llvm: fix jump table gen for labeled switch with single else prong

Avoids a null unwrap if there are no cases with explicit values present
while trying to construct a jump table for a labeled switch statement.

src/codegen/llvm.zig

@@ -6432,7 +6432,7 @@ pub const FuncGen = struct {
 
             // Don't worry about the size of the type -- it's irrelevant, because the prong values could be fairly dense.
             // If they are, then we will construct a jump table.
-            const min, const max = self.switchCaseItemRange(switch_br);
+            const min, const max = self.switchCaseItemRange(switch_br) orelse break :jmp_table null;
             const min_int = min.getUnsignedInt(zcu) orelse break :jmp_table null;
             const max_int = max.getUnsignedInt(zcu) orelse break :jmp_table null;
             const table_len = max_int - min_int + 1;
@@ -6595,7 +6595,7 @@ pub const FuncGen = struct {
         }
     }
 
-    fn switchCaseItemRange(self: *FuncGen, switch_br: Air.UnwrappedSwitch) [2]Value {
+    fn switchCaseItemRange(self: *FuncGen, switch_br: Air.UnwrappedSwitch) ?[2]Value {
         const zcu = self.ng.pt.zcu;
         var it = switch_br.iterateCases();
         var min: ?Value = null;
@@ -6619,6 +6619,10 @@ pub const FuncGen = struct {
                 if (high) max = vals[1];
             }
         }
+        if (min == null) {
+            assert(max == null);
+            return null;
+        }
         return .{ min.?, max.? };
     }
 

test/behavior/switch_loop.zig

@@ -297,3 +297,52 @@ test "switch loop with discarded tag capture" {
     S.doTheTest();
     comptime S.doTheTest();
 }
+
+test "switch loop with single catch-all prong" {
+    if (builtin.zig_backend == .stage2_spirv) return error.SkipZigTest;
+
+    const S = struct {
+        const E = enum { a, b, c };
+        const U = union(E) { a: u32, b: u16, c: u8 };
+
+        fn doTheTest() !void {
+            var x: usize = 0;
+            label: switch (E.a) {
+                else => {
+                    x += 1;
+                    if (x >= 5) continue :label .b;
+                    if (x == 10) break :label;
+                    continue :label .c;
+                },
+            }
+            try expect(x == 10);
+
+            label: switch (E.a) {
+                .a, .b, .c => {
+                    x += 1;
+                    if (x >= 15) continue :label .b;
+                    if (x == 20) break :label;
+                    continue :label .c;
+                },
+            }
+            try expect(x == 20);
+
+            label: switch (E.a) {
+                else => if (false) continue :label true,
+            }
+
+            const ok = label: switch (U{ .a = 123 }) {
+                else => |u| {
+                    const y: u32 = switch (u) {
+                        inline else => |y| y,
+                    };
+                    if (y == 456) break :label true;
+                    continue :label .{ .b = 456 };
+                },
+            };
+            try expect(ok);
+        }
+    };
+    try S.doTheTest();
+    try comptime S.doTheTest();
+}