From 363cdb7d5fc376dfb6fc0f67d983922fc4208aa1 Mon Sep 17 00:00:00 2001
From: William Sengir <william@sengir.com>
Date: Wed, 11 May 2022 03:36:58 -0700
Subject: [PATCH] Sema: add error for disjoint error sets in `zirErrSetCast`

---
 src/Sema.zig | 41 +++++++++++++++++++++++++++++++++++++++--
 1 file changed, 39 insertions(+), 2 deletions(-)

diff --git a/src/Sema.zig b/src/Sema.zig
index d12b85e097..5f584fe40c 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -14209,9 +14209,46 @@ fn zirErrSetCast(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstDat
     try sema.checkErrorSetType(block, dest_ty_src, dest_ty);
     try sema.checkErrorSetType(block, operand_src, operand_ty);
 
-    if (try sema.resolveDefinedValue(block, operand_src, operand)) |val| {
-        try sema.resolveInferredErrorSetTy(block, src, dest_ty);
+    // operand must be defined since it can be an invalid error value
+    const maybe_operand_val = try sema.resolveDefinedValue(block, operand_src, operand);
 
+    if (disjoint: {
+        // Try avoiding resolving inferred error sets if we can
+        if (!dest_ty.isAnyError() and dest_ty.errorSetNames().len == 0) break :disjoint true;
+        if (!operand_ty.isAnyError() and operand_ty.errorSetNames().len == 0) break :disjoint true;
+        if (dest_ty.isAnyError()) break :disjoint false;
+        if (operand_ty.isAnyError()) break :disjoint false;
+        for (dest_ty.errorSetNames()) |dest_err_name|
+            if (operand_ty.errorSetHasField(dest_err_name))
+                break :disjoint false;
+
+        if (dest_ty.tag() != .error_set_inferred and operand_ty.tag() != .error_set_inferred)
+            break :disjoint true;
+
+        try sema.resolveInferredErrorSetTy(block, dest_ty_src, dest_ty);
+        try sema.resolveInferredErrorSetTy(block, operand_src, operand_ty);
+        for (dest_ty.errorSetNames()) |dest_err_name|
+            if (operand_ty.errorSetHasField(dest_err_name))
+                break :disjoint false;
+
+        break :disjoint true;
+    }) {
+        const msg = msg: {
+            const msg = try sema.errMsg(
+                block,
+                src,
+                "error sets '{}' and '{}' have no common errors",
+                .{ operand_ty.fmt(sema.mod), dest_ty.fmt(sema.mod) },
+            );
+            errdefer msg.destroy(sema.gpa);
+            try sema.addDeclaredHereNote(msg, operand_ty);
+            try sema.addDeclaredHereNote(msg, dest_ty);
+            break :msg msg;
+        };
+        return sema.failWithOwnedErrorMsg(block, msg);
+    }
+
+    if (maybe_operand_val) |val| {
         if (!dest_ty.isAnyError()) {
             const error_name = val.castTag(.@"error").?.data.name;
             if (!dest_ty.errorSetHasField(error_name)) {