Zcu: fix switch prong source location resolution

Resolves: #22343

src/Zcu.zig

@@ -1864,15 +1864,16 @@ pub const SrcLoc = struct {
                         if (want_case_idx.isSpecial()) {
                             break case;
                         }
+                        continue;
                     }
 
                     const is_multi = case.ast.values.len != 1 or
                         node_tags[case.ast.values[0]] == .switch_range;
 
-                    if (!want_case_idx.isSpecial()) switch (want_case_idx.kind) {
+                    switch (want_case_idx.kind) {
                         .scalar => if (!is_multi and want_case_idx.index == scalar_i) break case,
                         .multi => if (is_multi and want_case_idx.index == multi_i) break case,
-                    };
+                    }
 
                     if (is_multi) {
                         multi_i += 1;

test/cases/compile_errors/invalid_switch_item.zig

@@ -0,0 +1,46 @@
+const E = enum { a, b, c };
+var my_e: E = .a;
+
+export fn f0() void {
+    switch (my_e) {
+        .a => {},
+        .b => {},
+        .x => {},
+        .c => {},
+    }
+}
+
+export fn f1() void {
+    switch (my_e) {
+        else => {},
+        .x, .y => {},
+    }
+}
+
+export fn f2() void {
+    switch (my_e) {
+        else => {},
+        .a => {},
+        .x, .y => {},
+        .b => {},
+    }
+}
+
+export fn f3() void {
+    switch (my_e) {
+        .a, .b => {},
+        .x, .y => {},
+        else => {},
+    }
+}
+
+// error
+//
+// :8:10: error: no field named 'x' in enum 'tmp.E'
+// :1:11: note: enum declared here
+// :16:10: error: no field named 'x' in enum 'tmp.E'
+// :1:11: note: enum declared here
+// :24:10: error: no field named 'x' in enum 'tmp.E'
+// :1:11: note: enum declared here
+// :32:10: error: no field named 'x' in enum 'tmp.E'
+// :1:11: note: enum declared here