langref: improve test_coerce_unions_enums.zig

Add more coercion examples to test_coerce_unions_enums.zig in the
"Type Coercion: unions and enums" section.

doc/langref.html.in

@@ -6448,14 +6448,37 @@ const U = union(E) {
     three,
 };
 
+const U2 = union(enum) {
+    a: void,
+    b: f32,
+
+    fn tag(self: U2) usize {
+        switch (self) {
+            .a => return 1,
+            .b => return 2,
+        }
+    }
+};
+
 test "coercion between unions and enums" {
     var u = U{ .two = 12.34 };
-    var e: E = u;
+    var e: E = u; // coerce union to enum
     try expect(e == E.two);
 
     const three = E.three;
-    var another_u: U = three;
-    try expect(another_u == E.three);
+    var u_2: U = three; // coerce enum to union
+    try expect(u_2 == E.three);
+
+    var u_3: U = .three; // coerce enum literal to union
+    try expect(u_3 == E.three);
+
+    var u_4: U2 = .a; // coerce enum literal to union with inferred enum tag type.
+    try expect(u_4.tag() == 1);
+
+    // The following example is invalid.
+    // error: coercion from enum '@TypeOf(.enum_literal)' to union 'test_coerce_unions_enum.U2' must initialize 'f32' field 'b'
+    //var u_5: U2 = .b;
+    //try expect(u_5.tag() == 2);
 }
       {#code_end#}
       {#see_also|union|enum#}