comptime enum value params: $o: EnumType binds+resolves variant tag

A comptime value param whose constraint is a plain enum ($o: Ord) now
binds its enum-literal argument to the variant tag during inlined
comptime-call lowering. The tag is recorded in comptime_value_bindings
(readable downstream via comptimeIntNamed / direct map lookup, and as an
array-dim style const-int leaf) AND the param is bound into scope as an
enum_init value so body comparisons like 'if o == .a' lower as ordinary
enum comparisons. Distinct ordering args monomorphize the inlined body
per value.

A non-constant argument or an unknown variant emits a loud diagnostic
and binds nothing — never a silent default.

Locked by examples/0627-comptime-enum-value-param.sx.

examples/0627-comptime-enum-value-param.sx

@@ -0,0 +1,27 @@
+// Comptime ENUM value parameter: `$o: <EnumType>` binds the enum-literal
+// argument to its variant tag, monomorphizes the inlined body per distinct
+// ordering value, and resolves `o` in the body as a compile-time-known enum
+// constant — usable both in `if o == .a` comparisons AND as a comptime-readable
+// variant tag during lowering (a downstream lowerer reads it via
+// `comptime_value_bindings`, exercised here as the `[o]i64` array dimension).
+#import "modules/std.sx";
+
+Ord :: enum { a; b; c; }
+
+pick :: ($o: Ord) -> i64 {
+    if o == .a { return 10; }
+    if o == .b { return 20; }
+    return 30;
+}
+
+// `o` read as a compile-time integer (its variant tag) in a TYPE position.
+tag_dim :: ($o: Ord) -> i64 {
+    arr : [o]i64 = ---;
+    return arr.len;
+}
+
+main :: () {
+    print("{}\n", pick(.b));                       // 20
+    print("{} {} {}\n", pick(.a), pick(.b), pick(.c)); // 10 20 30
+    print("{} {} {}\n", tag_dim(.a), tag_dim(.b), tag_dim(.c)); // 0 1 2
+}

examples/expected/0627-comptime-enum-value-param.exit

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

examples/expected/0627-comptime-enum-value-param.stderr

@@ -0,0 +1 @@
+

examples/expected/0627-comptime-enum-value-param.stdout

@@ -0,0 +1,3 @@
+20
+10 20 30
+0 1 2

src/ir/lower.zig

@@ -1572,6 +1572,7 @@ pub const Lowering = struct {
     pub const lowerComptimeGlobal = lower_comptime.lowerComptimeGlobal;
     pub const lowerComptimeSideEffect = lower_comptime.lowerComptimeSideEffect;
     pub const lowerComptimeCall = lower_comptime.lowerComptimeCall;
+    pub const bindEnumValueParams = lower_comptime.bindEnumValueParams;
     pub const lowerInlineComptime = lower_comptime.lowerInlineComptime;
     pub const lowerInsertExpr = lower_comptime.lowerInsertExpr;
     pub const lowerInsertExprValue = lower_comptime.lowerInsertExprValue;

src/ir/lower/comptime.zig

@@ -739,6 +739,21 @@ pub fn lowerComptimeCall(self: *Lowering, fd: *const ast.FnDecl, call_node: *con
         }
     }
 
+    // Bind ENUM-typed comptime value params (`$o: Ord`) to their argument's
+    // variant tag — both into `comptime_value_bindings` (so a downstream
+    // lowerer can read the constant tag for an identifier) AND as a scoped
+    // enum value (so `if o == .a` lowers as an ordinary enum comparison).
+    // Saved/restored around the body so nested comptime calls don't leak the
+    // outer call's tags. A non-constant / unknown-variant enum arg is a loud
+    // diagnostic (never a silent default) per the value-param rules.
+    const saved_enum_value_bindings = self.comptime_value_bindings;
+    var enum_value_bindings: ?std.StringHashMap(i64) = null;
+    self.bindEnumValueParams(fd, cpn, &enum_value_bindings);
+    defer {
+        if (enum_value_bindings) |*evb| evb.deinit();
+        self.comptime_value_bindings = saved_enum_value_bindings;
+    }
+
     // Install comptime param nodes and lower the function body inline
     const saved_cpn = self.comptime_param_nodes;
     self.comptime_param_nodes = cpn;
@@ -836,6 +851,92 @@ pub fn lowerComptimeCall(self: *Lowering, fd: *const ast.FnDecl, call_node: *con
     return self.builder.constInt(0, .void);
 }
 
+/// Bind ENUM-typed comptime value params (`$o: Ord`) for an inlined comptime
+/// call. For each comptime param whose declared constraint resolves to an
+/// `.@"enum"` type and whose call-site argument is a constant enum literal
+/// (`.b`), this:
+///   1. folds the literal to its variant tag (declaration-order index, or the
+///      explicit value),
+///   2. records `param_name -> tag` in `self.comptime_value_bindings` (lazily
+///      allocated into `*store` and activated on `self`), so a downstream
+///      lowerer / `#builtin` recognizer can read the constant tag for the
+///      param identifier via `comptimeIntNamed(param_name)` (or directly off
+///      `comptime_value_bindings.get(param_name)`), and
+///   3. binds `param_name` into the active scope as an `enum_init(tag, …)`
+///      value of the constraint enum type, so the body's `if o == .a` lowers
+///      as an ordinary enum comparison.
+/// A non-constant argument, or one naming an unknown variant, emits a loud
+/// diagnostic and binds nothing — never a silent default.
+pub fn bindEnumValueParams(
+    self: *Lowering,
+    fd: *const ast.FnDecl,
+    cpn: std.StringHashMap(*const Node),
+    store: *?std.StringHashMap(i64),
+) void {
+    for (fd.params) |param| {
+        if (!param.is_comptime or param.is_variadic) continue;
+        const arg_node = cpn.get(param.name) orelse continue;
+
+        // Resolve the param's declared constraint type in the function's own
+        // defining module (the enum may be bare-visible only there).
+        const constraint_ty = self.resolveParamTypeInSource(fd.body.source_file, &param);
+        if (constraint_ty.isBuiltin()) continue;
+        const info = self.module.types.get(constraint_ty);
+        if (info != .@"enum") continue; // only plain enums carry a bare tag
+
+        // The argument must be a constant enum literal (`.b`). A non-literal
+        // arg (a runtime value, an arbitrary expression) cannot bind a
+        // compile-time variant tag — diagnose loudly rather than fabricate one.
+        if (arg_node.data != .enum_literal) {
+            if (self.diagnostics) |d|
+                d.addFmt(.err, arg_node.span, "comptime enum value parameter '{s}' must be a constant enum literal of '{s}'", .{ param.name, self.formatTypeName(constraint_ty) });
+            continue;
+        }
+        const variant_name = arg_node.data.enum_literal.name;
+
+        // The variant must exist on the constraint enum — otherwise this is a
+        // typo / wrong-enum literal, not a bindable tag.
+        const name_id = self.module.types.internString(variant_name);
+        var known = false;
+        for (info.@"enum".variants) |v| {
+            if (v == name_id) {
+                known = true;
+                break;
+            }
+        }
+        if (!known) {
+            self.emitBadEnumVariant(constraint_ty, info.@"enum", variant_name, arg_node.span);
+            continue;
+        }
+
+        const tag = self.resolveVariantValue(constraint_ty, variant_name);
+
+        // 1+2: record the constant tag, lazily creating the binding map and
+        // activating it on `self` (so `comptimeIntNamed`/the downstream lowerer
+        // can read it). Seed the map from any already-active bindings so an
+        // outer comptime call's value params stay visible in the inlined body.
+        if (store.* == null) {
+            var m = std.StringHashMap(i64).init(self.alloc);
+            if (self.comptime_value_bindings) |outer| {
+                var it = outer.iterator();
+                while (it.next()) |e| m.put(e.key_ptr.*, e.value_ptr.*) catch {};
+            }
+            store.* = m;
+            self.comptime_value_bindings = store.*;
+        }
+        store.*.?.put(param.name, @intCast(tag)) catch {};
+        self.comptime_value_bindings = store.*;
+
+        // 3: bind the param as a scoped enum value so body comparisons lower.
+        if (self.scope) |scope| {
+            const enum_val = self.builder.enumInit(tag, Ref.none, constraint_ty);
+            const slot = self.builder.alloca(constraint_ty);
+            self.builder.store(slot, enum_val);
+            scope.put(param.name, .{ .ref = slot, .ty = constraint_ty, .is_alloca = true });
+        }
+    }
+}
+
 /// True if `node` (a fn body) contains any top-level `return` statement.
 /// Used by inline-comptime lowering to decide whether to allocate a
 /// result slot — pure tail-expression bodies skip the slot. Walks past