From 0a0f84443fcb987fb6652ea4edaf8b5581cf5b66 Mon Sep 17 00:00:00 2001
From: David Rubin <daviru007@icloud.com>
Date: Tue, 16 Jan 2024 14:30:20 -0800
Subject: [PATCH 1/2] add type check to `zirSwitchBlockErrUnion`

---
 src/Sema.zig | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/src/Sema.zig b/src/Sema.zig
index 941c91fcd0bd..fcd7f2a1d8cc 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -11255,10 +11255,18 @@ fn zirSwitchBlockErrUnion(sema: *Sema, block: *Block, inst: Zir.Inst.Index) Comp
     defer seen_errors.deinit();
 
     const operand_ty = sema.typeOf(raw_operand_val);
-    const operand_err_set_ty = if (extra.data.bits.payload_is_ref)
-        operand_ty.childType(mod).errorUnionSet(mod)
+    const operand_err_set = if (extra.data.bits.payload_is_ref)
+        operand_ty.childType(mod)
     else
-        operand_ty.errorUnionSet(mod);
+        operand_ty;
+
+    if (operand_err_set.zigTypeTag(mod) != .ErrorUnion) {
+        return sema.fail(block, switch_src, "expected error union type, found '{}'", .{
+            operand_ty.fmt(mod),
+        });
+    }
+
+    const operand_err_set_ty = operand_err_set.errorUnionSet(mod);
 
     const block_inst: Air.Inst.Index = @enumFromInt(sema.air_instructions.len);
     try sema.air_instructions.append(gpa, .{

From 4e81c74b01c0b0bf15ac8bd576f2c6d01488311b Mon Sep 17 00:00:00 2001
From: David Rubin <daviru007@icloud.com>
Date: Tue, 16 Jan 2024 19:58:15 -0800
Subject: [PATCH 2/2] add test case

---
 test/cases/compile_errors/switch_on_non_err_union.zig | 11 +++++++++++
 1 file changed, 11 insertions(+)
 create mode 100644 test/cases/compile_errors/switch_on_non_err_union.zig

diff --git a/test/cases/compile_errors/switch_on_non_err_union.zig b/test/cases/compile_errors/switch_on_non_err_union.zig
new file mode 100644
index 000000000000..87624b21dc27
--- /dev/null
+++ b/test/cases/compile_errors/switch_on_non_err_union.zig
@@ -0,0 +1,11 @@
+pub fn main() void {
+    false catch |err| switch (err) {
+        else => {},
+    };
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:23: error: expected error union type, found 'bool'