From ee69a4b45f67e929fe5780ab6bf44360f6511d26 Mon Sep 17 00:00:00 2001
From: John Schmidt <john.schmidt.h@gmail.com>
Date: Sat, 12 Feb 2022 20:03:16 +0100
Subject: [PATCH] stage2: improve compiler error message for bad union init

---
 src/Sema.zig | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/src/Sema.zig b/src/Sema.zig
index 7dbc36af37..480a6a1ca2 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -2631,9 +2631,24 @@ fn validateUnionInit(
     union_ptr: Air.Inst.Ref,
 ) CompileError!void {
     if (instrs.len != 1) {
-        // TODO add note for other field
-        // TODO add note for union declared here
-        return sema.fail(block, init_src, "only one union field can be active at once", .{});
+        const msg = msg: {
+            const msg = try sema.errMsg(
+                block,
+                init_src,
+                "cannot initialize multiple union fields at once, unions can only have one active field",
+                .{},
+            );
+            errdefer msg.destroy(sema.gpa);
+
+            for (instrs[1..]) |inst| {
+                const inst_data = sema.code.instructions.items(.data)[inst].pl_node;
+                const inst_src: LazySrcLoc = .{ .node_offset_back2tok = inst_data.src_node };
+                try sema.errNote(block, inst_src, msg, "additional initializer here", .{});
+            }
+            try sema.mod.errNoteNonLazy(union_obj.srcLoc(), msg, "union declared here", .{});
+            break :msg msg;
+        };
+        return sema.failWithOwnedErrorMsg(msg);
     }
 
     const field_ptr = instrs[0];