Add support for both '_' and 'else' prongs at the same time in switch statements

If both are used, 'else' handles named members and '_' handles
unnamed members. In this case the 'else' prong will be unrolled
to an explicit case containing all remaining named values.

test/behavior/switch.zig

@@ -1075,26 +1075,50 @@ test "switch on 8-bit mod result" {
 }
 
 test "switch on non-exhaustive enum" {
-    const E = enum(u32) {
+    if (builtin.zig_backend == .stage2_spirv) return error.SkipZigTest; // TODO
+
+    const E = enum(u4) {
         a,
         b,
         c,
         _,
+
+        fn doTheTest(e: @This()) !void {
+            switch (e) {
+                .a, .b => {},
+                else => return error.TestFailed,
+            }
+            switch (e) {
+                .a, .b => {},
+                .c => return error.TestFailed,
+                _ => return error.TestFailed,
+            }
+            switch (e) {
+                .a, .b => {},
+                .c, _ => return error.TestFailed,
+            }
+            switch (e) {
+                .a => {},
+                .b, .c, _ => return error.TestFailed,
+            }
+            switch (e) {
+                .b => return error.TestFailed,
+                else => {},
+                _ => return error.TestFailed,
+            }
+            switch (e) {
+                else => {},
+                _ => return error.TestFailed,
+            }
+            switch (e) {
+                inline else => {},
+                _ => return error.TestFailed,
+            }
+        }
     };
 
     var e: E = .a;
     _ = &e;
-    switch (e) {
-        .a, .b => {},
-        else => return error.TestFailed,
-    }
-    switch (e) {
-        .a, .b => {},
-        .c => return error.TestFailed,
-        _ => return error.TestFailed,
-    }
-    switch (e) {
-        .a, .b => {},
-        .c, _ => return error.TestFailed,
-    }
+    try E.doTheTest(e);
+    try comptime E.doTheTest(.a);
 }

test/behavior/switch_loop.zig

@@ -249,3 +249,27 @@ test "switch loop on larger than pointer integer" {
     }
     try expect(entry == 3);
 }
+
+test "switch loop on non-exhaustive enum" {
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_spirv) return error.SkipZigTest; // TODO
+
+    const S = struct {
+        const E = enum(u8) { a, b, c, _ };
+
+        fn doTheTest() !void {
+            var start: E = undefined;
+            start = .a;
+            const result: u32 = s: switch (start) {
+                .a => continue :s .c,
+                else => continue :s @enumFromInt(123),
+                .b, _ => |x| break :s @intFromEnum(x),
+            };
+            try expect(result == 123);
+        }
+    };
+    try S.doTheTest();
+    try comptime S.doTheTest();
+}

test/cases/compile_errors/switch_expression-non_exhaustive_inline.zig

@@ -0,0 +1,27 @@
+const E = enum(u8) {
+    a,
+    b,
+    _,
+};
+
+export fn f(e: E) void {
+    switch (e) {
+        .a => {},
+        inline _ => {},
+    }
+}
+
+export fn g(e: E) void {
+    switch (e) {
+        .a => {},
+        else => {},
+        inline _ => {},
+    }
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :10:16: error: cannot inline '_' prong
+// :18:16: error: cannot inline '_' prong

test/cases/compile_errors/switch_expression-non_exhaustive_unreachable_else.zig

@@ -0,0 +1,18 @@
+const E = enum(u8) {
+    a,
+    b,
+    _,
+};
+
+export fn f(e: E) void {
+    switch (e) {
+        .a, .b, _ => {},
+        else => {},
+    }
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :10:14: error: unreachable else prong; all explicit cases already handled

test/cases/compile_errors/switching_with_non-exhaustive_enums.zig

@@ -37,7 +37,7 @@ pub export fn entry3() void {
 // :12:5: error: switch must handle all possibilities
 // :3:5: note: unhandled enumeration value: 'b'
 // :1:11: note: enum 'tmp.E' declared here
-// :19:5: error: switch on non-exhaustive enum must include 'else' or '_' prong
+// :19:5: error: switch on non-exhaustive enum must include 'else' or '_' prong or both
 // :26:5: error: '_' prong only allowed when switching on non-exhaustive enums
 // :29:9: note: '_' prong here
 // :26:5: note: consider using 'else'