Fix edge case in tagName handling of unions

Closes #4226

src/ir.cpp

@@ -22354,6 +22354,15 @@ static IrInstruction *ir_analyze_instruction_enum_tag_name(IrAnalyze *ira, IrIns
 
     assert(target->value->type->id == ZigTypeIdEnum);
 
+    if (target->value->type->data.enumeration.src_field_count == 1 &&
+        !target->value->type->data.enumeration.non_exhaustive) {
+        TypeEnumField *only_field = &target->value->type->data.enumeration.fields[0];
+        ZigValue *array_val = create_const_str_lit(ira->codegen, only_field->name)->data.x_ptr.data.ref.pointee;
+        IrInstruction *result = ir_const(ira, &instruction->base, nullptr);
+        init_const_slice(ira->codegen, result->value, array_val, 0, buf_len(only_field->name), true);
+        return result;
+    }
+
     if (instr_is_comptime(target)) {
         if ((err = type_resolve(ira->codegen, target->value->type, ResolveStatusZeroBitsKnown)))
             return ira->codegen->invalid_instruction;

test/stage1/behavior/union.zig

@@ -629,3 +629,12 @@ test "union initializer generates padding only if needed" {
     var v = U{ .A = 532 };
     expect(v.A == 532);
 }
+
+test "runtime tag name with single field" {
+    const U = union(enum) {
+        A: i32,
+    };
+
+    var v = U{ .A = 42 };
+    expect(std.mem.eql(u8, @tagName(v), "A"));
+}