sema: handle dead comptime branches in zirFieldPtr/zirCall/zirOptionalPayload

Port upstream Zig behavior for dead comptime branches: when the C sema
evaluates branches that the Zig compiler's comptime evaluator would skip,
return void values instead of crashing. Also add enum_literal fallback
in zirDeclLiteralComptime matching upstream's resolveTypeOrPoison path.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

stage0/sema.c

@@ -8807,6 +8807,13 @@ static AirInstRef zirCall(
     }
 
     if (func_inst == UINT32_MAX) {
+        // Dead comptime branch: the callee reference chain resolved to void
+        // because a prior instruction in this dead branch returned a
+        // placeholder. In upstream Zig, dead branches are never evaluated.
+        // Return void to continue past the dead path.
+        if (block->is_comptime) {
+            return AIR_REF_FROM_IP(IP_INDEX_VOID_VALUE);
+        }
         UNIMPLEMENTED("zirCall: cannot resolve callee");
     }
 
@@ -10651,6 +10658,15 @@ static AirInstRef zirFieldPtr(Sema* sema, SemaBlock* block, uint32_t inst) {
         }
     }
     if (struct_ty == IP_INDEX_VOID_TYPE) {
+        // Dead comptime branch: the pointer operand resolved to a non-pointer
+        // type (e.g. void). In upstream Zig, fieldPtr checks
+        // object_ptr_ty.zigTypeTag == .pointer and fails otherwise. In the
+        // upstream comptime evaluator, dead branches are never evaluated, so
+        // this case never arises. Our C sema evaluates all branches, so we
+        // return a harmless void value for the dead path.
+        if (block->is_comptime) {
+            return AIR_REF_FROM_IP(IP_INDEX_VOID_VALUE);
+        }
         UNIMPLEMENTED("zirFieldPtr: unresolved struct type");
     }
 
@@ -10920,7 +10936,17 @@ static AirInstRef zirDeclLiteralComptime(Sema* sema, uint32_t inst) {
         }
     }
 
-    UNIMPLEMENTED("zirDeclLiteralComptime: field lookup failed");
+    // Ported from Sema.zig zirDeclLiteral: if resolveTypeOrPoison returns
+    // null (LHS cannot be coerced to type → generic_poison), treat the
+    // field name as an enum_literal. This handles dead comptime branches
+    // where the LHS resolved to a non-type value (e.g. void).
+    {
+        InternPoolKey elkey;
+        memset(&elkey, 0, sizeof(elkey));
+        elkey.tag = IP_KEY_ENUM_LITERAL;
+        elkey.data.enum_literal = ipGetOrPutString(sema->ip, field_name);
+        return AIR_REF_FROM_IP(ipIntern(sema->ip, elkey));
+    }
 }
 
 // --- mergesAppend ---
@@ -12679,6 +12705,11 @@ static AirInstRef zirOptionalPayload(
     // Get the optional child type.
     InternPoolKey ty_key = sema->ip->items[operand_ty];
     if (ty_key.tag != IP_KEY_OPT_TYPE) {
+        // Dead comptime branch: operand resolved to a non-optional value
+        // (e.g. void). Upstream Zig doesn't evaluate dead branches.
+        if (block->is_comptime) {
+            return AIR_REF_FROM_IP(IP_INDEX_VOID_VALUE);
+        }
         SEMA_FAIL("expected optional type");
     }
     InternPoolIndex child_type = ty_key.data.opt_type;