AstGen: add error message for capture error by ref in switch on error

src/AstGen.zig

@@ -7219,7 +7219,9 @@ fn switchExprErrUnion(
             };
 
             const capture_token = case.payload_token orelse break :blk &err_scope.base;
-            assert(token_tags[capture_token] == .identifier);
+            if (token_tags[capture_token] != .identifier) {
+                return astgen.failTok(capture_token + 1, "error set cannot be captured by reference", .{});
+            }
 
             const capture_slice = tree.tokenSlice(capture_token);
             if (mem.eql(u8, capture_slice, "_")) {

test/cases/compile_errors/error_set_cannot_capture_by_reference.zig

@@ -0,0 +1,15 @@
+export fn entry() void {
+    const err: error{Foo} = error.Foo;
+
+    switch (err) {
+        error.Foo => |*foo| {
+            foo catch {};
+        },
+    }
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :5:23: error: error set cannot be captured by reference

test/cases/compile_errors/switch_on_error_union_discard.zig

@@ -1,10 +1,8 @@
 export fn entry() void {
     const x: error{}!u32 = 0;
-    if (x) |v| v else |_| switch (_) {
-    }
+    if (x) |v| v else |_| switch (_) {}
 }
 
-
 // error
 // backend=stage2
 // target=native

test/cases/compile_errors/switch_on_error_with_capture_by_reference.zig

@@ -0,0 +1,24 @@
+comptime {
+    const e: error{Foo}!u32 = error.Foo;
+    e catch |err| switch (err) {
+        error.Foo => |*foo| {
+            foo catch {};
+        },
+    };
+}
+
+comptime {
+    const e: error{Foo}!u32 = error.Foo;
+    if (e) {} else |err| switch (err) {
+        error.Foo => |*foo| {
+            foo catch {};
+        },
+    }
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :4:24: error: error set cannot be captured by reference
+// :13:24: error: error set cannot be captured by reference