Make single-element enum default to u0

* Allow comptime_int as explicit enum tag type

Closes #2997

Author: LemonBoy

src/analyze.cpp

@@ -2406,8 +2406,6 @@ static Error resolve_enum_zero_bits(CodeGen *g, ZigType *enum_type) {
     ZigType *tag_int_type;
     if (enum_type->data.enumeration.layout == ContainerLayoutExtern) {
         tag_int_type = get_c_int_type(g, CIntTypeInt);
-    } else if (enum_type->data.enumeration.layout == ContainerLayoutAuto && field_count == 1) {
-        tag_int_type = g->builtin_types.entry_num_lit_int;
     } else {
         tag_int_type = get_smallest_unsigned_int_type(g, field_count - 1);
     }
@@ -2420,7 +2418,8 @@ static Error resolve_enum_zero_bits(CodeGen *g, ZigType *enum_type) {
         ZigType *wanted_tag_int_type = analyze_type_expr(g, scope, decl_node->data.container_decl.init_arg_expr);
         if (type_is_invalid(wanted_tag_int_type)) {
             enum_type->data.enumeration.resolve_status = ResolveStatusInvalid;
-        } else if (wanted_tag_int_type->id != ZigTypeIdInt) {
+        } else if (wanted_tag_int_type->id != ZigTypeIdInt &&
+                   wanted_tag_int_type->id != ZigTypeIdComptimeInt) {
             enum_type->data.enumeration.resolve_status = ResolveStatusInvalid;
             add_node_error(g, decl_node->data.container_decl.init_arg_expr,
                 buf_sprintf("expected integer, found '%s'", buf_ptr(&wanted_tag_int_type->name)));
@@ -2806,14 +2805,12 @@ static Error resolve_union_zero_bits(CodeGen *g, ZigType *union_type) {
                 union_type->data.unionation.resolve_status = ResolveStatusInvalid;
                 return ErrorSemanticAnalyzeFail;
             }
-            if (tag_int_type->id != ZigTypeIdInt) {
+            if (tag_int_type->id != ZigTypeIdInt && tag_int_type->id != ZigTypeIdComptimeInt) {
                 add_node_error(g, enum_type_node,
                     buf_sprintf("expected integer tag type, found '%s'", buf_ptr(&tag_int_type->name)));
                 union_type->data.unionation.resolve_status = ResolveStatusInvalid;
                 return ErrorSemanticAnalyzeFail;
             }
-        } else if (auto_layout && field_count == 1) {
-            tag_int_type = g->builtin_types.entry_num_lit_int;
         } else {
             tag_int_type = get_smallest_unsigned_int_type(g, field_count - 1);
         }

test/stage1/behavior/enum.zig

@@ -1014,3 +1014,19 @@ test "enum with one member and u1 tag type @enumToInt" {
     };
     expect(@enumToInt(Enum.Test) == 0);
 }
+
+test "enum with comptime_int tag type" {
+    const Enum = enum(comptime_int) {
+        One = 3,
+        Two = 2,
+        Three = 1,
+    };
+    comptime expect(@TagType(Enum) == comptime_int);
+}
+
+test "enum with one member default to u0 tag type" {
+    const E0 = enum {
+        X,
+    };
+    comptime expect(@TagType(E0) == u0);
+}

test/stage1/behavior/union.zig

@@ -324,7 +324,7 @@ test "union with only 1 field casted to its enum type" {
 
     var e = Expr{ .Literal = Literal{ .Bool = true } };
     const Tag = @TagType(Expr);
-    comptime expect(@TagType(Tag) == comptime_int);
+    comptime expect(@TagType(Tag) == u0);
     var t = Tag(e);
     expect(t == Expr.Literal);
 }
@@ -335,7 +335,7 @@ test "union with only 1 field casted to its enum type which has enum value speci
         Bool: bool,
     };
 
-    const Tag = enum {
+    const Tag = enum(comptime_int) {
         Literal = 33,
     };
 
@@ -469,7 +469,7 @@ test "union no tag with struct member" {
 }
 
 fn testComparison() void {
-    var x = Payload{.A = 42};
+    var x = Payload{ .A = 42 };
     expect(x == .A);
     expect(x != .B);
     expect(x != .C);
@@ -494,3 +494,19 @@ test "packed union generates correctly aligned LLVM type" {
     };
     foo[0].f1();
 }
+
+test "union with one member defaults to u0 tag type" {
+    const U0 = union(enum) {
+        X: u32,
+    };
+    comptime expect(@TagType(@TagType(U0)) == u0);
+}
+
+test "union with comptime_int tag" {
+    const Union = union(enum(comptime_int)) {
+        X: u32,
+        Y: u16,
+        Z: u8,
+    };
+    comptime expect(@TagType(@TagType(Union)) == comptime_int);
+}