issue 0140) and reject a bare
+/// `declare()` never completed by `define()` (a zero-field nominal slot that
+/// would otherwise panic at codegen). `span` locates both diagnostics.
+pub fn runComptimeTypeFunc(self: *Lowering, func_id: FuncId, span: ast.Span) ?TypeId {
     var interp = interp_mod.Interpreter.init(self.module, self.alloc);
     defer interp.deinit();
     if (self.diagnostics) |d| if (d.import_sources) |sm| interp.setSourceMap(sm);
@@ -469,7 +501,7 @@ pub fn evalComptimeType(self: *Lowering, expr: *const Node) ?TypeId {
         // unannounced.
         if (self.diagnostics) |d| {
             const detail = interp_mod.Interpreter.last_bail_detail orelse @errorName(err);
-            d.addFmt(.err, expr.span, "comptime type construction failed: {s}", .{detail});
+            d.addFmt(.err, span, "comptime type construction failed: {s}", .{detail});
         }
         return null;
     };
@@ -483,7 +515,7 @@ pub fn evalComptimeType(self: *Lowering, expr: *const Node) ?TypeId {
         const info = self.module.types.get(tid);
         if (info == .tagged_union and info.tagged_union.fields.len == 0) {
             if (self.diagnostics) |d|
-                d.addFmt(.err, expr.span, "type '{s}' is declared but never defined — complete it with define(handle, info)", .{self.module.types.getString(info.tagged_union.name)});
+                d.addFmt(.err, span, "type '{s}' is declared but never defined — complete it with define(handle, info)", .{self.module.types.getString(info.tagged_union.name)});
             return null;
         }
     }
@@ -799,6 +831,15 @@ pub fn fnBodyHasReturn(node: *const Node) bool {
 /// Creates a temporary function marked `is_comptime = true` that wraps
 /// the given expression as its return value. Returns the FuncId.
 pub fn createComptimeFunction(self: *Lowering, prefix: []const u8, expr: *const Node, ret_ty: TypeId) FuncId {
+    return self.createComptimeFunctionWithPrelude(prefix, &.{}, expr, ret_ty);
+}
+
+/// Like `createComptimeFunction`, but lowers `prelude` statements (e.g. a
+/// type-fn body's local declarations) into the comptime function's scope BEFORE
+/// the result `expr`, so the expr can reference names they bind. Used to
+/// comptime-evaluate a generic type-fn body that has locals before its `return`
+/// (the non-prelude path only sees the return expression).
+pub fn createComptimeFunctionWithPrelude(self: *Lowering, prefix: []const u8, prelude: []const *const Node, expr: *const Node, ret_ty: TypeId) FuncId {
     var buf: [64]u8 = undefined;
     const name = std.fmt.bufPrint(&buf, "{s}_{d}", .{ prefix, self.comptime_counter }) catch prefix;
     self.comptime_counter += 1;
@@ -883,6 +924,11 @@ pub fn createComptimeFunction(self: *Lowering, prefix: []const u8, expr: *const
     var ct_scope = Scope.init(self.alloc, saved_scope);
     self.scope = &ct_scope;
 
+    // Lower any prelude statements (type-fn body locals) so the result
+    // expression can reference the names they bind. Empty for the common
+    // single-expression case.
+    for (prelude) |stmt| self.lowerStmt(stmt);
+
     // Lower the expression and return it
     const result = self.lowerExpr(expr);
     if (ret_ty == .void) {

src/ir/lower/generic.zig

@@ -1759,7 +1759,21 @@ pub fn instantiateTypeFunction(self: *Lowering, alias_name: []const u8, template
     // `resolveTypeWithBindings` can't evaluate a Type-returning call.
     if (findReturnTypeExpr(fd.body)) |ret_node| {
         if (self.returnExprMintsType(ret_node)) {
-            const tid = self.evalComptimeType(ret_node) orelse return .unresolved;
+            // A body with LOCALS before its `return` (e.g. `vs := …; return
+            // make_enum(…, vs)`) needs its full body comptime-evaluated so those
+            // locals resolve; the bare return-expr path leaves them unresolved.
+            // A no-prelude body stays on the simpler `evalComptimeType` path.
+            const has_prelude = fd.body.data == .block and blk: {
+                for (fd.body.data.block.stmts) |stmt| {
+                    if (stmt.data == .return_stmt) break :blk false;
+                    break :blk true; // a non-return statement precedes the return
+                }
+                break :blk false;
+            };
+            const tid = (if (has_prelude)
+                self.evalComptimeTypeBody(fd.body, ret_node)
+            else
+                self.evalComptimeType(ret_node)) orelse return .unresolved;
             // Re-key to the instantiation's mangled (or alias) name so the
             // cache check at the top dedups a second instantiation — Contract 1.
             self.renameNominalType(tid, if (has_alias) alias_name else mangled_name);