stage1: Make sure union(enum(T)) is valid

The T type should be wide enough to fit values in the  0...num field
range.

Closes #6988

src/stage1/analyze.cpp

@@ -3178,6 +3178,22 @@ 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) {
+                BigInt bi;
+                bigint_init_unsigned(&bi, field_count - 1);
+                if (!bigint_fits_in_bits(&bi,
+                            tag_int_type->data.integral.bit_count,
+                            tag_int_type->data.integral.is_signed))
+                {
+                    ErrorMsg *msg = add_node_error(g, enum_type_node,
+                            buf_sprintf("specified integer tag type cannot represent every field"));
+                    add_error_note(g, msg, enum_type_node,
+                            buf_sprintf("type %s cannot fit values in range 0...%" PRIu32,
+                                buf_ptr(&tag_int_type->name), field_count - 1));
+                    union_type->data.unionation.resolve_status = ResolveStatusInvalid;
+                    return ErrorSemanticAnalyzeFail;
+                }
+            }
         } else {
             tag_int_type = get_smallest_unsigned_int_type(g, field_count - 1);
         }

test/compile_errors.zig

@@ -2,6 +2,37 @@ const tests = @import("tests.zig");
 const std = @import("std");
 
 pub fn addCases(cases: *tests.CompileErrorContext) void {
+    cases.add("union with too small explicit signed tag type",
+        \\const U = union(enum(i2)) {
+        \\    A: u8,
+        \\    B: u8,
+        \\    C: u8,
+        \\    D: u8,
+        \\};
+        \\export fn entry() void {
+        \\    _ = U{ .D = 1 };
+        \\}
+    , &[_][]const u8{
+        "tmp.zig:1:22: error: specified integer tag type cannot represent every field",
+        "tmp.zig:1:22: note: type i2 cannot fit values in range 0...3",
+    });
+
+    cases.add("union with too small explicit unsigned tag type",
+        \\const U = union(enum(u2)) {
+        \\    A: u8,
+        \\    B: u8,
+        \\    C: u8,
+        \\    D: u8,
+        \\    E: u8,
+        \\};
+        \\export fn entry() void {
+        \\    _ = U{ .E = 1 };
+        \\}
+    , &[_][]const u8{
+        "tmp.zig:1:22: error: specified integer tag type cannot represent every field",
+        "tmp.zig:1:22: note: type u2 cannot fit values in range 0...4",
+    });
+
     cases.add("unreachable executed at comptime",
         \\fn foo(comptime x: i32) i32 {
         \\    comptime {